Tick-borne encephalitis virus vaccines

SECTION TWO: Licensed vaccines

Tick-borne encephalitis virus vaccines P. Noel Barrett Daniel Portsmouth Hartmut J. Ehrlich

Tick-borne encephalitis virus (TBEV) is a member of the family Flaviviridae, which comprises approximately 70 viruses that cause many serious diseases including yellow fever, Japanese encephalitis, and dengue fever. TBEV is one of the major human pathogenic flaviviruses, with disease being caused by three subtypes (European, Far Eastern, and Siberian).1 The effects of the European subtype were first described in 1931 by Schneider,2 who reported a seasonal outbreak of meningitis cases in the district of Neunkirchen in Lower Austria. This was the first report of TBE in the literature. Shortly afterward, the disease was reported from the far eastern part of Russia and from 1939 onward, also in its European part. The disease is now reported to occur in Western, Central, Eastern and Northern Europe, Russia, and Asia, corresponding to the distribution of ixodid tick species, the chief vectors of TBEV. The disease has also been referred to as spring-summer meningoencephalitis, central European encephalitis, Far Eastern encephalitis, Taiga encephalitis, Russian spring-summer encephalitis, biundulating meningoencephalitis, diphasic milk fever, Kumlinge disease, and Schneider disease. Among the flaviviruses, TBEV has one of the highest impacts as a human pathogen. According to a study conducted in 1958, 56% of all viral central nervous system (CNS) diseases in Austria were caused by TBEV infection.3 Before the start of the vaccination program, it was the most important and most frequent disease of this type in adults, with several hundred hospitalization cases reported each year.4 During the last 20 years between 5,000 and 12,000 clinical TBE cases have been reported worldwide each year. However, the incidence of TBE is underestimated, even in countries where the disease is known, and there must still be regions where TBE occurs and is not properly diagnosed. In addition, the increasing mobility in people's lifestyle exacerbates the risk of infection. Therefore, TBE is a growing public health problem in Europe. As an efficient and safe vaccine is available, it is a preventable disease.

Background Clinical description The clinical course of TBE is monophasic or biphasic. The incubation period, which is clinically silent, may last between 2 and 28 days, but in most cases it is between 7 and 14 days. The first stage, which may last for 1 to 8 days, corresponds to the viremic phase. It is associated with nonspecific systemic signs and symptoms such as fatigue, headache, aching back and

34

limbs, nausea, and general malaise, with temperatures rising to 38°C or higher in most cases. Sometimes exceptionally high initial temperatures may occur, rising to more than 40°C.5 An afebrile interval follows the first stage of TBE and lasts 1 to 20 days, during which patients are usually free of symptoms. Another sudden rise of temperature marks the beginning of the second stage of the disease. The clinical manifestations in this second febrile episode are far more serious. Patients have temperatures that are higher than the average temperatures in other forms of viral meningitis or meningoencephalitis. In about 50% of cases there is CNS involvement in the form of meningitis with lymphocytosis and elevated cerebrospinal fluid protein levels. About 40% of cases have more severe disease with signs of encephalitis, including paralysis, stupor, and pyramidal tract signs. Meningoencephalomyelitis, the most severe form of the disease, occurs in about 10% of patients.6 In paralytic forms of the disease, paralysis, especially in the region of the shoulder girdle, develops 5 to 10 days after the remission of fever. Paralysis may progress up to 2 weeks, followed by a moderate tendency toward improvement. The risk of developing longterm sequelae increases according to the severity of the neurologic symptoms.5 A recent study showed that 80% of patients with TBE with encephalomyelitis did not recover in a 10-year follow-up period.7 About 5% of patients require intensive care owing to respiratory paralysis or serious disorder of consciousness.5 Hospitalization varies between 3 and 40 weeks,8,9 depending on the severity of the illness. Long-term rehabilitation is required for about 20% to 30% of patients.5 In children and juveniles, meningitis is the predominant form of the disease; this is why the infection usually takes a milder course than that observed in adults. However, about 35% to 58% of patients younger than 15 years have long-term cognitive or neuropsychiatric sequelae,6 and a number of cases of severe TBE have been reported, even in young children.10–13 After the approximate age of 40 years, patients affected by TBE increasingly have the encephalitic form of the disease. The incidence of TBE is substantially higher in elderly people, and, especially in persons older than 60 years, the disease increasingly takes a severe course. Severe symptoms such as paralysis and seizures are more common, and the case fatality rate is higher.5,14 In Austria, Germany, Sweden, Switzerland, and Lithuania, more than 50% of TBE cases are reported in people 50 years or older.15 The high incidence and substantially increased morbidity in elderly people makes this population a special target group for immunization.16 Not all persons infected with TBE experience the entire course of the disease. In approximately 65% of cases, the infection remains silent or the patient shows the clinical picture of the initial phase of TBE but the symptoms subside without

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SECTION TWO • Licensed vaccines

developing into full-blown TBE. The remaining 35% of people infected experience the second phase of TBE, the majority of whom have the typical biphasic course. In the remainder, the infection is inapparent during the first stage and the onset of clinical illness coincides with the beginning of the second phase of the disease.17 The clinical picture of the disease with the Far Eastern variety differs from the European form. The onset of illness is more often gradual than acute, with a prodromal phase including fever, headache, anorexia, nausea, vomiting, and photophobia. These symptoms are followed by stiff neck, sensorial changes, visual disturbances, and variable neurologic dysfunctions, including paresis, paralysis, sensory loss, and seizures. In fatal cases, death usually occurs within the first week after onset. The case-fatality rate is more than 20%,16,18 compared with 1% to 2% for the European form, but these rates may be biased by differences in the reporting of less severe cases and the different standards of medical treatment available in Western Europe and eastern regions. In contrast with the European form, the disease caused by the Far Eastern variety is more severe in children than in adults. Neurologic sequelae occur in 30% to 80% of survivors, especially residual flaccid paralyses of the shoulder girdle and arms. Fatal cases of TBE with an unusual hemorrhagic syndrome have also been associated with the Far Eastern TBEV subtype.19 The Siberian subtype is considered less virulent than the Far Eastern subtype, with reports of case fatality rates between 2% and 8%.16,18,20 However, it has been reported that severe human cases of TBE are becoming more frequently associated with Siberian strains.20

Virology and pathogenesis Electron microscopy of negatively stained TBEV shows it to be spherical, with a diameter of about 50 nm, carrying a fringe of small projections on its surface. The virus particle consists of an electron-dense spherical nucleocapsid of approximately 30 nm in diameter that is surrounded by a lipid bilayer. In sucrose density gradients, purified virus sediments homogeneously at about 200 S and bands after equilibrium centrifugation at a density of about 1.19 g/cm (for review, see Burke et al21). The virus genome consists of a single, positive-stranded RNA molecule about 11,000 nucleotides long. Mature virions are composed of three structural proteins termed envelope (E), core (C), and membrane (M) protein, with molecular weights of 55, 15, and 8 kDa, respectively. The envelope proteins E and M are type 1 membrane proteins embedded in the lipid bilayer by C-terminal hydrophobic anchors. In addition, a precursor of the membrane protein (prM) is present in immature intracellular virus particles. The M protein is derived by a furin-mediated cleavage from the glycosylated precursor protein prM. C is the only protein constituent of the isometric nucleocapsid that contains the virion RNA. The virus RNA also codes for seven nonstructural (NS) proteins that can be detected only in infected cells. The coding sequence of the positively stranded RNA is 5′-C-prM-ENS1-NS2A-NS2B-NS3-NS4A-NS4B-NS5-3′. All viral proteins are encoded within a single open reading frame. The individual proteins are released from a precursor polyprotein by cotranslational and posttranslational cleavage.22 Glycoprotein E contains the important antigenic determinants responsible for hemagglutination inhibition (HI) and neutralization and is responsible for induction of immunological responses in the infected host. Structural elements of the E protein determinants are involved in the binding of virions to cell receptors and in intraendosomal fusion at low pH.23 By isolating a soluble, crystallizable form of the TBEV protein E,24 it was possible to elucidate its three-dimensional structure using X-ray diffraction analysis.25 Structural analysis has shown that protein E, unlike the envelope proteins of many other lipid-enveloped viruses, does not form spikelike projections, but is aligned parallel to the viral surface.

Antigen analyses using monoclonal antibodies and comparisons of sequences of various virus isolates have shown that TBEVs are closely related within and between subtypes. European subtype viruses are quite homogeneous (maximum E-protein amino acid sequence divergence, 2.2%), and Siberian and Far-Eastern s­ ubtypes are also closely related to the European subtype, h ­ aving 94 to 96% identity in E-protein sequences.26 After the bite of an infected tick, the virus usually replicates in the dermal cells at the site of the bite. From there, the virus is transferred by afferent lymphatic vessels to the regional lymph nodes. After further replication in lymphoid tissue, the virus is spread via the lymphatic system and the bloodstream, and it invades other susceptible organs or tissues, especially the reticuloendothelial system. Massive virus replication takes place there, and only after this stage is it possible for the virus to reach the CNS. High production of virus in the primarily affected organs is a prerequisite for the virus to cross the blood-brain barrier because the capillary endothelium is not easily infected. It has recently been shown that TBEV rearranges intracellular membranes to prevent recognition of double-stranded RNA (formed during virus replication) by cytoplasmic pathogen recognition receptors.27 This allows for an approximate 24-hour delay in interferon induction. The interferon response is further reduced by a block of JAK/STAT (Janus kinase–signal transducer and activator of transcription) signaling mediated by the nonstructural TBEV protein NS5.28,29 TBEV replicates and enters the CNS by seeding through the capillary endothelium into the brain tissue. TBEV also may spread along nerve fibers.30 However, in arthropod-borne infections, neural spread of the virus is of little importance. Recent research in animal models indicates that CD4+ T cells may have a role in limiting the severity of TBE, whereas, in contrast, CD8+ T cells seem to be mediators of immuno­pathology.31

Diagnosis Because the clinical manifestations of TBE are not specific and are usually insufficient for diagnosis, TBE can be diagnosed definitively only by means of laboratory techniques. In the viremic phase of the initial stage of the disease, the virus can be identified by blood culture in a suitable cell line or in suckling mice.32 Polymerase chain reaction technology has mainly replaced such culture technologies for virus identification.33 By the onset of the second phase of the disease, the virus has been cleared from the blood and is also difficult to detect in cerebrospinal fluid.33 Because the symptoms that affect the CNS usually are not observed until 2 to 4 weeks after the tick bite, antibodies against the virus are nearly always present at the time of admission to a hospital and can be detected readily by standard serologic tests. Initially, a recent infection with TBEV was established by an increase of the titer in the HI test, neutralization test, or complement fixation test or by titer reduction in the HI test by 2-mercaptoethanol treatment in one serum sample, suggesting the presence of IgM antibodies.33 These tests are now used mainly for confirmatory purposes and are being replaced by rapid, sensitive, and reliable enzyme-linked immunosorbent assay (ELISA) systems based on the detection of IgM antibodies in the early phase of TBE. A four-layer ELISA system for the detection of TBEV-specific IgM has been developed that is extremely sensitive and prevents interference when high-titer, virus-specific IgG antibodies for TBEV are present.34 At an early stage after the onset of illness, TBEV-specific IgM could be detected in serum dilutions up to 1:10,000. A commercial development of this system (Immunozym FSME [TBE], Progen Biotechnik, Heidelberg) allows measurement of IgM and IgG antibodies. However, in cases of other Flavivirus contacts (eg, vaccinations against yellow fever or Japanese encephalitis;

Tick-borne encephalitis virus vaccines

dengue virus infections), the use of a neutralization assay (eg, RFFIT, rapid fluorescent focus inhibition test) is necessary for assessing immunity due to the interference of Flavivirus crossreactive antibodies in ELISA and HI testing.33,35 An excellent correlation was found between ELISA IgG units and the antibody titers obtained by the HI and the neutralization assay, provided there was no other exposure to Flavivirus antigens except TBE vaccination.36,37

Treatment No specific therapy for TBE has been established. The treatment of patients with TBE with RNAse obtained from bovine pancreas38 and with emetine39 has not been generally accepted. Corticosteroids apparently lead to a rapid temperature decrease and an improvement of subjective symptoms40 but at the same time seem to prolong the period of hospitalization compared with patients receiving only symptomatic treatment. Because there is no specific treatment targeting the virus itself, symptomatic treatment of patients with TBE is required. The most important measures in the clinical management of patients are maintenance of water and electrolyte balance, provision of sufficient caloric intake, and the administration of analgesics, vitamins, and antipyretics, as well as antiseizure agents if required. Physiotherapy of paralyzed limbs is essential to prevent muscle atrophy.6 Because person-to-person transmission of the virus has never been observed, there is no need to isolate patients with TBE.

Epidemiology Incidence and prevalence data TBEV is almost exclusively restricted to areas of Europe and Asia, with no incidence in other areas of the world. The distribution of TBEV covers almost the entire southern part of the nontropical Eurasian forest belt, from Alsace-Lorraine in the west to Vladivostok and northern and eastern regions of China in the East through to Hokkaido in Japan. Available data indicate that TBE is endemic in a number of Asian countries such as China, Japan, Kazakhstan, Kyrgyzstan, Mongolia, and South Korea.1 From 1980 to 1998, 2,202 cases of TBE were registered in China, although the true incidence is likely much higher because TBE is not a notifiable disease in China.41 In Europe, eight species of ticks have been identified that are capable of transmitting TBEV. Ixodes ricinus, the common castor bean tick, is the chief vector for the European subtype and Ixodes persulcatus for the Siberian and Far Eastern subtypes. Other tick species have also been associated with local TBE outbreaks in some areas of Siberia and the Far East.1 I. ricinus is found in much of Europe, extending to Turkey, northern Iran and the Caucasus. I. persulcatus occurs mainly in eastern Europe, Russia, China, and Japan. Both species are present in the area between the northern Baltics and the Urals.1 The distribution of TBEV subtypes is closely correlated to the distribution of these tick species. Thus, the European subtype is most prevalent in western, northern, and eastern Europe; the Siberian subtype in Russia; and the Far Eastern subtype in eastern parts of Russia, Japan, and China. In some regions both I. ricinus and I. persulcatus are present; thus, the European, Siberian and Far Eastern TBE subtypes can cocirculate in these areas. Moreover, the Siberian subtype has been isolated from I. ricinus in eastern Europe,42 and the European subtype has recently been found in I. persulcatus in Finland.43 The I. persulcatus ticks carrying both Siberian and Far East TBEV subtypes have also recently been found in Finland.44 The Siberian subtype seems to be gradually replacing the Far Eastern subtype in several regions of Russia.1,42

34

Considerable data are available on the prevalence of TBEV in the general population and among inhabitants of endemic areas. In most endemic areas of Austria and southern Germany, TBEV prevalence has been found to be 4% to 8% in unvaccinated persons. In the most severely affected areas in the east and southeast of Austria, prevalence as high as 14% was observed before the introduction of vaccination. Prevalence is also extremely high in Russia, followed by the Baltic states, Czech and Slovak republics, eastern Germany, Sweden, and Finland. The incidence of TBE in many affected countries has increased considerably during the last 20 years, with more than 170,000 clinical cases of TBE reported in Europe and Russia during this period.1 Figure 34-1 shows the high-risk areas in Europe. The largest number of reported TBE cases come from Russia, but the Czech Republic, Estonia, Germany, Hungary, Latvia, Lithuania, Poland, Slovenia, Sweden, and Switzerland have each reported more than 200 annual cases. Ticks and TBEV are being reported at higher altitudes than was previously the case and are also spreading north in Sweden, Norway, Finland, and Germany, and new disease foci have emerged in Denmark, Finland, Sweden, Norway, Austria, Germany, and Switzerland.1 The increased incidence and expanding geographic distribution of TBE is probably due to a multitude of factors, including climate change, migration of populations to suburban areas, change of leisure habits, changes in agricultural practices, and increased reforestation.

Risk groups A much higher percentage of TBEV antibody-positive persons has been observed among high-risk groups, such as persons working in agriculture and forestry, hikers, ramblers, people engaged in outdoor sports, and collectors of mushrooms and berries. However, a higher percentage of the population seems to be joining the recognized high-risk groups with changes in human behavior that have brought more people into contact with infected ticks. In addition, increased tourism of nonvaccinated persons to central and eastern Europe exposes large numbers of tourists to significant risk of TBEV infection.45 It is estimated that around 50 million travelers visit TBE-endemic regions every year.46 Unfortunately, TBE is rarely mentioned in any travel information available to tourists who spend their vacation in endemic areas, so that awareness among tourists and clinicians outside endemic regions is limited. This was highlighted in recently published case studies of UK and US citizens who contracted TBE during visits to Europe, Russia, and China.47,48 Genetic risk factors for TBE have also been reported. A mutation in the CCR5 gene (CCR5∆32)49 and a wild-type TLR3 gene were found to be associated with an increased risk of TBE.50

Modes of transmission and reservoirs of infection Ticks are the chief vectors and reservoir hosts of TBEV in nature. The other reservoir hosts are vertebrates that amplify the virus by acting as a source of infection for ticks.1 The virus can be transmitted to humans or other hosts by larvae, nymphs, and adult ticks. Virus can spread between viremic host and tick and from infected tick to uninfected tick feeding on the same host by passage of infected cells.51 Transovarial transmission of the virus has also been described. However, TBEV is usually transferred to the host with the saliva of the infected tick. On humans, ticks attach themselves to the hair-covered portion of the head, the ears, the arm and knee joints, and the hands and feet. The epidermis is punctured with the chelicerae, and the hypostome is inserted. Owing to the anesthetizing effect of the tick's saliva, the bite causes no pain and often passes unnoticed by the host. This may be the main reason why persons

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SECTION TWO • Licensed vaccines

Regions of TBE risk TBE risk unknown >5,000,000 inhabitants >1,000,000 inhabitants > 500,000 inhabitants >100,000 inhabitants

Helsinki

Oslo

Stockholm Tallinn

Riga Copenhagen London

Moscow

Vilnus Minsk

Amsterdam Berlin

Warsaw

Brussels Paris

Kiev Prague

Bern Budapest

Chisinău

Ljubljana Zagreb

Bucharest Belgrade

Sarajevo Lisbon

Madrid

Pristina Rome Tirana

Sofia

Skopje

Ankara

Figure 34-1  Regions of tick-borne encephalitis risk in Europe.

with manifest TBE sometimes cannot recall having been ­bitten by a tick. During the viremic stage, milk from goats, cows, and sheep may contain the virus and may be a source of infection for humans. Infection by the alimentary route as a result of the ingestion of raw milk and milk products has been reported from several European countries. More than 50 cases of TBE occurred in Slovakia after the patients had eaten cheese made from raw sheep milk,52 and an epidemic of TBE in Estonia was linked to raw goat milk.53 Unpasteurized milk and cheese was reported to be responsible for 64 cases of TBE in the Czech Republic between 1997 and 2008.54 TBEV was found to be present in more than 10% of cows milk and more than 20% of sheep and goat milk in an endemic region of Poland, as detected by reverse transcription–polymerase chain reaction.55 Outbreaks of TBE were also recently reported in Hungary56 and Austria57 following consumption of unpasteurized goat milk and cheese. Both of these reports emphasized that although relatively rare, alimentary transmission is highly efficient if humans consume unpasteurized milk products containing TBEV. Although it has not been observed, human-to-human transmission is a theoretical possibility when blood from a viremic donor is transfused to a patient. Transmission by human milk from a mother to an infant is also a theoretical possibility.16

Significance as a public health problem Table 34-1 lists the number of TBEV infections in the European countries in which the disease poses a major problem to public health and/or such figures have been collected for a long time. The course of the disease can be severe with a high frequency of sequelae, leading to substantial costs for public health ­systems, with long periods of hospitalization, long-lasting neurologic symptoms, and long-term working disability.5–7,16

Passive immunization An immune globulin concentrate containing specific γ-globulin against TBEV had previously been used prophylactically or as a postexposure treatment. However, a number of cases of suspected enhancement of infection were reported after use of TBE immune globulin in children.58 Although there was no conclusive evidence of enhancement of TBEV infection after correct use of immune globulin in humans and such enhancement has been demonstrated not to occur in a mouse model,59 postexposure treatment using TBE immune globulin is no longer recommended in almost all European countries.

Table 34-1 Number of Reported Cases of TBE From Various European Countries and Russia 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

84

102

178

109

128

99

62

41

60

54

60

82

54

100

84

45

86

79

Albania

8

Austria*

89

128

2

20

50

66

97

67

78

26

23

61

18

25

Croatia

23

60

27

76

87

59

57

25

24

26

18

27

30

36

38

28

20

12

20

Czech Republic*

193

356

338

629

613

744

571

415

422

490

719

411

647

606

500

642

1,029

542

630

816

1

4

3

1

1

4

8

4

2

1

1

Belarus

Denmark Estonia*

37

Finland*

9

France

2

68

163

166

177

175

177

404

387

185

272

215

90

237

182

164

171

140

90

179 26

25

16

23

10

19

17

12

41

33

38

16

31

17

18

20

23

2

5

4

6

1

1

2

5

0

0

2

6

7

0

6

7

10

44

142

118

306

226

114

211

148

115

133

253

226

278

274

431

546

238

285

313

222

288

206

329

258

234

224

99

84

51

45

76

80

114

89

52

56

62

70

64

2

2

8

6

8

8

11

5

15

19

6

14

23

22

14

4

34

32

Latvia*

122

227

287

791

1,366

1,341

716

874

1,029

350

544

303

153

365

251

142

170

171

181

328

Lithuania*

9

14

17

198

284

426

309

645

548

171

419

298

168

763

425

242

462

234

220

617

1

1

2

1

2

1

2

3

3

13

9

8

Poland*

8

4

8

249

181

267

257

201

209

101

170

205

126

339

262

174

316

233

202

335

Russia*

5,486

5,225

6,301

7,893

5,593

5,982

9,548

6,539

6,987

9,955

5,931

6,339

515

477

4,235

4,551

3,510

3,098

2,817

3,721

Slovakia*

14

24

16

51

60

89

101

76

54

57

92

76

62

74

70

28

91

46

77

71

Slovenia*

235

245

210

194

492

260

406

274

136

150

190

260

262

275

204

297

373

199

246

307

Sweden*

54

75

83

51

116

68

44

76

64

53

133

128

105

105

160

130

163

190

224

211

Switzerland*

26

37

66

44

97

60

62

123

68

112

91

107

53

116

138

206

259

113

127

118

Germany* Hungary* Italy

Norway*

Ukraine *Registration of tick-borne encephalitis (TBE) cases in these countries is mandatory. Data courtesy of Prof. J. Suess, Friedrich-Loeffler-Institute, National Reference Laboratory for Tick-borne Diseases, Jena, Germany.

12

Tick-borne encephalitis virus vaccines

14 1

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SECTION TWO • Licensed vaccines

Active immunization History of vaccine development and manufacture The first TBE vaccinations were done in the Russian Army in 1937, just a few months after identification of the infectious agent. This vaccine was the first Flavivirus vaccine to be used in humans and was only the third human virus vaccine. First attempts to develop a vaccine against the European subtype of TBEV were made in Czechoslovakia in the 1960s.60,61 This formalin-inactivated preparation was grown in primary avian fibroblast cultures. The vaccine was shown to be effective in a variety of laboratory animals and in human volunteers.62,63 In 1971, a cooperative project for the development of an inactivated vaccine that could be produced commercially in large quantities was initiated between the Institute of Virology in Vienna, Austria, and the Microbiological Research Establishment in Porton Down, England. The vaccine was based on the Austrian TBE strain Neudörfl, which was passaged in the brains of specific pathogen-free (SPF) baby mice and grown for vaccine production on suspensions of primary SPF chicken embryo cells. The vaccine was prepared by clarifying the virus by centrifugation followed by inactivation with formalin and purification by hydroxylapatite chromatography. Aluminum hydroxide was added as an adjuvant. More than 400,000 people were vaccinated in Austria, and serologic tests revealed seroconversion rates of more than 90% after two vaccinations, as measured by the HI test.64 However, despite its efficacy, local and systemic side effects were common. The vaccine was subsequently further developed in a collaboration with Immuno AG (now Baxter AG, Vienna, Austria) by including a purification step by continuous-flow zonal densitygradient centrifugation.65 This resulted in a 90-fold increase in vaccine purity and a drastic reduction in vaccine-associated side effects.64 In 1999, the preservative thimerosal was removed from the final formulation to fulfill the requirements of the European Pharmacopoeia. In the year 2000, several changes were made to the manufacturing process. A production virus seed that was free of potential contaminating mouse brain protein was generated by subjecting the master virus seed to two sequential passages in primary chick embryo cells. The new version of the vaccine was also free of human serum albumin (HSA). Unexpectedly, the removal of HSA from the final formulation led to an increase in the rate of high fever in infants and small children, and a number of cases of febrile seizures were observed in children up to 24 months old. The HSA was subsequently reintroduced into the current vaccine formulation for the year 2001, and there was a dramatic reduction in the rate of adverse drug reaction reports with this amended formulation (FSME-IMMUN).66 A second European TBE vaccine, similar to FSME-IMMUN, was licensed in Germany in 1991 (Encepur, Novartis, formerly Chiron Behring, Marburg, Germany) and was subsequently introduced to a number of other European countries.67 This vaccine was based on the K23 virus strain isolated in southern Germany, which has a nucleic acid sequence close to that of the Neudörfl strain.26 Like the original vaccine, the virus was grown on primary chick embryo cells, inactivated by formaldehyde, and purified by continuous-flow density-gradient centrifugation. The vaccine was stabilized with processed bovine gelatin and adsorbed onto aluminum hydroxide.68 Formulations were available for adults and children.69 However, the first pediatric version of Encepur resulted in cases of immediate allergic reactions in children.70 A detailed evaluation of the reported cases suggested that these were most likely IgE-mediated reactions to the vaccine's polygeline stabilizer. As a consequence, the first pediatric TBE vaccine was voluntarily withdrawn from all markets in early 1998.71 A new TBE vaccine formulation was d ­ eveloped that contained an increased amount of sucrose, making the use of an additional protein-derived stabilizer unnecessary.70,72

Two chick embryo cell–derived, formalin-inactivated TBE vaccines have been developed and marketed in Russia. The vaccine TBE-Moscow is manufactured by the M.P. Chumakov Insitute of Poliomyelitis and Viral Encephalitides (IPVE) of the Russian Academy of Medical Sciences using the Far Eastern prototype strain, Sofjin.73 The second Russian vaccine, EnceVir (Microgen) is based on the Far Eastern subtype strain 205.74 A TBE vaccine derived from the Far Eastern strain Senzhang is also produced and marketed in China by the Changchun Institute of Biological Products (CIBP).75

Manufacture of vaccine Schematic diagrams describing the manufacturing process for the current Baxter vaccine are given in Figures 34-2 and 34-3. The master seed virus was prepared from one isolate made from a pool of five ticks from the area of Neudörfl in Austria (Figure 34-2). This isolate was passaged once by intracerebral injection of SPF mice, and the virus recovered from the mouse brain suspension was cloned on primary chicken embryo cells. The cloned virus was then subjected to four further passages in SPF baby mice to make a master seed virus that consists of a 2% mouse brain suspension. This material is subjected to two further passages in chick embryo cells to generate the production virus seed. This virus seed is then used as an inoculum for a primary culture of chicken embryo cells derived from SPF eggs (Figure 34-3). This production cell culture is infected with the production virus, and, after an adsorption period, the infected cells are washed and further incubated at 37°C for a period of 96 to 114 hours. The virus-containing supernatant is separated from the cells by centrifugation and then inactivated with formaldehyde at a concentration of 0.185 g/L for 33 hours at 37°C. The inactivated virus harvest is then treated with protamine sulfate to precipitate cell debris and further purified using sucrose densitygradient ultracentrifugation. The virus-containing sucrose fraction is then stored at −20°C before thawing and pooling of a number of different purified virus harvests to give the bulk vaccine preparation. The bulk vaccine is then diluted with a human albumin–­ containing buffer to give the required virus antigen concentration. An aluminum hydroxide suspension is then added to this diluted pool to give an end ­concentration of 2 mg/mL. The final bulk vaccine is then filled into syringes before labeling and packaging.

Producers TBE vaccines are produced commercially by five manufacturers: Baxter AG (Vienna), Novartis Vaccines (Marburg), IPVE (Moscow), Microgen (Tomsk, Russia), and CIBP (Changchun, China). Both manufacturers of European strain vaccines use essentially the same process to produce the vaccine, the major differences being the use of different strains and the addition of different stabilizers. An overview of the characteristics of the European and Russian vaccines is shown in Table 34-2. HSA is used as a stabilizer by Baxter, IPVE, and Microgen, whereas Novartis uses an increased amount of sucrose as a stabilizer. Both manufacturers of European strain vaccines provide adult (Baxter, FSME-IMMUN; Novartis, Encepur) and pediatric (Baxter, FSME-IMMUN Junior; Novartis, Encepur-Children) formulations for children older than 1 year. FSME-IMMUN is marketed as TicoVac in some countries. TBE-Moscow has been approved for use in adults since 1982 and has been used to ­vaccinate more than 25 million people in Russia and ­neighboring countries.73 EnceVir was licensed in the Russian Federation in 2001. Russian manufacturers do not offer ­pediatric ­formulations, and neither vaccine is licensed for use in children younger than 3 years. TBE-Moscow has been approved for use in children 3 years or older since 1999.73 EnceVir was ­administered to children 3 to 16 years old, but this was temporarily stopped by the Russian Ministry of Healthcare in 2010 owing to postvaccination complications. No information is

Tick-borne encephalitis virus vaccines

5 TICKS Ixodes ricinus

Mouse brain passage (SPF baby mice) VIRUS PREPARATION Ptb 1 Cloning on chick embryo cells Mouse brain passage (SPF baby mice) Lyophilization VIRUS PREPARATION PC1H Suspension Mouse brain passage (SPF baby mice) Lyophilization VIRUS PREPARATION P1/78 Suspension Mouse brain passage (SPF baby mice) Storage at –60°C VIRUS PREPARATION MH I/80 Thawing Mouse brain passage (SPF baby mice) Storage at –60°C MASTER VIRUS SEED (MH II/83) Storage at –130°C Chick embryo cell passage

WORKING VIRUS SEED

Chick embryo cell passage PRODUCTION VIRUS SEED (“virus inoculum”) Figure 34-2 Preparation of master virus, working virus, and production virus seeds. SPF, specific pathogen-free.

available about the manufacturing process, formulation, or dose schedule for the Chinese vaccine, but it is reported to have passed phase 1, 2, and 3 clinical trials.76

Dosage and route An overview of dosing schedules for European and Russian vaccines is provided in Table 34-2. The standard immunization schedule for both European vaccines is similar. This consists of three immunizations administered intramuscularly on day 0 and the second dose 1 to 3 months later. A third dose is administered 5 to 12 months (FSME-IMMUN) or 9 to 12 months (Encepur) after the second immunization. The first booster should be administered within 3 years. In most countries, subsequent booster doses are usually recommended every 5 years

34

for persons younger than 60 years and every 3 years for persons of 60 years or older. In Germany, 3-yearly booster vaccinations are recommended for persons of 50 years or older; 3-yearly booster vaccinations are also recommended for Encepur recipients of 50 years or older in some other countries. In Austria, TBE vaccination is routinely recommended for all individuals of at least one year of age, and, in highly endemic regions such as parts of Carinthia, Styria or Upper Austria, children are vaccinated from the age of 6 months onwards. Rapid immunization schedules are also available for persons requiring more rapid protection, eg, when traveling to TBE-endemic areas or when the tick season has already begun. The rapid immunization schedule for Encepur consists of three immunizations to be administered on days 0, 7, and 21,72,77,78 with a fourth vaccination after 12 to 18 months. The rapid immunization schedule for FSME-IMMUN allows a shortened interval for the administration of the second dose, ie, 2 weeks after the first dose, and a third dose to complete the primary vaccination program after 5 to 12 months.79 Encepur is also licensed for a schedule with doses administered on days 0 and 14, with a third dose after 9 to 12 months. The standard primary immunization schedule for both Russian vaccines is two doses administered 1 to 7 months apart with a booster 12 months after the second dose and every 3 years thereafter. A rapid immunization schedule for the emergency use of EnceVir prescribes the administration of the second dose 5 to 7 days after the first dose. No information is available on the recommended dosing schedules of the Chinese vaccine.

Vaccine stability and storage FSME-IMMUN may be stored at 2°C to 8°C for 30 or 24 months (for prefilled syringes with or without needle attached, respectively). Encepur and TBE-Moscow have shelf lives of 24 months and EnceVir 36 months at this temperature. No stability data are available for the Chinese vaccine.

Vaccine immunogenicity The TBE vaccines produced by the European and Russian manufacturers are reported to be highly immunogenic. A recent review published by the Cochrane Collaboration of 11 clinical trials involving a total of more than 8,000 participants reported that all vaccines gave seroconversion rates of more than 87%.80 A substantial data set from clinical trials is available for FSME-IMMUN and Encepur. However, study results are not always directly comparable because different assay systems have been used for the assessment of immunogenicity. Published FSMEIMMUN immunogenicity studies most frequently measure TBEV-neutralizing antibodies or total IgG antibodies against the homologous TBEV Neudörfl strain using an NT assay according to Adner et al81 or Holzmann et al36 or a commercially available ELISA kit (Immunozym FSME [TBE]-IgG, Progen Biotechnik, Heidelberg, Germany). Studies with Encepur measure neutralizing antibodies (NT assay) and total IgG antibodies (Enzygnost TBE, Dade Behring, Marburg) against the homologous K23 strain. For FSME-IMMUN, a prospective, randomized, phase 2, double-blind dose-finding study with FSME-IMMUN for adults was performed in volunteers 16 to 65 years old to evaluate the immunogenicity and safety of two vaccinations with three different vaccine doses, and the effects of the third vaccination were investigated in a follow-up study in the same study population.82 Seroconversion rates (ELISA) in the different dose groups (0.6, 1.2, and 2.4 μg of antigen) were 85.1%, 96.2%, and 97.0%, respectively, 21 days after the second vaccination. Seroconversion rates after the third vaccination increased to

779

780

SECTION TWO • Licensed vaccines

Cell preparation (disintegrator)

Embryo harvest Egg incubator

Virus inoculation (isolator) Virus propagation (fermentation)

Virus adsorption

Cell separation (centrifugation)

Inactivation (formaldehyde)

Pooling of purified virus

Formulation

Purification

Filling

Figure 34-3  Schematic for tick-borne encephalitis vaccine production.

96.0%, 99.2%, and 100%, respectively. Detailed results are shown in Table 34-3. The 2.4-μg dose was identified as optimal from safety and immunogenicity perspectives. This dose was subsequently tested in two large phase 3 clinical studies powered for the analysis of safety (n = 3,966, and, in the follow-up study, n = 3,705), which confirmed the high seroconversion rates and geometric mean concentrations in the subgroup of 564 volunteers studied for this parameter.83 The conventional vaccination schedule for Encepur results in seroconversion (ELISA) in 50%, 98%, and 99%, respectively, after the first, ­second, and third doses.84 For the pediatric formulation of FSME-IMMUN, two randomized, phase 2, double-blind, multicenter studies were conducted to identify the optimal dose of the vaccine in 639 children 1 to 5 years old and 639 children 6 to 15 years old. Two vaccinations with 0.3, 0.6, or 1.2 μg of antigen were given 21 to 35 days apart and resulted in seroconversion rates of 92.3%, 98.1%, and 100% for the 1- to 5-years-olds and 87.4%, 95.3%, and 98.5% for the 6- to 15-year-olds, respectively.85 According to the prospectively defined criteria, 1.2 μg was determined to be the optimal dose of FSME-IMMUN in both pediatric study groups. Immunogenicity of the pediatric formulation was further investigated in children and adolescents 1 to 15 years old in a multicenter prospective study.85 Seroconversion rates after

the second vaccination were 96.0% and 95.7% as determined by ELISA and NT, respectively, rising to 100% and 99.4% after the third vaccination. The seroconversion rate after the second vaccination in adolescents 12 to 15 years old was 96.9%. Further analyses demonstrated that adolescents receiving the pediatric formulation attained similarly high seroconversion rates as young adults receiving the adult formulation, justifying the ­conclusion that the FSME-IMMUN Junior vaccine formulation is appropriate not only for children younger than 12 years, but also for adolescents younger than 16 years.85 As discussed earlier, rapid immunization schedules exist for FSME-IMMUN and Encepur. Neutralizing antibodies developed rapidly after two vaccinations (12 ± 2 days apart) with FSMEIMMUN in adults 16 to 65 years old, with seropositivity rates of 0% before vaccination and, after the second dose, of 89.3% (day 3), 96.4% (day 7), 98.2% (day 14), and 100% (days 21 and 42).79 In a larger study (n = 340), seropositivity rates in adults 16 to 49 years old were 76.5%, 94.8%, and 96.7% as measured by NT at days 7, 14, and 21 after the second vaccination. The corresponding seropositivity rates in subjects 50 years or older were 48.4%, 80.9%, and 88.0%. Seropositivity rates increased to 100% for young adults and 98.7% in the older age group after the third vaccination.86 For Encepur, the rapid immunization schedule has been used in several clinical trials performed in

Table 34-2 Characteristics of Currently Marketed TBE Vaccines FSME-IMMUN

FSME-IMMUN junior

Encepur

Encepur children

TBE-Moscow

EnceVir

Chinese

Manufacturer

Baxter

Baxter

Novartis

Novartis

IPVE

Microgen

CIBP

Virus strain

Neudörfl

Neudörfl

K23

K23

Sofjin

205

Senzhang

Virus subtype

European

European

European

European

Far Eastern

Far Eastern

Far Eastern

Amount of antigen (μg)

2.4

1.2

1.5

0.75

0.50-0.75

2.0-2.5

NA

Age indication (y)

≥16

1-15*

≥12

1-11*

≥3

≥3

NA

Aluminum hydroxide (mg)

1

0.5

1

0.5

‡

0.3-0.5

NA

Stabilizer

HSA

HSA

Sucrose

Sucrose

HSA, sucrose

HSA, sucrose

NA

Shelf life (mo)

30

30

24

24

36

24

NA

Volume (mL)

0.5

0.25

0.5

0.25

0.5

0.5

NA

Second dose (mo)

1-3

1-3

1-3

1-3

1-7

1-7

NA

Third dose (mo)

5-12

5-12

9-12

9-12

12

12

NA

First booster (y)

3

3

3

3

3

3

NA

5 (3 y if aged ≥60 )

5

5 (3 y if aged ≥60**)

5

3

3

NA

Second dose (d)

14

14

7/14††

7/14††

Third dose

5-12 mo

5-12 mo

First booster

3y

3y

Subsequent boosters (y)

5 (3 y if aged ≥60 )

3-5

§

§

†

Vaccination schedules¶ Conventional

Subsequent boosters (y)

||

Rapid 5-7

NA

14 d/9-12 mo

14 d/9-12 mo

NA

12 mo

NA

12-18 mo/3 y††

12-18 mo/3 y††

NA

3y

NA

5 (3 y if aged ≥60**)

5

NA

3

NA

CIBP, Changchun Institute of Biological Products; HSA, human serum albumin; IPVE, M.P. Chumakov Institute of Poliomyelitis and Viral Encephalitides of the Russian Academy of Medical Sciences; NA, not applicable or information not available; TBE, tick-borne encephalitis. *Children ≥6 months old who are in high-risk groups may be vaccinated. † Administration of EnceVir was temporarily stopped in 2010 in children 3-16 years old owing to postvaccination complications. ‡ Contains aluminum hydroxide, but no information available on amount. § 24 months for prefilled syringes without needle attached. ¶ Shown are the recommended intervals since the previous dose. || In Germany, ≥50 y. **In some countries, ≥50 y. †† For rapid/accelerated conventional vaccination schedules.

Tick-borne encephalitis virus vaccines

||

NA ††

††

34 781

SECTION TWO • Licensed vaccines

Table 34-3 Immunological Response to FSME-IMMUN After the Second and Third Vaccinations (Compared With Baseline) as Determined by Enzyme-Linked Immunosorbent Assay Baseline

Second vaccination

Third vaccination

Antigen Dose (μg)

GMC (95% CI of GMC)

GMC (95% CI of GMC)

Seroconversion rate (%)

GMC (95% CI of GMC)

Seroconversion rate (%)

0.6

9.8 (8.9-10.7)

277.4 (237.7-323.7)

114/134 (85.1)

668.4 (564.3-791.7)

121/126 (96.0)

1.2

11.0 (9.9-12.2)

465.8 (414.7-523.1)

126/131 (96.2)

1267.7 (1,067.7-1,505.3)

127/128 (99.2)

2.4

9.8 (8.9-10.8)

631.3 (561.3-710.0)

128/132 (97.0)

1,503.0 (1,253.5-1,802.2)

118/118 (100.0)

CI, confidence interval, GMC, geometric mean concentration.

ELISA (Neudörfl) 160

3000

140

2500

100

120

2000

100

1500

80 60

1000 50

40

500

20

0

0

0 Encepur Children

150

FSME-IMMUN Junior

200

ELISA (K23) 180

3500

250

Encepur Children

As described, it has been difficult to compare the results of immunogenicity studies performed with vaccines from different manufacturers owing to differences in the assays used to detect TBEV-specific antibodies. Recently, however, several head-to-head studies have been published that report the direct comparison of these vaccines using identical assay techniques. The first of these studies compared FSME-IMMUN Junior and Encepur Children in children 1 to 11 years old.93 This study found Encepur Children to be more immunogenic than FSME-IMMUN Junior, as determined by NT using the K23 or Neudörfl strain. In a subsequent multicenter, randomized

NT (Neudörfl)

FSME-IMMUN Junior

Comparative studies

clinical study of children 1 to 11 years old, the immunogenicity and safety of FSME-IMMUN Junior and Encepur Children after the first two doses of a conventional primary immunization schedule were compared.94 Both vaccines were found to induce a high level of seropositivity, and, in contrast with the first study, FSME-IMMUN Junior was shown to be noninferior to Encepur Children with respect to NT seropositivity rates after the second dose (100% for FSME-IMMUN and 97.8% for Encepur Children), as determined using the Neudörfl strain. Both vaccines also elicited similar seropositivity rates as determined using the Immunozym ELISA (based on the Neudörfl strain) or the Enzygnost ELISA (based on the K23 strain). At day 180, seropositivity rates determined by NT, Immunozym ELISA, and Enzygnost ELISA were 95.4%, 95.1%, and 93.2%, respectively, for FSME-IMMUN Junior and 91.0%, 62.6%, and 80.5%, respectively, for Encepur Children. The magnitude of the immunological responses induced by FSME-IMMUN Junior were found to be consistently higher than those induced by Encepur Children, regardless of the immunological test or viral antigen used (Figure 34-4). A potential and logical ­explanation

Encepur Children

adults and children. In immunogenicity studies in 404 children 1 to 11 years old, seroconversion (defined as baseline seronegative subjects who developed neutralizing antibody titers ≥2) or baseline seropositive subjects with a fourfold or greater increase in neutralizing antibody titers was reported in 100% of subjects 21 days after the third vaccination (ie, in total 42 days after the first vaccination).87 Similarly, immunogenicity of Encepur using the rapid immunization schedule was measured in a subset of 200 subjects as part of a clinical program including a total of more than 3,000 adults and adolescents 12 to 76 years old. Seroconversion (defined as seronegative subjects who developed neutralizing antibodies or seropositive subjects showing a ≥4fold increase in neutralizing antibody titers) was reported in 100% of subjects 21 days after the third vaccination.88 A small number of immunogenicity studies have been reported for the Russian vaccines. A study carried out in 325 children and adolescents in 2003 by the Russian National Regulatory Authority showed fourfold or greater increases in HI titer after two doses, 2 months apart, of 89% to 96% for the TBE-Moscow and 84% to 97% for EnceVir. No immunogenicity data have been reported for the Chinese vaccine. Because different virus subtypes exist and sometimes also coexist in the same TBE-endemic region, it is important that TBE vaccines are able to elicit cross-protective antibody responses. Owing to the high degree of antigenic similarity between TBEV subtypes, all currently marketed TBE vaccines are expected to provide good cross-protection against TBE viruses. Various studies with FSME-IMMUN and Encepur demonstrate some degree of cross-neutralization against European, Far Eastern, and Siberian TBEV subtypes.89–91 A novel assay system was recently described based on hybrid viruses that allows unbiased comparisons of virus neutralization that are not influenced by the differences in infectivity and replication of individual strains. By using this assay, it could be shown that FSME-IMMUN induces equivalent levels of neutralizing antibodies against European, Far Eastern, and Siberian TBEV subtypes and that high neutralizing antibody titers are also induced against Omsk hemorrhagic fever virus.92

FSME-IMMUN Junior

782

Figure 34-4  Geometric mean antibody response 28 days after the second vaccination with FSME-IMMUN Junior or Encepur Children as determined by NT or enzyme-linked immunosorbent assay (ELISA) based on the tick-borne encephalitis strains Neudörfl (Immunozym) or K23 (Enzygnost). (Reprinted from Poellabauer EM, Pavlova BG, Loew-Baselli A, et al. Comparison of immunogenicity and safety between two paediatric TBE vaccines. Vaccine 28:4680-4685, 2010, with permission from Elsevier.)

Tick-borne encephalitis virus vaccines

for the stronger immune responses reported for FSME-IMMUN Junior compared with Encepur Children is the higher amount of TBEV antigen (1.2 μg) in the FSME-IMMUN Junior vaccine formulation compared with that of Encepur Children (0.75 μg). Because antigen immunogenicity is dependent on tertiary and quaternary structure, the differences in immune response could also be related to differences in the aggregated fraction of the final vaccine preparation. The inclusion of HSA in the FSMEIMMUN Junior vaccine formulation may have caused aggregation of TBEV antigen particles, which could contribute to stronger antibody responses by inducing more efficient antigen uptake and processing. A comparison of the adult formulations of FSME-IMMUN and Encepur has also been reported for a retrospective singlecenter observational study of subjects older than 60 years.95 This study found significantly higher geometric mean concentrations and seropositivity rates in subjects vaccinated with FSME-IMMUN as determined by the Immunozym ELISA. Significantly higher seropositivity rates were also determined by NT based on the Neudörfl strain. No significant differences were found when the Enzygnost ELISA was used. However, this nonrandomized, uncontrolled study had several limitations,96 and, hence, the interpretation of these observations is not straightforward. Despite the differences in immunogenicity, all studies in children and adults demonstrate that both vaccines induce a high level of seropositivity.

Duration of immunity and protection Previously, regular booster doses were recommended every 3 years following the primary immunization regimen, ie, after three doses of FSME-IMMUN and three (regular schedule) or four (rapid schedule) doses of Encepur. However, based on published serologic data,97–99 the Austrian immunization board changed this recommendation in 2004 to 5 year booster intervals for persons younger than 60 years, with the exception of the first booster, which continues to be administered 3 years after completion of the primary immunization. These recommendations apply to standard and rapid immunization schedules. Slightly different recommendations may be given in some other European countries, eg, in Germany, it is recommended that persons 50 years or older continue to receive a booster every 3 years. The current dose schedules for each vaccine are given in Table 34-2. Since the changes in vaccine schedule recommendations, several seropersistence studies have been undertaken using FSME-IMMUN and Encepur. The seropersistence of TBEV antibodies in adults 18 to 67 years old, 2 and 3 years after primary vaccination (first two doses, FSME-IMMUN or Encepur; third dose, FSME-IMMUN) was evaluated in a multicenter prospective study (n = 347).100 Seropositivity rates 2 and 3 years after the primary vaccination were 96.8% and 95.4%, respectively, as determined by neutralizing antibody titers. Following the booster vaccination, seropositivity rates increased to 100%. Another study analyzed antibody titers before and after booster vaccination in subjects who had received their last FSMEIMMUN vaccination 3 to 7 years previously.101 Before booster vaccination, antibody titers were significantly higher in subjects younger than 30 years compared with subjects 50 years or older. The response to a booster vaccination was significantly higher in the group younger than 30 years compared with subjects 50 years or older. However, there was no difference in booster response in the age groups 50 years or older. In a study of 430 adults with complete primary immunization (and additional regular booster) with FSME-IMMUN, TBEV antibody concentrations were measured 3 to more than

34

8 years after the last vaccination. Rates of seropositivity declined over time; however, the majority of subjects still had TBEV antibodies far above the detection limit even when the previous TBE vaccination had surpassed the current recommendation for booster vaccinations.102 Subjects from this study were given a booster dose of Encepur,103 and seropersistence was evaluated for 2 to 6 years after the booster.104–106 Neutralizing antibody titers of 10 or more were detected in 95.9%, 96.7%, and 93.8% of subjects after 2, 3, and 4 years, respectively. Geometric mean titers were significantly lower in subjects 50 years or older as determined after 3 years and were also significantly lower in subjects 60 years or older as determined after 4 years. After 4 years, the proportion of subjects with NT titers of 10 or more dropped to 93.0% and 91.7% for people 50 to 60 years old and older than 60 years, respectively. After 5 and 6 years, NT titers of 10 or more were detected in 96% and 94% of subjects younger than 60 years ofage, respectively. For the group 60 years or older, the proportion of subjects with NT titers of 10 or more dropped to 89% and 86% as determined 5 and 6 years after the booster. In another study, the seropersistence of antibodies induced by Encepur was determined in 222 adults 19 to 51 years old after primary vaccination using a rapid schedule.107 Neutralizing antibody titers of 10 or more were detected in 99% of subjects 3 and 5 years after the first booster vaccination. Seropersistence of TBEV antibodies induced by Encepur Children was also investigated in 335 children 1 to 11 years old after primary vaccination using a rapid schedule. Neutralizing antibody titers of 10 or more were detected in 99% and 100% of subjects 3 and 5 years after the first booster vaccination, respectively.108

Cellular responses Few data are available on the generation or role of T-cell responses in immunized persons, but it has been reported that immunization with inactivated TBE vaccine induces primarily a CD4+ T-cell response with very low CD8+ responses.109,110

Correlates of protection Based on data generated from 20 years of field experience, serum IgG levels of 127 Vienna units (VIEU)/mL or greater are used as a surrogate marker for protection, and levels of 63 to 126 VIEU/ mL are borderline.111 Provided there has been no exposure to other flavivirus antigens, there is an excellent and highly significant correlation between ELISA IgG units and NT titers.36

Special groups TBE vaccines have been investigated in several special patient groups, such as human immunodeficiency virus (HIV)-positive patients, patients with cancer undergoing chemotherapy, children who have undergone thymectomy, and patients with chronic asthma. In children who have undergone thymectomy, lower TBEV antibody levels were observed after two doses of FSME-IMMUN compared with healthy children, but normal antibody levels were elicited after the third vaccination.112 After four doses of FSME-IMMUN at 0, 1, 2, and 9 months, 44.8% of HIV-infected adults developed seroprotective antibody levels.113 The HIV-positive patients who developed seroprotective antibody titers also had higher CD4+ T-cell counts compared with patients who did not produce an adequate immune response. Patients receiving cancer chemotherapy and heart transplant recipients receiving cyclosporine treatment also responded poorly to TBE vaccination, but TBE vaccination was effective when administered before the initiation of chemotherapy.114,115 Children with chronic asthma vaccinated with FSMEIMMUN showed immunogenicity results virtually identical to those of healthy children.116

783

784

SECTION TWO • Licensed vaccines

Efficacy and effectiveness of vaccine No controlled clinical trials have been done to demonstrate the efficacy of vaccination in protecting against disease. However, the extensive diagnostic service for TBE in Austria permits calculation of the protection rate of the vaccine using clinical and epidemiologic data from the Austrian population, where the Baxter vaccine has been in use for more than 30 years (Table 34-4). The protection rate of the vaccine for the years 2000 to 2006 has been estimated to be approximately 99% in regularly vaccinated persons, with no statistically significant difference between age groups.117 During this period, approximately 90% to 95% of TBE vaccines administered in Austria were FSME-IMMUN. Vaccine effectiveness is at least as high between the second and third doses of the primary vaccination course, with no vaccine breakthroughs recorded in this group, but is significantly lower (≈96%) in persons with an irregular vaccination history. The chance of acquiring TBE was calculated to be threefold to eightfold increased in persons with an irregular vaccination history, highlighting the importance of adhering to the recommended vaccination schedule. It is estimated that about 2,800 TBE cases and 20 deaths were prevented by vaccination in Austria between 2000 and 2006.117 A similarly high protection rate (96%-98% after ≥three vaccinations) was reported in an earlier Austrian analysis for the years 1994 to 2001.118 Since vaccination uptake is highest in endemic regions and lower in less-affected areas of Austria, exposure to TBEV is likely to be higher in the vaccinated population; therefore, field effectiveness may, in fact, be even higher than calculated. These data demonstrate that the 2004 decision by the Austrian Advisory Board for Vaccinations to increase the interval between booster immunizations to 5 years for persons younger than 60 years did not lead to decreased vaccine effectiveness. Impressively, whereas the incidence of TBE in Austria has declined markedly since the advent of generalized vaccination, that of neighboring Czech Republic, where only a small percentage of the population is immunized (16% in 2009) has actually increased (Figure 34-5). A similar situation exists in Slovenia, which had a vaccination rate of only 12% in 2009 and an incidence of 9.90 per 100,000 in 2007.119 The most affected region of this country, which borders on Austria, reported a TBE incidence of 47 per 100,000 in 2006,120 whereas the mean annual incidence rate in Austria from 2003 to 2007 was 0.82 per 100,000.41 A mass immunization program initiated in 1996 in the Sverdlovsk Region in Russia demonstrated the effectiveness of the Russian TBE vaccines (predominantly TBE-Moscow was used, 80%; EnceVir, 6%; FSME-IMMUN, 12%; Encepur, 2%). The incidence of TBE cases per 100,000 inhabitants decreased from 42.1 in 1996 to 9.7 in 2000 and to 5.1 in 2006. Vaccine effectiveness estimates ranged from 62% in 2000 to 89% in 2006.121 Although vaccination is highly effective, rare cases of TBE have occasionally been reported after partial or complete basic TBE vaccination.122–125 The majority of reported vaccine breakthroughs occurred in persons older than 50 years.

Safety As with all intramuscularly administered vaccines, mild local reactions such as reddening, swelling, and pain may occur following vaccination with TBE vaccines. Systemic adverse events noted following the administration of TBE vaccines include headache, fatigue, malaise, muscle pain, joint pain, and fever. Interestingly, these symptoms occur more frequently after the first vaccination compared with subsequent vaccinations, indicating that the induced immune response may protect ­

s­ ubjects from such reactions. This observation was confirmed in a large phase 3 study investigating the safety and immunogenicity of FSME-IMMUN in adults.83 In this clinical study in about 3,000 volunteers, the overall rates of fever (typically mild) following the first and second vaccinations were 0.8% and 0.6%, respectively. The rate of occurrence of the most frequently reported systemic symptoms after the first two injections is shown in Table 34-5. The incidence of these reactions decreased after the second vaccination. Notably, most of the reactions were mild and the rates observed after the third vaccination were similar to the rates after the second vaccination. The current formulation of Encepur has been reported to induce lower rates of reactions than the previous Encepur version, with headache in adults being the most frequent reaction, declining from 17% after the first dose to 9% after the third.88 The safety profile of TBE vaccines is similar in children, although fever (typically mild) occurs more frequently in younger than in older children, and children 1 or 2 years old are most susceptible.66,126 In the largest study involving children (n = 2,417), the rate of fever after the first vaccination with FSMEIMMUN Junior was 9.7%, and this decreased markedly to 2.3% after the second vaccination.85 Two comparative head-to-head studies between FSMEIMMUN Junior and Encepur Children showed that both ­vaccines were well tolerated in all age groups. In one study, 2.4% of Encepur recipients experienced fever, temperature more than 39°C, after the first dose compared with 1.2% receiving FSME-IMMUN. Encepur was also associated with greater overall rates of injection site pain (36% vs 25%), malaise (15% vs 7.5%), and headache (12% vs 6%) than FSME-IMMUN.93 In the other comparative study, both vaccines were again well tolerated, but local reactions after the first and second vaccinations were found to be significantly higher with Encepur Children than with FSME-IMMUN Junior.94 The presence of HSA in FSME-IMMUN Junior may inhibit excessive immunological reactions (particularly the production of tumor necrosis factor α).127 It is thus possible that the presence of HSA in FSMEIMMUN Junior improves the local safety profile of the vaccine compared with Encepur Children. There are no published large-scale, randomized, controlled, safety trials for the Russian vaccines. Small-scale studies suggest that both vaccines are moderately reactogenic. A study done in 2002-2003 in 325 children and 400 adults found both Russian vaccines to be similarly moderately reactogenic, and no severe adverse events were recorded. No safety data have been published for the Chinese vaccine. Single cases of very rare adverse events associated with TBE vaccines have occasionally been reported, including inflammation of the nerves to the gravity muscles in the legs and feet,128 neurologic complications after simultaneous immunization with TBE and tetanus vaccines,129 smell impairment,130 and reactivation of immune thrombocytopenic purpura.131 It cannot be totally excluded that TBE vaccination may result in an aggravation of autoimmune diseases such as multiple sclerosis or iridocyclitis in some patients. However, data published recently suggested no link to any association between TBE vaccination and magnetic resonance imaging–detected disease in patients with multiple sclerosis.132 Although there is no evidence to support a hypothesis that vaccination can cause autoimmunity, an unfavorable influence of the vaccination on a preexisting autoimmune ­disease must be weighed against the risk of TBE. The safety of TBE vaccines for use during pregnancy and lactation has not been established in controlled clinical trials, and, therefore, vaccines should be given only with caution after individual consideration of potential risks and benefits. No safety issues with regard to special patient populations have been identified.

Table 34-4 FE and Number of Cases Prevented by Tick-borne Encephalitis Vaccination in Austria, 2000 to 2006 Unvaccinated

Regularly vaccinated

Irregularly vaccinated

Age group (y)

Incidence

Cases

Incidence

Observed cases

Expected cases

FE

Incidence

Observed cases

Expected cases

FE

Cases prevented by vaccination

0-15

1.44

22

0.04

3

101

97.0

0.066

1

22

95.4

119

16-49

4.96

157

0.01

2

1,132

99.8

0.156

11

349

96.8

1,468

50-60

6.44

71

0.06

3

327

99.1

0.162

3

120

97.5

441

>60

6.79

189

0.08

6

542

98.9

0.329

12

248

95.2

772

Total

5.92

439

0.04

14

2,102

99.3

0.212

27

739

96.4

2,800

FE, field effectiveness. From Heinz FX, Holzmann H, Essl A, et al. Field effectiveness of vaccination against tick-borne encephalitis. Vaccine 25:7559-7567, 2007, with permission from Elsevier.

Tick-borne encephalitis virus vaccines

34 785

SECTION TWO • Licensed vaccines Figure 34-5  Comparison of tick-borne encephalitis cases in Austria and Czech Republic since the introduction of immunization in Austria. (Data courtesy of

1100 1000

Czech Republic

Prof. F.X. Heinz, Department of Virology, Medical University of Vienna, Austria.)

900 800 Number of cases

786

700 600 500 400 300 200 Austria

100 0 1979

1983

1987

1991

1995 Years

1999

2003

2007

2011

Table 34-5 Probability of Occurrence of Specifically Queried Symptoms of Systemic Reactions Following FSME-IMMUN Vaccination* Fever

Headache

Muscle pain

Joint pain

Fatigue

Malaise

First vaccination (n = 2,977)

0.8

5.7

4.8

1.3

6.2

4.5

Second vaccination (n = 2,950)

0.6

3.8

3.6

1.0

3.8

3.1

*Values are given as percentages.

Indications for vaccination Both European vaccines are indicated for the active (prophylactic) immunization of children (FSME-IMMUN Junior 0.25 mL, 1-15 years; Encepur Children 0.25 mL, 1-12 years) and adolescents and adults (FSME-IMMUN 0.5 mL, persons 16 years or older; Encepur 0.5 mL, persons 12 years or older). Russian vaccines (TBE-Moscow and EnceVir) are indicated for adults and children 3 years or older, although the administration of EnceVir to children 3 to 16 years old was temporarily stopped in 2010 owing to postvaccination complications. In particular, vaccination is warranted for persons living in endemic areas, people working under high-risk conditions (foresters, w ­ oodcutters, farmers, military personnel, laboratory workers), and t­ ourists engaged in highrisk activities (eg, outdoor activities such as field work, camping, hunting, and hiking). The World Health Organization recommends that vaccination be offered to all age groups, including children, in areas in which TBE is highly endemic.132a

Contraindications and precautions Persons with acute disorders that require treatment should not be vaccinated until at least 2 weeks after full recovery. Allergies to components of the vaccines or severe reactions to egg ingestion constitute contraindications. Nonsevere allergy to egg protein does not usually constitute a contraindication to TBE vaccination. However, persons with such allergies should be vaccinated only under appropriate supervision. It has been speculated that previous exposure to other flaviviruses could affect TBE vaccination. Such problems are difficult to study because of cross-reactive Flavivirus antibodies. However, stronger TBEV antibody responses in subjects previously vaccinated against yellow fever virus have been reported.133 Interestingly, vaccination with TBEV has also been shown to enhance the immune response to an inactivated Japanese encephalitis vaccine.134

Public health considerations Epidemiologic results of vaccination The TBE vaccine is widely used in western and central Europe (>100 million doses administered since 1980), and the vaccine has been a major success in preventing TBEV infections in this region. In Austria, where a vaccination campaign using the FSME-IMMUN vaccine was initiated more than 25 years ago, vaccination rates have increased from 6% in 1980 to 88% in 2010, with 68% of the total population abiding with the recommended vaccination schedule. This has resulted in a dramatic reduction of disease incidence. In the prevaccination era, Austria had a high recorded morbidity of TBE, probably at that time the highest in Europe. Figure 34-6 shows that, since the beginning of the vaccination campaign in Austria, the annual number of cases of TBE has been reduced from a high of 677 in 1979 to between 41 and 100 cases since 1997. The Austrian experience shows that containment of TBE is feasible by routine universal vaccination.

Disease control strategies Adequate clothing may help to make access to the skin more difficult for ticks. Protective clothes must be completely closed to be effective, but this would be unacceptable to people spending their leisure time or holidays in endemic areas in the warm season. Skin should be checked daily for the presence of ticks, and children should be checked two to three times a day. Ticks should be removed using fine-tipped tweezers. In former Czechoslovakia, forestry workers were given protective clothing impregnated with d ­ ichlorodiphenyltrichloroethane (DDT) and were regularly disinfested after work. Furthermore,

34

Tick-borne encephalitis virus vaccines

100 677 84 612

78

600 71

Number of cases

500

60

438

63

65

78

86

87

87

87

88

88

88

87

86

85

79

90 80

74

67

70

TBE cases in Austria Vaccination rate (in percent)

56

60

51

400 300

294

300

50

46

336

40

258

240

215

200

201

30

178 131

128 89

100

84

109

102

Vaccination rate (percent)

700

20

128 99 62

41

60

54

100

82

60

86

84

54

80

46

63

10 0

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

1984

1983

1982

1981

1979

1980

0

Figure 34-6 Vaccination rate and tick-borne encephalitis (TBE) cases in Austria, 1979 through 2010. (TBE incidence data courtesy of Prof. F.X. Heinz, Department of Virology, Medical University of Vienna, Austria.)

a variety of repellents were used, such as diethyltoluamide, indalone, dimethyl carbate, dimethyl phthalate, and benzyl benzoate. These preparations provided protection for only a few hours, however. Moreover, there have been reports from Russia of ticks becoming resistant to repellents.135

Eradication or elimination Efforts to eradicate the disease in the past were concentrated on the extermination of the tick population in TBE-endemic areas. In former Czechoslovakia and USSR, large-scale eradication measures using tetrachlorvinphos, DDT, or hexachlor did not produce the desired effect. Because the virus persists not only in ticks, but also in a large number of wild animals, such measures are unlikely to eradicate or even control the disease. The most effective method to prevent infection is vaccination.

Future vaccines All currently available TBE vaccines consist of purified, inactivated, whole-virus particles. A number of candidate vaccines using recombinant or live attenuated technology are at various stages of development. Chimeric live attenuated TBEV vaccines have been developed by replacing the membrane precursor and glycoprotein E genes of dengue type 4 (DEN4) virus with the corresponding genes of a Far Eastern TBEV strain. Another chimeric candidate TBEV vaccine was based on the antigenically distinct Langat virus. A single dose of either of these chimeric vaccines was efficacious in protecting monkeys against wild-type Langat virus challenge.136 The Langat/DEN4 was subsequently shown to be safe in a phase 1 clinical trial.137 However, low levels of viremia were detected in one subject, and antibody responses to TBEV were poor. In a later report, this vaccine candidate was shown to replicate to moderate titers in the CNS of monkeys, causing pathological lesions in two and illness in one of four

animals.138 Further studies on a TBEV/DEN4 vaccine candidate have reported the introduction of defined mutations in several regions of the viral genome that have led to improved attenuation in mice and arthropods, which represent the natural vector for the two wild-type viruses.139,140 It was also shown that neurovirulence of the TBEV/DEN4 chimera could be reduced by insertion into the viral genome of target sequences for cellular microRNAs expressed in the CNS.141 Although live viral vaccines have in many cases proven to be an extremely effective tool for the prevention of disease, the production of conventional live vaccines in cell culture has many disadvantages. These include the potential for contamination with adventitious agents and genetic alterations during propagation, making it necessary to do extensive testing before release. A number of promising experimental recombinant subunit and nucleic acid vaccines were reported in the 1990s and in the first half of the last decade,109,142–146 but there have been no published follow-up data or registered clinical trials for the majority of these proposed vaccine candidates. However, a novel TBE vaccine candidate, Replivax-TBE, is currently being developed.147 This single-cycle hybrid viral vector vaccine contains membrane precursor and glycoprotein E genes of TBEV and the nonstructural protein genes of TBEV or another Flavivirus. A deletion in the capsid protein gene prevents replication; thus, the vaccine is generated in helper cells that supply this protein in trans.148 The RepliVax-TBE vaccine has been reported to be nonneurovirulent, highly immunogenic and efficacious in mice, and capable of boosting antibody titers induced in mice by an inactivated TBE vaccine.147 The vaccine was also reported to be immunogenic and prevent viremia following challenge in monkeys.

Acknowledgments We thank Kathrin Wanderer and Cornelia Rosenmayer for assistance in preparing the manuscript and Dr Bernd Unger, Dr Robert Petermann, and Mag. Viola Moritz for support.

787

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83. Loew-Baselli LA, Konior R, Pavlova BG, et al. Safety and immunogenicity of the modified adult tick-borne encephalitis vaccine. Vaccine 24:5256–5263, 2006. 93. Wittermann C, Schondorf I, Gniel D. Antibody response following administration of two paediatric tick-borne encephalitis vaccines using two different vaccination schedules. Vaccine 27:1661–1666, 2009. 94. Poellabauer EM, Pavlova BG, Loew-Baselli A, et al. Comparison of immunogenicity and safety between two paediatric TBE vaccines. Vaccine 28:4680–4685, 2010. 117. Heinz FX, Holzmann H, Essl A, et al. Field effectiveness of vaccination against tick-borne encephalitis. Vaccine 25:7559–7567, 2007. 132a. Vaccines against tick-borne encephalitis: WHO position paper– Recommendations. Vaccine 29:8769–8770, 2011.