High levels of antibody in adults three years after vaccination with a reduced antigen content diphtheria-tetanus-acellular pertussis vaccine

Vaccine 23 (2004) 380–385

High levels of antibody in adults three years after vaccination with a reduced antigen content diphtheria-tetanus-acellular pertussis vaccine Peter B. McIntyrea,∗ , Fiona M. Turnbullb,1 , Anne-Marie Egana , Margaret A. Burgessa , Joanne M Wolterc , Lode M Schuermanc a

Centre for Immunisation Research, University of Sydney and the Royal Alexandra Hospital for Children, Locked Bag 4001, Westmead, NSW 2145, Australia b George Institute for International Health, University of Sydney, Sydney, NSW, Australia c GlaxoSmithKline Biologicals, Rue de l’institut 89, Rixensart B1330, Belgium Received 30 December 2002; received in revised form 14 May 2004; accepted 24 May 2004 Available online 12 July 2004

Abstract There is increasing interest in prevention of pertussis in adults by vaccination, but little is known about the duration of the antibody response to pertussis, diphtheria or tetanus in reduced antigen content vaccines formulated for adult use. Follow-up of a clinical trial including 550 adults comparing responses to reduced antigen content diphtheria-tetanus-acellular pertussis (dTpa) vaccine, or a licensed Td vaccine, provided the opportunity to evaluate this. Blood samples were collected at 0, 1, 12, 24 and 36 months following vaccination; of the original cohort of 550, 387 subjects (dTpa group N = 310, Td + pa group N = 77) were tested at month 36. Following a decrease in antibody levels against all vaccine antigens between one and 24 months following vaccination, levels stabilized during the third year, remaining higher at 36 months than pre-vaccination for all vaccine antigens. In particular, more than 90% of subjects remained seropositive for pertussis toxin and pertactin antibodies at 36 months after vaccination, suggesting ongoing protection against pertussis. Adult-formulated dTpa vaccines could replace Td for routine booster vaccination of older individuals. © 2004 Elsevier Ltd. All rights reserved. Keywords: Antibody persistence; Diphtheria; Tetanus; Pertussis

1. Introduction In the pre-vaccination era, pertussis was primarily a disease of childhood. However, in highly vaccinated communities it has recently become apparent that a shift in epidemiology has occurred, with an increasing incidence of disease in older children, adolescents and adults [1–6]. This shift is attributed to the fact that immunity to pertussis, whether nat∗

Corresponding author. Tel.: +612 9845 1257; fax: +612 9845 3082. E-mail address: [email protected] (P.B. McIntyre). 1 Formerly, Centre for Immunisation Research, University of Sydney and the Royal Alexandra Hospital for Children, Locked Bag 4001, Westmead, NSW 2145, Australia. 0264-410X/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2004.05.030

ural or vaccine-induced, is not life-long [7–9]. Although not as severe as in young children, pertussis causes significant morbidity in adolescents and adults [10–13] and is underdiagnosed [13,14]. Associated with adult disease is the risk of transmission of pertussis from adults to neonates and infants unprotected by a full course of primary vaccination [1,7,15–21]. Until recently pertussis vaccination of adults has not been feasible due to the unacceptable reactogenicity of whole-cell pertussis vaccines when administered beyond childhood. However, the advent of adult-type acellular pertussis vaccines with reduced antigen content has seen calls for widespread pertussis vaccination of older age groups [22]. GlaxoSmithKline (GSK) Biologicals’ reduced antigen content diphtheria-tetanus-acellular pertussis vaccine (dTpa,

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Table 1 Vaccine formulation Vaccine/dose (0.5 ml)

PT (␮g)

FHA (␮g)

PRN (␮g)

Diphtheria toxoid

Tetanus toxoid

paa

8 8 –

8 8 –

2.5 2.5 –

– ≥2 IU (2.5 Lf) ≥2 IU (2.0 Lf)

– ≥20 IU (5 Lf) ≥20 IU (6 Lf)

dTpab Tdc

PT: pertussis toxoid, FHA: filamentous haemaglutinin, PRN: pertactin. a pa = experimental reduced-antigen-content acellular pertussis vaccine from GSK Biologicals Belgium. b dTpa = reduced-antigen-content combined diphtheria-tetanus-acellular pertussis vaccine BoostrixTM from GSK Biologicals Belgium. c Td = diphtheria-tetanus vaccine (adult formulation) ADT from CSL vaccines Australia.

BoostrixTM ) has been licensed for use from 4 years of age in several countries and from 10 years of age in Australia. In studies in which the dTpa vaccine was compared to licensed combined reduced antigen content diphtheria and tetanus (Td) vaccines, the response to both tetanus and diphtheria components compared well, although geometric mean concentrations (GMCs) for tetanus were higher in the groups that received Td [23,24]. Since the majority of subjects in clinical trials demonstrated anti-tetanus antibody concentrations of ≥1.0 IU/ml, which is at least 100 times higher than the accepted protective concentration for tetanus [25], this is unlikely to be clinically significant. Although longer term efficacy data are available for participants in acellular pertussis vaccine trials commencing in infancy [26], published data on duration of immunity following acellular pertussis vaccines given to adults are limited [2,24,27,28]. In order to assess long-term antibody persistence after vaccination with dTpa, adult subjects enrolled in a clinical study comparing a booster dose of dTpa (BoostrixTM , GSK Biologicals, Rixenart, Belgium) with a licensed Td vaccine (ADT® vaccine, CSL Vaccines Limited, Melbourne, Australia) have been contacted for yearly blood sampling following vaccination. We report results of three successive years of serologic follow-up, which are relevant to long-term planning for use of adult-formulated vaccines.

2. Methods 2.1. Subjects The single-blinded, randomised booster vaccination study was conducted at the Centre for Immunisation Research, Children’s Hospital at Westmead, NSW, Australia, and has been reported in depth previously [24]. Briefly, adults aged 18 years or above were randomized to receive either a single injection of dTpa vaccine (BoostrixTM ) or Td vaccine (ADT) followed by stand alone reduced antigen content acellular pertussis (pa) vaccine (GSK Biologicals, Rixenart, Belgium) 1 month later; or the pa vaccine followed 1 month later by Td. As the sequence of administration of Td and pa vaccines did not influence the immune response 1 month after vaccination, data for the groups receiving Td + pa and pa + Td have been pooled for the analysis presented in this report. Results from blood samples collected before vaccination and

1 month and 12 months after vaccination (for a randomly selected subgroup of 100 subjects) have been reported previously [24]. Subsequently, all subjects from the vaccination study were invited to return for additional blood sampling at approximately 24 and 36 months after vaccination. 2.2. Vaccines The composition of the three study vaccines is given in Table 1. Vaccines were administered by intramuscular injection. 2.3. Serologic assays Serum samples were stored at −20 to −70 ◦ C until blinded analyses were conducted at GSK Biologicals’ laboratory, Rixensart, Belgium. Pertussis IgG antibodies (anti-PT, antiFHA and anti-PRN) were determined by an ELISA assay and were expressed in ELISA Units (EU/ml). Seropositivity for pertussis antigens was defined as a concentration above the assay cut-off of 5 EU/ml. Anti-diphtheria and anti-tetanus toxoid concentrations were measured by a modified sandwich ELISA and expressed in International Units per millilitre (IU/ml), with respect to a reference serum. The assay cut-off was 0.1 IU/ml. It is generally accepted for both antibodies that concentrations ≥0.01 IU/ml, as measured by in vivo neutralisation tests, are protective. It has been previously demonstrated that a good correlation exists between in vivo neutralisation tests and the ELISA tests [29,30] for antibodies to diphtheria and tetanus toxoids, but this correlation may be reduced at antibody concentrations below 0.1 IU/ml. Samples with anti-diphtheria ELISA concentrations below 0.1 IU/ml were re-tested using the more sensitive in vitro neutralisation assay on Vero cells [31,32]. Antibody concentrations of ≥0.016 IU/ml using the Vero-cell assay were considered indicative of protection. For calculation of geometric mean concentration (GMC), antibody levels below the cut-off of the assay were given an arbitrary value of half the cut-off. 2.4. Statistical analysis Analyses were performed using SAS (version 6.12) under Windows and StatXact (version 3.0). Descriptive analyses were performed on all subjects enrolled in the 36-month

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follow-up. It was planned to exclude subjects having evidence of confirmed pertussis, diphtheria or tetanus or vaccination since the previous study visit. However, no subject needed to be excluded on these grounds. Seropositivity/seroprotection rates and GMCs with exact 95% confidence intervals (CIs) were calculated by vaccine group (dTpa group and Td (+pa) group) for each antigen at each blood sampling time point (pre-vaccination, 1, 12, 24 and 36 months after vaccination). Since the study was descriptive in nature, statistical comparisons between groups were not performed, except for comparison of 95% CIs.

3. Results 3.1. Study population

Table 2 Demography and time from first vaccination to blood sampling of the Month 24 and 36 cohorts

Month 24 cohort N Mean age (years) ± S.D. Range (years) Mean time (months) ± S.D. from vaccination to blood sampling Range (months) Month 36 cohort N Mean age (years) ± S.D. Range (years) Mean time (months) ± S.D. from vaccination to blood sampling Range (months)

dTpa group

Td (+pa) group

320 39.8 ± 10.88 20–69 27.3 ± 2.2

79 41.3 ± 10.20 22–57 24.3 ± 1.9

22–32

23–31

310 39.8 ± 10.91 20–69 36.0 ± 1.7

77 41.2 ± 10.12 22–57 35.9 ± 1.8

34–40

34–40

SD: standard variation.

Three hundred and ninety-nine (73%) and 387 (70%) subjects of the 550 enrolled in the original vaccination study returned for the 24 and 36 month blood sampling (Table 2). Results from the Month 36 cohort at each blood sampling point (pre-vaccination, 1, 12, 24 and 36 months after vaccination) are shown in Tables 3 and 4. The subset of subjects in the month 36 cohort was representative of the total cohort, as the pre-vaccination and one month post-vaccination antibody levels for the month 36 cohort were found to be similar to the concentrations observed for the total cohort enrolled in the booster study [24].

tinued to have protective levels (≥0.016 IU/ml). For tetanus, the proportion of subjects with seroprotective antibody concentrations (≥0.1 IU/ml by ELISA) remained high at 36 months in both groups (94.8% in the dTpa group and 93.5% in the Td group). Anti-tetanus antibody GMCs were not significantly different (overlapping 95% CIs) between groups after 36 months, although a higher point estimate after Td was still observed. Fig. 1 graphically illustrates the reduction in antibody concentrations to all vaccine antigens over time after vaccination.

3.2. Diphtheria and tetanus antibodies 3.3. Pertussis antibodies Thirty six months after vaccination, the proportion of subjects seroprotected (≥0.1 IU/ml by ELISA) against diphtheria was similar for dTpa and Td vaccinees (71.2 and 73.7%, respectively, Table 3). When re-tested with the Vero-cell assay, 97.4% in the dTpa group and 98.6% in the Td group con-

Thirty-six months after vaccination with dTpa or pa vaccine, the proportion of subjects with antibodies against the 3 pertussis antigens was similar between groups, and remained higher than before vaccination (Table 4).

Table 3 Diphtheria and tetanus antibodies: seroprotection rates and GMCs prior to and at 1, 12, 24 and 36 months following vaccination with dTpa or Td (+pa) (month 36 cohort) Antibody

Anti-diphtheria ELISA

ELISA + VEROa Anti-tetanus ELISA

Timing

dTpa group

Td (+pa) group

N

% Seroprotection

N

% Seroprotection

Pre Post 1 month Post 12 month Post 24 month Post 36 month

301 309 34 310 309

57.1 (51.3–62.8] 98.7 (96.7–99.6) 73.5 (55.6–87.1) 74.2 (68.9–79.0) 71.2 (65.8–76.2)

75 77 40 76 76

54.7 (42.7–66.2) 97.4 (90.9–99.7) 85.0 (70.2–94.3) 76.3 (65.2–85.3) 73.7 (62.3–83.1)

Post 36 month

309

97.4 (95.6–99.2)

76

98.6 (95.9–100)

Pre Post 1 month Post 12 month Post 24 month Post 36 month

310 310 34 309 310

85.2 (80.7–88.9) 100.0 (98.8–100) 94.1 (80.3–99.3) 95.1 (92.1–97.3) 94.8 (91.8–97.0)

77 77 40 77 77

83.1 (72.9–90.7) 100.0 (95.3–100) 97.5 (86.8–99.9) 93.5 (85.5–97.9) 93.5 (85.5–97.9)

a ELISA + VERO refers to % of with seroprotective diphtheria antibody levels by either ELISA or Vero assay (antibody concentration ≥0.1 IU/ml or ≥0.016 IU/ml, respectively).

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Table 4 Pertussis antibody seropositivity rates and GMCs prior to and at 1, 12, 24 and 36 months following vaccination with dTpa or Td + pa (Month 36 cohort) Antibody

Timing

dTpa group N

(Td +) pa group % Seropositive

N

% Seropositive

Anti-PT

Pre-vaccination Post 1 month Post 12 month Post 24 month Post 36 month

307 309 33 310 310

67.4 (61.9–72.6] 99.0 (97.2–99.8) 97.0 (84.2–99.9) 90.6 (86.8–93.6) 90.6 (86.8–93.6)

76 76 37 77 77

59.2 (47.3–70.4) 97.4 (90.8–99.7) 94.6 (81.8–99.3) 84.4 (74.4–91.7) 84.4 (74.4–91.7)

Anti-FHA

Pre-vaccination Post 1 month Post 12 month Post 24 month Post 36 month

300 308 34 309 309

98.3 (96.2–99.5) 100.0 (98.8–100) 100.0 (89.7–100) 100.0 (98.9–100) 100.0 (98.8–100)

77 76 40 74 77

98.7 (93.0–100) 100.0 (95.3–100) 100.0 (91.2–100) 100.0 (95.1–100) 100.0 (95.3–100)

Anti-PRN

Pre-vaccination Post 1 month Post 12 month Post 24 month Post 36 month

310 310 34 310 309

71.0 (65.6–76.0) 99.4 (97.7–99.9) 100.0 (89.7–100) 94.5 (91.4–96.8) 94.8 (91.7–97.0)

77 77 40 77 77

75.3 (64.2–84.4) 100.0 (95.3–100) 87.5 (73.2–95.8) 90.9 (82.2–96.3) 90.9 (82.2–96.3)

Seropositive: antibody concentration ≥5 EU/ml.

Fig. 1. Antibody concentrations up to 36 months following vaccination with dTpa or Td + pa (month 36 cohort).

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Antibody concentrations to all antigens in both groups fell several-fold between 1 month and 24 months after vaccination, and then appeared to stabilise during the third year after vaccination. Antibody concentrations to all vaccine antigens continued to be higher at 36 months than prior to vaccination.

4. Discussion In recent years, the shift in the incidence of pertussis to older age groups observed in many developed countries [1–5] and the availability of less reactogenic acellular pertussis vaccines formulated for adult use [23,24] has raised the issue of prevention of disease in adults by vaccination [22]. This has been further supported by the results of a NIH-sponsored study conducted in the USA to assess the protective efficacy of pa vaccine (GSK Biologicals). After 2 years of followup of more than 2500 adolescents and adults randomized to receive either pa or a control vaccine (GSK Biologicals’ Havrix® ), the estimated efficacy against pertussis was 77% (95% CI (−9; 98)) [33]. However, this does not address the issue of the duration of protection against pertussis by booster vaccine administered to adults. Infant vaccination serologic follow-up with efficacy evaluation is currently the only means to assess this [26]. In this study, carried out three years after vaccination with dTpa, antibody concentrations to all three antigens were higher than those observed after a three-dose primary series in infants in whom efficacy was demonstrated [34,35], suggesting that the dTpa vaccine will be similarly efficacious against pertussis disease [36]. These adults vaccinated with a dTpa also showed similar levels of antibody, correlated with protection to diphtheria and tetanus, to a commercial Td vaccine. Immunogenicity and reactogenicity of the combined dTpa vaccine was also comparable to the licensed Td vaccines [23,24,27]. Although anti-tetanus antibody GMCs were higher after vaccination with Td compared to dTpa, 36 months later the absolute difference in anti-tetanus antibody concentrations initially observed had fallen markedly. Similarly, 36 months after vaccination both vaccines continued to provide similar protection against diphtheria. Although antibody levels to the three pertussis antigens fell over time, antibody concentrations 36 months after vaccination continued to be higher than pre-vaccination levels, with over 90% of subjects having detectable antibody to PT, and PRN three years after vaccination. This is in contrast to seropositivity rates of 64–78% for PT and 25–51.2% for PRN, reported in a seroepidemiologic study of adults in Belgium [37]. The presence of detectable PRN and PT antibody has been correlated with a reduced risk of pertussis disease in exposed individuals [38]. Seropositivity rates for FHA antibodies were uniformly high in subjects vaccinated in this study, and in unvaccinated individuals [37], reflecting crossreactivity of FHA with other organisms. In the absence of clear serologic correlates of protection or sufficiently long trial follow-up, the optimal interval be-

tween pertussis boosters is difficult to define. In a paediatric population, persistence of pertussis vaccine efficacy has been demonstrated up to 6 years following the primary vaccination series, even though more than 50% of subjects had undetectable levels of PT [26]. In this context, the observation that the majority of vaccinated subjects in this study continued to have detectable antibody three years after vaccination suggests that protection against pertussis is ongoing. One way of achieving regular pertussis re-vaccination is by combining the pertussis antigens with the tetanus and diphtheria toxoid vaccines already recommended at regular intervals in older age groups. Although the National Health and Medical Research Council in Australia now only recommends booster doses of Td at 15 and 50 years of age [39], in accord with earlier recommendations in the United Kingdom, a 10-yearly booster interval remains common practice in many other regions of the world. Recently an International Consensus Group on Pertussis Immunisation has advocated universal vaccination of all age groups against pertussis [22]. As a first step, this strategy proposes universal adolescent booster vaccination, and vaccination targeted at adults most likely to have contact with very young babies, as well as those whose medical conditions or advanced age place them at risk of severe disease [22]. This recommendation mirrors that recently endorsed in Australia by the National Health and Medical Research Council [40]. The data presented in this report provide reassurance that, in addition to pertussis, long-term protection afforded against diphtheria and tetanus from dTpa vaccine is likely to be similar to Td vaccines. Continued follow-up of this cohort, in addition to monitoring the impact of the recent Australian recommendations, will provide valuable information to inform decisions on the optimal program to control pertussis in comparable developed countries.

Acknowledgements We are grateful to Dr. Miriam Hynes and Dr. Cheryl McCoy for help in the preparation of the manuscript. This study was supported by a grant from GSK Biologicals Rixensart Belgium.

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