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Alternative Early Life Vaccination Programs for Companion Animals
ARTICLE IN PRESS J. Comp. Path. 2007,Vol. 137, S67^S71
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Alternative Early Life Vaccination Programs for Companion Animals H. Poulet Merial Research and Development, 254 rue Marcel MeŁrieux, 69007 Lyon, France
Summary An experimental challenge study of multicomponent vaccination of kittens is reported. Seven-to-nine week old, speci¢c pathogen-free kittens received two injections (4 weeks apart) of non-adjuvanted, multicomponent vaccine formulated at the minimum protective dose. Kittens were challenged at 4 weeks or 1 year post-vaccination with individual infectious agents.Vaccination induced complete protection against challenge from feline parvovirus on both occasions, but at 1 year, the protection against feline herpesvirus, feline calicivirus and Chlamydophila felis was not as strong as 4 weeks after vaccination. This demonstration of a decline in protective immunity at the normal time of administration of the ¢rst booster vaccine suggests that earlier administration of this booster (at 4^6 months of age) may provide better protection.The e¡ect of maternally derived antibody (MDA) on kitten vaccination was determined by conducting an identical experiment but with kittens born to queens vaccinated during pregnancy. Serum antibody titres to speci¢c vaccine components were measured in these kittens on day 0 (time of ¢rst vaccination), day 28 (time of second vaccination) and day 42. There was heterogeneity in transfer of MDA to kittens within a litter, and between litters. MDA may neutralize the serological response of kittens on the ¢rst, and occasionally the second, occasion of vaccination when vaccination is performed at 8 and 12 weeks of age. This ¢nding underpins recent recommendations that the ¢nal vaccination in the primary series be administered at 16 weeks of age. r 2007 Elsevier Ltd. All rights reserved. Keywords: duration of immunity; maternal antibodies; vaccine; cat
Introduction It is usually recommended that companion animals receive their ¢rst dose of vaccine at 8 ^ 9 weeks of age. However, sometimes primary vaccination starts even earlier with, for example, vaccines against canine distemper virus (CDV) and parvovirus (CPV) that are licensed for administration to 6-week-old puppies. In older animals it has been common practice to give annual boosters with combination vaccines, raising safety concerns associated with over-vaccination. Consequently, issues such as duration of immunity (DOI) and frequency of administration of boosters continue to be widely discussed by academia, the veterinary profession and the vaccine industry (Coyne et al., 2001; Correspondence to: H. Poulet. (e-mail: [email protected]). 0021-9975/$ - see front matter
doi:10.1016/j.jcpa.2007.04.020
Schultz, 2006). It is generally accepted that the frequency of administration of boosters has been too high in pet animals which are routinely vaccinated. Instead, e¡orts should be made to vaccinate individual animals less frequently, but to vaccinate a larger proportion of the overall population of dogs and cats to improve the general level of protective immunity. For reasons of convenience, combination vaccines have become very popular, and are routinely used for both primary and booster vaccination. As a result, vaccine antigens are mixed and used according to a single regime, even though they may perform di¡erently in terms of protection, onset and duration of immunity, or e⁄cacy in the presence of maternally derived antibodies (MDA). For example, vaccines against feline herpesvirus (FHV), feline calicivirus (FCV) and Chlamydophila felis (C. felis) usually have weaker claims r 2007 Elsevier Ltd. All rights reserved.
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H. Poulet
(reduction of clinical signs) than vaccines against feline parvovirus (FPV), feline leukaemia virus (FeLV) or rabies (prevention of disease). Although most live vaccines are expected to provide protection after a single injection, this is not the case for live FHV, FCVor C. felis vaccines which require two injections to induce adequate protection. There are fundamental di¡erences between a live FPV vaccine strain, which, after administration at a low titre, actively replicates and widely disseminates in the vaccinated host, and a live FHV vaccine strain, which has limited and localized replication after parenteral immunization, thereby requiring a high titre of virus to be administered. Vaccination regimes are based on e⁄cacy and safety data generated during vaccine development. In the USA, the clinical development of vaccines has generally been limited to studies of immediate e⁄cacy in speci¢c pathogen-free (SPF) animals, supported by ¢eld safety studies. In Europe, in addition to immediate e⁄cacy and safety, DOI and e⁄cacy in the presence of MDA must be supported by appropriate studies. This paper reports experimental studies of feline early life vaccination in the context of DOI and e⁄cacy in the presence of MDA.
Materials and Methods Animals and Vaccines
For these studies, 7^9-week-old SPF kittens were randomly allocated to vaccinated and control groups of at least 10 animals each (except for FPV; n ¼ 5).Vaccination consisted of two subcutaneous injections 4 weeks apart with a non-adjuvanted combination vaccine containing attenuated FHV, FPV and C. felis strains, recombinant canarypox-vectored FeLV and inactivated FCVantigens. All components were formulated at the minimum protective dose. To test e⁄cacy in presence of MDA, 8-week-old kittens born to seropositive queens were vaccinated according to the same protocol. The queens were vaccinated against all components and rabies twice before pregnancy, and revaccinated against FHV, FCV and C. felis during pregnancy. Challenge Studies
Challenge with a virulent strain of FPV, FHV, FCVor C. felis was performed 4 weeks after vaccination (immediate e⁄cacy studies) or at least 1year after vaccination (DOI studies), via the intraperitoneal, oculonasal, oronasal or ocular route for each infectious agent, respectively. The challenge protocol was the same for immediate e⁄cacy and DOI studies. For FCV studies, the challenge was with a heterologous strain. A European Pharmacopoeia-like clinical scoring system was used
and the relative e⁄cacy of vaccines was calculated as the reduction of clinical score ([mean score of controlsmean score of vaccinates]/mean score of controls). For each pathogen, duplicate immediate e⁄cacy and DOI studies were performed to con¢rm the consistency of the model. Serology
Antibodies against FHV, FPV, FCV and C. felis were measured by enzyme-linked immunosorbent assay (ELISA). Brie£y, microplates (Maxisorp; Nunc, Rochester, NY) were coated with a capture antibody for 18 h at 5 1C, then rinsed. In separate dilution plates, 50 ml of each test and control serum sample were serially diluted and a ¢xed concentration of puri¢ed antigen was added to each serum dilution. These dilution plates were incubated for 18 h at 5 1C. The serum/antigen mixture was then transferred to the ELISA plates coated with the capture antibody. These plates were incubated for 3 h at 37 1C and then rinsed. A detecting monoclonal antibody speci¢c for the test antigen and labelled with horseradish peroxidase (HRP) was added to each well. After 1h at 37 1C, the plates were rinsed and incubated with HRP substrate for 30 min at 20 1C. The reaction was stopped with 2 M sulphuric acid and optical densities (OD) were read at 450^630 nm. The 50 per cent serum titre corresponded to the inverse of the dilution of the test serum which gave an OD of 50 per cent (optical density equal to 50 per cent of the maximal optical density). Rabies neutralizing antibody titres were measured as described elsewhere (Cliquet et al.,1998).
Results and Discussion Duration of Immunity
All challenges induced clinical signs in adults as well as kittens. Unlike an oronasal challenge, FPV infection via the intraperitoneal route induced severe clinical signs in adults. Comparison of the protection against a challenge performed 4 weeks or1year after vaccination showed similar e⁄cacy for FPV, but a time-related reduction of performance for FHV, FCV or C. felis vaccines. In FPV vaccinated cats, complete protection against clinical signs (relative e⁄cacy X95 per cent) and leucopenia was observed, even when challenge occurred 3 years after vaccination. This protection was associated with the persistence of high and stable neutralizing antibody titres in vaccinated cats. Consistent with the known performance of FHV, FCVand C. felis vaccines, whether live or killed, protection against challenge with these agents was not complete and consisted of reduction of clinical signs.
ARTICLE IN PRESS Early Life Vaccination of Companion Animals
The relative e⁄cacy of the tested vaccines was comparable to published data, notwithstanding their formulation at minimum protective dose (Scott, 1977; Povey and Wilson, 1978; Povey et al., 1980; Gaskell et al., 1982). For FHV, the relative e⁄cacy decreased from 74 to 50 per cent within 1 year (Table 1). Of note was the fact that this reduction of protection occurred in the face of fairly stable antibody titres (Table 2) suggesting that the use of serology to monitor the need for booster vaccination may be questionable in some models. Protection against FHV is known not to be correlated with antibody titre. Similar reduction in relative e⁄cacy was observed with C. felis vaccine and to a lesser extent with FCV vaccine (Table 1). Although such direct comparison between relative e⁄cacy 4 weeks and 1 year after vaccination with the same vaccine in standardized conditions has not previously been reported, recently published data showed that the relative e⁄cacy of an FHV modi¢ed-live vaccine 3 years after vaccination was reduced to 30 per cent (Gore et al., 2006), raising concerns about the weakness of the observed protection. Indeed, DOI should not be de¢ned as the maximum duration for which clinical scores between Table 1 Immediate e⁄cacy and duration of immunity for feline vaccine components Reduction of clinical score (%) Vaccine component
Study number
FHV
1 2 3 4 5 6 7 8
FCV FPV C. felis
Immediate e⁄cacy
Duration of immunity (1 year)
77 71 73 84 97 96 65 95
54 45 53 63 99 95* 52 40
Each vaccine component was formulated at minimum dose; *3 year DOI study; FHV, feline herpesvirus; FCV, feline calicivirus; FPV, feline parvovirus.
Table 2 Speci¢c antibody titres Antibody titre by ELISA Mean 7 standard deviation (log10) Antigen Day 56 FCV FHV FPV
Day 392 1.8370.59 1.9470.64 3.4770.83
1.7570.48 1.6470.57 3.7370.93
Day 56 is time of challenge in immediate e⁄cacy studies; day 392 is time of challenge for 1 year DOI studies; FCV, feline calicivirus; FHV, feline herpesvirus; FPV, feline parvovirus
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vaccinates and controls are signi¢cantly di¡erent, but should also take into consideration the quality of the protection. While 3 year DOI can be readily demonstrated for rabies or FPV vaccines, acceptable 3 year protection by FHVand FCV vaccines is questionable. For FCV, an additional di⁄culty is the strong antigenic diversity of this virus, and its impact on vaccine performance. Protection depends on the cross-antigenicity between the vaccine strain and the ¢eld virus. The use of a homologous challenge strain to demonstrate the DOI (Scott et al., 1999) is therefore not representative of the ¢eld situation. These results suggest that the protection against FHV, C. felis and to a lesser extent FCV, may somewhat decline by the time of the ¢rst annual revaccination, and that there might be a period when cats could be more susceptible to those infectious diseases. Although the studies were performed with vaccines of low antigen content (below the minimum release dose), similar reduction in relative e⁄cacy may be expected with vaccines at commercial dose, as suggested by the results obtained by others (Scott et al., 1999; Gore et al., 2006). In an investigation of FCV vaccination in the ¢eld, it was found that most FCV vaccine breakdowns occurred about 5^6 months after primary vaccination (Dawson et al.,1993). The basic immunization programme for cats consists of one or two primary injections of vaccine and a ¢rst booster which is usually given 1 year later (Horzinek, 2006). Subsequent boosters are then given at regular intervals, from 1 to 3 years depending on the vaccine. DOI is generally studied in animals which have received only primary vaccination. A better and longer relative e⁄cacy may be expected after the ¢rst annual revaccination (complete basic immunization), especially in ¢eld conditions where maternal antibodies can interfere with primary vaccination in young animals. Maternally Derived Antibody
MDA play an important role in the protection of neonates against infectious diseases. Depending on their speci¢city, MDA may disappear within 8 weeks (e.g. MDA against FHV), or persist beyond 12 weeks of age (e.g. MDA against FPVand FCV) and interfere with early vaccination (Scott et al.,1970). This interference may be responsible for a partial or complete neutralization of the vaccine and occasionally result in lack of e⁄cacy in the ¢eld. Measurement of antibody titres in kittens born to queens vaccinated according to the same protocol highlights the heterogeneity in antibody transfer within and between litters (Fig. 1). Following vaccination of kittens with MDA against FHV, FCV or FPV, the
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Fig. 1. Antibody titres in individual kittens born to two SPF queens vaccinated according to the same protocol and with the same vaccine. Antibody titre of the queen is indicated within brackets under the x-axis for each of the components. FCV, FHVand FPVantibodies determined by ELISA and reported as log10 titre. Rabies neutralizing antibody (VN Ab) titre is reported as IU/ml.
Fig. 2. Logit model of protection against feline parvovirus (FPV) as a function of vaccine potency and FPVantibody titre (determined by ELISA) at time of vaccination. FPV vaccine titre: A ¼ 104.0 CCID50/dose; B ¼ 103.5 CCID50/dose.
Fig. 3. (a) In seronegative kittens, two injections (V1 and V2) of a modi¢ed-live FHV vaccine result in signi¢cant seroconversion. (b and c) In seropositive 8 ^ 9-week-old kittens, depending on maternally derived antibody titre at the time of primary vaccination, the ¢rst (V1) and sometimes the second (V2) vaccine injections may be neutralized. The same observation can be made for FPV and FCV (data not shown).
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relationship between seroconversion and MDA titre in kittens at the time of vaccination could be modelled for each component, as illustrated for FPV (Fig. 2). Similar models could be de¢ned for FHV and FCV (not shown). Seroconversion depends on the MDA titre at the time of vaccination, and on the antigen content (infectious titre or potency) of the vaccine. Inactivated vaccines can overcome high MDA titres, provided the amount of antigen in the vaccine is su⁄cient. For live vaccines, ability to overcome MDA also depends on the replicative capacity of the vaccine strain. By combining these models with a survey of the prevalence of MDA in young kittens, the seroconversion rates after primary vaccination with a given vaccine can be predicted. MDA may neutralize the ¢rst and occasionally the second vaccine injection when it is given by or at 12 weeks of age (Fig. 3). Although some kittens with MDA do not seroconvert after vaccination, they may have been primed for a secondary antibody response, as suggested by the seroconversion after the second injection in seronegative kittens (Fig. 3b). After completion of primary vaccination at 12 weeks of age, a signi¢cant proportion of kittens do not seroconvert against FHV, FCVor FPV (Dawson et al., 2001), and this has led to a recent revision of some vaccination guidelines recommending that the last injection of primary vaccination be given at 16 weeks of age.
Conclusions Establishment of appropriate vaccination programs is hampered by the lack of epidemiological data about the prevalence of infectious diseases in companion animals. In humans, establishment of vaccination programs relies partly on epidemiological surveillance of infectious diseases. Unfortunately, there is no co-ordinated monitoring of infectious diseases in companion animals. In particular, it is di⁄cult to determine the rate of vaccine breakdowns and the timing of their occurrence. However, time-related decline in vaccine e⁄cacy against FHV and FCV, and possible interference of MDA in young kittens provides some evidence that primary vaccination may result in less than optimal immunity in a fraction of the population, until the cats receive their ¢rst annual revaccination.To obtain a more homogeneous and overall higher immunity in the cat population, it might therefore be preferable to complete the basic vaccination programme (primary vaccination plus ¢rst revaccination) at a younger age, by giving the ¢rst recall vaccination 4^6 months after primary vaccination, instead of 1 year later. In this case, the period of sub-optimal immunity would be shortened. Subsequent boosters would then be
administered every 1, 2 or 3 years depending on the vaccines and epidemiological considerations.
References Coyne, M. J., Burr, J. H.,Yule,T. D., Harding, M. J.,Tresnan, D. B. and McGavin, D. (2001). Duration of immunity in cats after vaccination or naturally acquired infection. Veterinary Record, 149, 545^548. Cliquet, F., Aubert, M. F. and SagneŁ, L. (1998). Development of a £uorescent antibody virus neutralizing test (FAVN test) for the quantitation of rabies-neutralising antibody. Journal of Immunological Methods, 212,79^87. Dawson, S., McArdle, F., Bennett, D., Carter, S. D., Bennett, M., Ryvar, R. and Gaskell, R. M. (1993). Investigation of vaccine reactions and breakdowns after feline calicivirus vaccination.Veterinary Record, 132, 346^350. Dawson, S., Willoughby, K., Gaskell, R. M., Wood, G. and Chalmers, W. S. (2001). A ¢eld trial to assess the e¡ect of vaccination against feline herpesvirus, feline calicivirus and feline panleucopenia virus in 6-week-old kittens. Journal of Feline Medicine and Surgery, 3,17^22. Gaskell, C. J., Gaskell, R. M., Dennis, P. E. and Wooldridge, M. J. A. (1982). E⁄cacy of an inactivated feline calicivirus (FCV) vaccine against challenge with United Kingdom ¢eld strains and its interaction with the FCVcarrier state. Research inVeterinary Science, 32, 23^26. Gore, T. C., Lakshmanan, N., Williams, J. R., Jirjis, F. F., Chester, S. T., Duncan, K. L., Coyne, M. J., Lum, M. A. and Sterner, F. J. (2006).Three-year duration of immunity in cats following vaccination against feline rhinotracheitis virus, feline calicivirus, and feline panleukopenia virus. VeterinaryTherapeutics, 7, 213^222. Horzinek, M. C. (2006). Vaccine use and disease prevalence in dogs and cats.Veterinary Microbiology, 117, 2^8. Povey, R. C. andWilson, M. R. (1978). A comparison of inactivated feline viral rhinotracheitis and feline caliciviral disease vaccines with live modi¢ed viral vaccines. Feline Practice, 8, 35^42. Povey, R. C., Koones, H. and Hays, M. B. (1980). Immunogenicity and safety of an inactivated vaccine for the prevention of rhinotracheitis, caliciviral disease, and panleukopenia in cats. Journal of the American Veterinary Medical Association, 177, 347^350. Schultz, R. D. (2006). Duration of immunity for canine and feline vaccines: a review. Veterinary Microbiology, 117, 75^79. Scott, F. W., Csiza, C. K. and Gillespie, J. H. (1970). Maternally derived immunity to feline panleukopenia. Journal of the AmericanVeterinary Medical Association, 156, 439^453. Scott, F.W. (1977). Evaluation of a feline viral rhinotracheitisfeline calicivirus disease vaccine. American Journal of Veterinary research, 38, 229^234. Scott, F. W., Cordell, M. and Geissinger, B. S. (1999). Longterm immunity in cats vaccinated with an inactivated trivalent vaccine. American Journal of Veterinary Research, 60, 652^658.
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Alternative Early Life Vaccination Programs for Companion Animals H. Poulet Merial Research and Development, 254 rue Marcel MeŁrieux, 69007 Lyon, France
Summary An experimental challenge study of multicomponent vaccination of kittens is reported. Seven-to-nine week old, speci¢c pathogen-free kittens received two injections (4 weeks apart) of non-adjuvanted, multicomponent vaccine formulated at the minimum protective dose. Kittens were challenged at 4 weeks or 1 year post-vaccination with individual infectious agents.Vaccination induced complete protection against challenge from feline parvovirus on both occasions, but at 1 year, the protection against feline herpesvirus, feline calicivirus and Chlamydophila felis was not as strong as 4 weeks after vaccination. This demonstration of a decline in protective immunity at the normal time of administration of the ¢rst booster vaccine suggests that earlier administration of this booster (at 4^6 months of age) may provide better protection.The e¡ect of maternally derived antibody (MDA) on kitten vaccination was determined by conducting an identical experiment but with kittens born to queens vaccinated during pregnancy. Serum antibody titres to speci¢c vaccine components were measured in these kittens on day 0 (time of ¢rst vaccination), day 28 (time of second vaccination) and day 42. There was heterogeneity in transfer of MDA to kittens within a litter, and between litters. MDA may neutralize the serological response of kittens on the ¢rst, and occasionally the second, occasion of vaccination when vaccination is performed at 8 and 12 weeks of age. This ¢nding underpins recent recommendations that the ¢nal vaccination in the primary series be administered at 16 weeks of age. r 2007 Elsevier Ltd. All rights reserved. Keywords: duration of immunity; maternal antibodies; vaccine; cat
Introduction It is usually recommended that companion animals receive their ¢rst dose of vaccine at 8 ^ 9 weeks of age. However, sometimes primary vaccination starts even earlier with, for example, vaccines against canine distemper virus (CDV) and parvovirus (CPV) that are licensed for administration to 6-week-old puppies. In older animals it has been common practice to give annual boosters with combination vaccines, raising safety concerns associated with over-vaccination. Consequently, issues such as duration of immunity (DOI) and frequency of administration of boosters continue to be widely discussed by academia, the veterinary profession and the vaccine industry (Coyne et al., 2001; Correspondence to: H. Poulet. (e-mail: [email protected]). 0021-9975/$ - see front matter
doi:10.1016/j.jcpa.2007.04.020
Schultz, 2006). It is generally accepted that the frequency of administration of boosters has been too high in pet animals which are routinely vaccinated. Instead, e¡orts should be made to vaccinate individual animals less frequently, but to vaccinate a larger proportion of the overall population of dogs and cats to improve the general level of protective immunity. For reasons of convenience, combination vaccines have become very popular, and are routinely used for both primary and booster vaccination. As a result, vaccine antigens are mixed and used according to a single regime, even though they may perform di¡erently in terms of protection, onset and duration of immunity, or e⁄cacy in the presence of maternally derived antibodies (MDA). For example, vaccines against feline herpesvirus (FHV), feline calicivirus (FCV) and Chlamydophila felis (C. felis) usually have weaker claims r 2007 Elsevier Ltd. All rights reserved.
ARTICLE IN PRESS S68
H. Poulet
(reduction of clinical signs) than vaccines against feline parvovirus (FPV), feline leukaemia virus (FeLV) or rabies (prevention of disease). Although most live vaccines are expected to provide protection after a single injection, this is not the case for live FHV, FCVor C. felis vaccines which require two injections to induce adequate protection. There are fundamental di¡erences between a live FPV vaccine strain, which, after administration at a low titre, actively replicates and widely disseminates in the vaccinated host, and a live FHV vaccine strain, which has limited and localized replication after parenteral immunization, thereby requiring a high titre of virus to be administered. Vaccination regimes are based on e⁄cacy and safety data generated during vaccine development. In the USA, the clinical development of vaccines has generally been limited to studies of immediate e⁄cacy in speci¢c pathogen-free (SPF) animals, supported by ¢eld safety studies. In Europe, in addition to immediate e⁄cacy and safety, DOI and e⁄cacy in the presence of MDA must be supported by appropriate studies. This paper reports experimental studies of feline early life vaccination in the context of DOI and e⁄cacy in the presence of MDA.
Materials and Methods Animals and Vaccines
For these studies, 7^9-week-old SPF kittens were randomly allocated to vaccinated and control groups of at least 10 animals each (except for FPV; n ¼ 5).Vaccination consisted of two subcutaneous injections 4 weeks apart with a non-adjuvanted combination vaccine containing attenuated FHV, FPV and C. felis strains, recombinant canarypox-vectored FeLV and inactivated FCVantigens. All components were formulated at the minimum protective dose. To test e⁄cacy in presence of MDA, 8-week-old kittens born to seropositive queens were vaccinated according to the same protocol. The queens were vaccinated against all components and rabies twice before pregnancy, and revaccinated against FHV, FCV and C. felis during pregnancy. Challenge Studies
Challenge with a virulent strain of FPV, FHV, FCVor C. felis was performed 4 weeks after vaccination (immediate e⁄cacy studies) or at least 1year after vaccination (DOI studies), via the intraperitoneal, oculonasal, oronasal or ocular route for each infectious agent, respectively. The challenge protocol was the same for immediate e⁄cacy and DOI studies. For FCV studies, the challenge was with a heterologous strain. A European Pharmacopoeia-like clinical scoring system was used
and the relative e⁄cacy of vaccines was calculated as the reduction of clinical score ([mean score of controlsmean score of vaccinates]/mean score of controls). For each pathogen, duplicate immediate e⁄cacy and DOI studies were performed to con¢rm the consistency of the model. Serology
Antibodies against FHV, FPV, FCV and C. felis were measured by enzyme-linked immunosorbent assay (ELISA). Brie£y, microplates (Maxisorp; Nunc, Rochester, NY) were coated with a capture antibody for 18 h at 5 1C, then rinsed. In separate dilution plates, 50 ml of each test and control serum sample were serially diluted and a ¢xed concentration of puri¢ed antigen was added to each serum dilution. These dilution plates were incubated for 18 h at 5 1C. The serum/antigen mixture was then transferred to the ELISA plates coated with the capture antibody. These plates were incubated for 3 h at 37 1C and then rinsed. A detecting monoclonal antibody speci¢c for the test antigen and labelled with horseradish peroxidase (HRP) was added to each well. After 1h at 37 1C, the plates were rinsed and incubated with HRP substrate for 30 min at 20 1C. The reaction was stopped with 2 M sulphuric acid and optical densities (OD) were read at 450^630 nm. The 50 per cent serum titre corresponded to the inverse of the dilution of the test serum which gave an OD of 50 per cent (optical density equal to 50 per cent of the maximal optical density). Rabies neutralizing antibody titres were measured as described elsewhere (Cliquet et al.,1998).
Results and Discussion Duration of Immunity
All challenges induced clinical signs in adults as well as kittens. Unlike an oronasal challenge, FPV infection via the intraperitoneal route induced severe clinical signs in adults. Comparison of the protection against a challenge performed 4 weeks or1year after vaccination showed similar e⁄cacy for FPV, but a time-related reduction of performance for FHV, FCV or C. felis vaccines. In FPV vaccinated cats, complete protection against clinical signs (relative e⁄cacy X95 per cent) and leucopenia was observed, even when challenge occurred 3 years after vaccination. This protection was associated with the persistence of high and stable neutralizing antibody titres in vaccinated cats. Consistent with the known performance of FHV, FCVand C. felis vaccines, whether live or killed, protection against challenge with these agents was not complete and consisted of reduction of clinical signs.
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The relative e⁄cacy of the tested vaccines was comparable to published data, notwithstanding their formulation at minimum protective dose (Scott, 1977; Povey and Wilson, 1978; Povey et al., 1980; Gaskell et al., 1982). For FHV, the relative e⁄cacy decreased from 74 to 50 per cent within 1 year (Table 1). Of note was the fact that this reduction of protection occurred in the face of fairly stable antibody titres (Table 2) suggesting that the use of serology to monitor the need for booster vaccination may be questionable in some models. Protection against FHV is known not to be correlated with antibody titre. Similar reduction in relative e⁄cacy was observed with C. felis vaccine and to a lesser extent with FCV vaccine (Table 1). Although such direct comparison between relative e⁄cacy 4 weeks and 1 year after vaccination with the same vaccine in standardized conditions has not previously been reported, recently published data showed that the relative e⁄cacy of an FHV modi¢ed-live vaccine 3 years after vaccination was reduced to 30 per cent (Gore et al., 2006), raising concerns about the weakness of the observed protection. Indeed, DOI should not be de¢ned as the maximum duration for which clinical scores between Table 1 Immediate e⁄cacy and duration of immunity for feline vaccine components Reduction of clinical score (%) Vaccine component
Study number
FHV
1 2 3 4 5 6 7 8
FCV FPV C. felis
Immediate e⁄cacy
Duration of immunity (1 year)
77 71 73 84 97 96 65 95
54 45 53 63 99 95* 52 40
Each vaccine component was formulated at minimum dose; *3 year DOI study; FHV, feline herpesvirus; FCV, feline calicivirus; FPV, feline parvovirus.
Table 2 Speci¢c antibody titres Antibody titre by ELISA Mean 7 standard deviation (log10) Antigen Day 56 FCV FHV FPV
Day 392 1.8370.59 1.9470.64 3.4770.83
1.7570.48 1.6470.57 3.7370.93
Day 56 is time of challenge in immediate e⁄cacy studies; day 392 is time of challenge for 1 year DOI studies; FCV, feline calicivirus; FHV, feline herpesvirus; FPV, feline parvovirus
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vaccinates and controls are signi¢cantly di¡erent, but should also take into consideration the quality of the protection. While 3 year DOI can be readily demonstrated for rabies or FPV vaccines, acceptable 3 year protection by FHVand FCV vaccines is questionable. For FCV, an additional di⁄culty is the strong antigenic diversity of this virus, and its impact on vaccine performance. Protection depends on the cross-antigenicity between the vaccine strain and the ¢eld virus. The use of a homologous challenge strain to demonstrate the DOI (Scott et al., 1999) is therefore not representative of the ¢eld situation. These results suggest that the protection against FHV, C. felis and to a lesser extent FCV, may somewhat decline by the time of the ¢rst annual revaccination, and that there might be a period when cats could be more susceptible to those infectious diseases. Although the studies were performed with vaccines of low antigen content (below the minimum release dose), similar reduction in relative e⁄cacy may be expected with vaccines at commercial dose, as suggested by the results obtained by others (Scott et al., 1999; Gore et al., 2006). In an investigation of FCV vaccination in the ¢eld, it was found that most FCV vaccine breakdowns occurred about 5^6 months after primary vaccination (Dawson et al.,1993). The basic immunization programme for cats consists of one or two primary injections of vaccine and a ¢rst booster which is usually given 1 year later (Horzinek, 2006). Subsequent boosters are then given at regular intervals, from 1 to 3 years depending on the vaccine. DOI is generally studied in animals which have received only primary vaccination. A better and longer relative e⁄cacy may be expected after the ¢rst annual revaccination (complete basic immunization), especially in ¢eld conditions where maternal antibodies can interfere with primary vaccination in young animals. Maternally Derived Antibody
MDA play an important role in the protection of neonates against infectious diseases. Depending on their speci¢city, MDA may disappear within 8 weeks (e.g. MDA against FHV), or persist beyond 12 weeks of age (e.g. MDA against FPVand FCV) and interfere with early vaccination (Scott et al.,1970). This interference may be responsible for a partial or complete neutralization of the vaccine and occasionally result in lack of e⁄cacy in the ¢eld. Measurement of antibody titres in kittens born to queens vaccinated according to the same protocol highlights the heterogeneity in antibody transfer within and between litters (Fig. 1). Following vaccination of kittens with MDA against FHV, FCV or FPV, the
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Fig. 1. Antibody titres in individual kittens born to two SPF queens vaccinated according to the same protocol and with the same vaccine. Antibody titre of the queen is indicated within brackets under the x-axis for each of the components. FCV, FHVand FPVantibodies determined by ELISA and reported as log10 titre. Rabies neutralizing antibody (VN Ab) titre is reported as IU/ml.
Fig. 2. Logit model of protection against feline parvovirus (FPV) as a function of vaccine potency and FPVantibody titre (determined by ELISA) at time of vaccination. FPV vaccine titre: A ¼ 104.0 CCID50/dose; B ¼ 103.5 CCID50/dose.
Fig. 3. (a) In seronegative kittens, two injections (V1 and V2) of a modi¢ed-live FHV vaccine result in signi¢cant seroconversion. (b and c) In seropositive 8 ^ 9-week-old kittens, depending on maternally derived antibody titre at the time of primary vaccination, the ¢rst (V1) and sometimes the second (V2) vaccine injections may be neutralized. The same observation can be made for FPV and FCV (data not shown).
ARTICLE IN PRESS S71
Early Life Vaccination of Companion Animals
relationship between seroconversion and MDA titre in kittens at the time of vaccination could be modelled for each component, as illustrated for FPV (Fig. 2). Similar models could be de¢ned for FHV and FCV (not shown). Seroconversion depends on the MDA titre at the time of vaccination, and on the antigen content (infectious titre or potency) of the vaccine. Inactivated vaccines can overcome high MDA titres, provided the amount of antigen in the vaccine is su⁄cient. For live vaccines, ability to overcome MDA also depends on the replicative capacity of the vaccine strain. By combining these models with a survey of the prevalence of MDA in young kittens, the seroconversion rates after primary vaccination with a given vaccine can be predicted. MDA may neutralize the ¢rst and occasionally the second vaccine injection when it is given by or at 12 weeks of age (Fig. 3). Although some kittens with MDA do not seroconvert after vaccination, they may have been primed for a secondary antibody response, as suggested by the seroconversion after the second injection in seronegative kittens (Fig. 3b). After completion of primary vaccination at 12 weeks of age, a signi¢cant proportion of kittens do not seroconvert against FHV, FCVor FPV (Dawson et al., 2001), and this has led to a recent revision of some vaccination guidelines recommending that the last injection of primary vaccination be given at 16 weeks of age.
Conclusions Establishment of appropriate vaccination programs is hampered by the lack of epidemiological data about the prevalence of infectious diseases in companion animals. In humans, establishment of vaccination programs relies partly on epidemiological surveillance of infectious diseases. Unfortunately, there is no co-ordinated monitoring of infectious diseases in companion animals. In particular, it is di⁄cult to determine the rate of vaccine breakdowns and the timing of their occurrence. However, time-related decline in vaccine e⁄cacy against FHV and FCV, and possible interference of MDA in young kittens provides some evidence that primary vaccination may result in less than optimal immunity in a fraction of the population, until the cats receive their ¢rst annual revaccination.To obtain a more homogeneous and overall higher immunity in the cat population, it might therefore be preferable to complete the basic vaccination programme (primary vaccination plus ¢rst revaccination) at a younger age, by giving the ¢rst recall vaccination 4^6 months after primary vaccination, instead of 1 year later. In this case, the period of sub-optimal immunity would be shortened. Subsequent boosters would then be
administered every 1, 2 or 3 years depending on the vaccines and epidemiological considerations.
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