Antibody responses to FPV vaccines in kittens: comparison of formulations and injection sites
Journal of feline medicine and surgery · 2026
In plain language
This study in kittens compared antibodies to feline panleukopenia virus (FPV) after different vaccines were given at different injection sites. At T1, 80 of the 88 evaluable kittens reached the antibody level the study considered indicative of protection, but protection was not tested through experimental exposure to the virus. The modified-live vaccine produced higher antibody titres at two of the three assessment points; no statistically significant differences were detected between injection sites for the inactivated vaccine. All kittens remained FeLV-negative on the tests performed, but this study does not demonstrate protection against FeLV or effects in FeLV-infected cats.
Scientific detail
This in vivo study, published in the Journal of feline medicine and surgery in 2026, assessed FPV antibody responses following vaccination with a non-adjuvanted modified-live formulation or an adjuvanted inactivated formulation. It recruited 100 kittens. METHODS describes 69 in the modified-live vaccine group and 12, 10 and 9 in the groups receiving the inactivated vaccine in the tail, left hindlimb and scruff, respectively. Of the recruited animals, 77 returned on day 365. RESULTS reports the exclusion of eight baseline haemagglutination inhibition (HI)-positive kittens from the principal analysis, with evaluable denominators of 88 at T1, 84 at T2 and 70 at T12. Allocation is described inconsistently: the abstract calls it random, whereas METHODS describes consecutive allocation.
The claims in the knowledge object are classified as level C evidence. The modified-live vaccine group had higher FPV antibody titres than the pooled inactivated-vaccine groups at T1 and T12, but not at T2. These differences concern titre magnitude, not demonstrated differences in clinical protection. At T1, 80 of 88 evaluable kittens met the predefined HI threshold for presumptive FPV protection, with no statistically significant differences by formulation or inactivated-vaccine injection site. Protection was inferred from serology rather than established through an experimental FPV challenge reported in the article.
No statistically significant differences in FPV antibody titres were detected between the three inactivated-vaccine injection sites. This finding does not demonstrate equivalence between sites or comparable protection against FeLV. The inactivated-vaccine site groups were small. In addition, some hindlimb injections may have been given at the stifle rather than distal to it. The comparison between formulations also involved different injection-site distributions and concurrent separate FeLV vaccination only in the modified-live FPV vaccine group.
The authors could not identify and exclude animals that may have experienced natural FPV exposure during follow-up because HI detects antibodies resulting from both natural exposure and vaccination. This limits attribution of the responses exclusively to vaccination without establishing which animals experienced natural exposure. Unknown vaccination before recruitment could not be excluded either. Other limitations included incomplete follow-up and insufficient samples for some tests at certain time points. The quoted passage does not provide confidence intervals for the percentage differences mentioned in the titre comparison.
FeLV-containing vaccines were part of the vaccination groups, and the authors refer to a separate study of FeLV antibody responses in the same cohort. The present article reports that all kittens remained FeLV-negative by antigen testing and proviral DNA qPCR throughout the study. This characterizes the study population but does not demonstrate that vaccination prevented infection: FeLV exposure and a corresponding efficacy comparison were not demonstrated. The FPV findings do not establish protection against FeLV, effects in FeLV-infected cats or differences in feline injection-site sarcoma (FISS) incidence.
What we know
The level C evidence presented shows higher FPV antibody titres with the modified-live vaccine than with the pooled inactivated-vaccine groups at T1 and T12, but not at T2. At T1, 80 of 88 evaluable kittens met the serological threshold for presumptive protection, with no statistically significant differences by formulation or inactivated-vaccine injection site. No significant titre differences were detected between the three inactivated-vaccine injection sites either; this does not demonstrate equivalence. All kittens remained FeLV-negative by antigen testing and proviral DNA qPCR.
What we don't know
It is not known which animals may have experienced natural FPV exposure or whether any had received an unrecorded vaccination before recruitment. Serology cannot attribute the detected antibodies exclusively to vaccination. Differences in clinical protection against FPV were not demonstrated through an experimental challenge, nor was equivalence between injection sites established. This study does not establish efficacy against FeLV, effects in FeLV-infected cats or differences in FISS incidence.
Evidence
EvidenceLevel C — Preliminary evidenceConfidence 100/100
The modified-live FPV vaccine group had higher reported FPV antibody titres than the pooled inactivated-vaccine groups at T1 and T12, but not at T2.
Excerpt verified in the source — verbatim, in the source's own language“Vaccination of kittens with the MLV vaccine into the scruff (group 1) produced higher FPV antibody titres than vaccination of kittens with the inactivated vaccine into the three different sites combined (groups 2–4) at T1 (23% higher antibody titres; P = 0.006) and T12 (20% higher antibody titres; P = 0.001), but not T2 ( P = 0.90) ( Figure 3 ).”
The reported differences concern titre magnitude, not demonstrated differences in clinical protection. Confidence intervals for these percentage differences are not supplied in the quoted passage. The comparison also involved different formulations, injection-site distributions and concurrent vaccine arrangements.
EvidenceLevel C — Preliminary evidenceConfidence 100/100
At T1, 80 of 88 evaluable kittens met the study's HI-based threshold for presumptive FPV protection, with no statistically significant difference by vaccine formulation or inactivated-vaccine injection site.
Excerpt verified in the source — verbatim, in the source's own language“At T1 (1 month after a single FPV vaccination), based on the assigned HI cut-off, 91% of kittens had protective titres of antibodies against FPV (n = 80/88; four samples missing). This comprised 55/62 (89%) cats vaccinated with the MLV vaccine and 25/26 (96%) administered the inactivated vaccine (no difference in protection between vaccine type [MLV vs inactivated] or site of vaccination [inactivated vaccine only]; P = 0.43 and P = 0.62, respectively; two-tailed Fisher’s exact tests).”
Protection was inferred from the predefined antibody threshold rather than demonstrated by experimental challenge. These figures describe the evaluable study population and are not a clinical recommendation.
EvidenceLevel C — Preliminary evidenceConfidence 100/100
The authors could not identify and exclude animals exposed to natural FPV challenge during follow-up because HI detects antibodies from both natural exposure and vaccination.
Excerpt verified in the source — verbatim, in the source's own language“HI detects antibodies produced by both natural exposure and vaccination; therefore, it was impossible to identify and exclude individuals that had been exposed to field challenge during the study period.”
This is an explicitly acknowledged limitation on attributing antibody responses exclusively to vaccination. It does not establish which animals experienced natural exposure.
EvidenceLevel C — Preliminary evidenceConfidence 100/100
All kittens were reported to remain FeLV-negative by antigen testing and proviral DNA qPCR throughout the study.
Excerpt verified in the source — verbatim, in the source's own language“All kittens were FeLV-negative (via antigen and proviral DNA qPCR testing) for the duration of the study.”
This characterizes the study population. It does not establish that vaccination prevented FeLV infection because FeLV exposure and a corresponding efficacy comparison were not demonstrated.
EvidenceLevel C — Preliminary evidenceConfidence 100/100
The study found no statistically significant difference in anti-FPV antibody titres between kittens receiving the inactivated vaccine at the three anatomical sites.
Excerpt verified in the source — verbatim, in the source's own language“This included there being no difference between kittens vaccinated with the inactivated vaccine in the three different anatomical locations ( P = 0.17).”
This is a failure to detect a difference in this study, not proof of equivalence between sites. The site groups were small, the endpoint was FPV serology, and the finding does not establish comparable FeLV protection.
Provenance
Source: Antibody responses to inactivated (adjuvanted) and attenuated (non-adjuvanted) feline panleukopaenia virus vaccination, including in different injection sites.
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