Recently the results of a study on the efficacy of the Fel-O-Vax vaccine in preventing feline immunodeficiency virus (FIV) infection in cats in New Zealand were published in ‘Veterinary Microbiology’.
This study was supported by Healthy Pets New Zealand and is a great example of the benefits of having a New Zealand companion animal charity supporting New Zealand-based research.
In the study, the FIV infection status of 185 privately owned cats from throughout New Zealand was determined. The health status of none of the cats was known. Of these cats, 26 (14%) were infected by FIV.
Surprisingly, the infected cats included 7 of 82 (8.5%) unvaccinated cats and 19 of 103 (18.4%) cats that had been vaccinated against FIV according to the recommendations of the manufacturer. Of the 19 FIV-positive vaccinated cats, 11 had been vaccinated as kittens and so had not been confirmed to be uninfected prior to vaccination. However, even with these cats excluded, 8.7% of cats that had been confirmed to be FIV-negative prior to vaccination subsequently became infected by FIV.
As the rates of FIV infection were roughly the same in vaccinated and unvaccinated cats, the study provided no evidence that Fel-O-Vax prevented FIV infection in cats in New Zealand. This is consistent with a study in Australia that also showed no significant effect of Fel-O-Vax vaccination in preventing FIV infection. Furthermore a review of the nine previously-performed laboratory-based studies of this vaccine revealed an efficacy that varied from 0% to 100% with an overall efficacy of just 66%.
Considering the apparent lack of protective effect of vaccination against FIV infection in New Zealand, it is perhaps surprising that there are no reports of ‘vaccine breakdown’. As suggested by the authors of the New Zealand study, this could be because vaccination does not prevent infection, but may instead prevent the infection from causing disease. However, it is probably more likely that vaccine breakdown is not observed because infection with FIV rarely, if ever, causes significant disease in cats in New Zealand.
While cats experimentally infected with FIV developed diseases, natural infection with FIV has not been associated with any disease manifestations. Indeed, several recent studies in North America have failed to detect significant differences in the life-span of cats infected by FIV and the life-span of uninfected cats.
Furthermore, similar rates of FIV infection were reported in cats with clinical disease as in apparently healthy cats in Australia. If FIV does not cause disease, the failure of the vaccine to prevent infection would have no health consequence to the cat and so the lack of efficacy would remain undetected.
Another interesting finding of the New Zealand study was that around 15% of cats are infected with FIV. Considering the high frequency of infection in New Zealand cats and the lack of clear association between FIV infection and disease in this country, it is very difficult to interpret the results of a FIV test in clinical practice.
Overall, the results of this study did not show that Fel-O-Vax prevents FIV infection of cats in New Zealand. Given the lack of significant protection against FIV infection and the uncertainty regarding whether or not FIV infection causes disease, there is currently little scientific evidence supporting the use of this vaccine in New Zealand.