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The fact that the FIP vaccine is a modified-live vaccine doesn’t have anything to do with the possibility that it could increase the likelihood that a cat could come down with the disease. The mechanism behind this phenomenon is called antibody dependent enhancement (ADE). To be honest, I’m not up on all the nitty-gritty details of ADE, but my basic understanding is that when it occurs, the antibodies that develop in response to a vaccine can bind to the virus and actually make it more rather than less likely that the virus will be able to invade host cells.
Now the evidence is hardly rock solid that ADE occurs with the FIP vaccine. Laboratory experiments pointed to its existence, but studies that took place under more natural settings failed to corroborate the effect. Interestingly, the FIP vaccine is only labeled for use in cats 16 weeks of age or older in an attempt to reduce the chances of ADE. For ADE to be caused by a vaccine, it has to be given before exposure to the virus. The great majority of cats will have already come in contact with the coronavirus that can mutate into an FIP-causing form by 16 weeks of age. This creates a bit of a catch-22, however. The only way this vaccine can have any positive effect is for it to be given before coronavirus exposure occurs, but the label prevents us from doing that because of the risk of ADE.
In truth, the real reason why I can’t recommend the FIP vaccine has nothing to do with ADE. It is simply based on the fact that the vaccine doesn’t work in real world situations. Preventative vaccination is based on the principal that we give the patient a version of a microbe that can’t induce disease but can stimulate immunity before exposure to the microbe occurs. As I said above, the label restricts the FIP vaccine’s use to cats 16 weeks of age or older. Even if we could give it earlier, the vast majority of cats become infected with the coronavirus that might mutate into the FIP-causing form when they are very young, often before they are weaned and start their vaccination series.
The last question I received regarding the FIP vaccine was, “Why is it still being used at all?” I suppose if I had a feline patient who was over 16 weeks old, tested negative for exposure to coronavirus, and was at an incredibly high risk for FIP exposure (living with an FIP positive Hannibal Lecter-type cat perhaps), I could see a potential role for the vaccine, but outside of that type of extremely limited circumstance, I’m afraid I don’t have a good answer. Greed and/or ignorance would be my best guesses.
Dr. Jennifer Coates writing for PetMD