This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Yes. In another, recent study cats were tested in the same manner as in the vaccine manufacturer's tests. At the end of an 8-week period, 30% of cats vaccinated, and 60% of the controls demonstrated FIP-positive conditions from tissue examinations. This demonstrates a 50% "preventable percentage."
Another recent field trial ran for 16 months using 500 cats in a no-kill shelter with endemic FIP. The fact that this is a shelter makes it a different makeup than a cattery (and some multi-cat households) because the average age of a cat was approximately 2 years old, and there were no kittens under 16 weeks old. However ALL cats tested were seronegative prior to exposure in the shelter. During the time of the study, 0.8% of the vaccinated cats died and 3.25% of controls died of FIP. [This is statistically significant at p=.048, which means that there is a 95.2% probability that this result is not random] So, for seronegative cats over the age of 16 weeks, this study shows a 75% efficacy rate. Vaccination after exposure (after a cat is already seropositive) is not likely to be helpful in preventing the disease.