This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
In sum, if you know your cats are seronegative, and they are older than 16 weeks, the vaccine is recommended by both the Cornell Feline Health Center and the consensus reached at the Winn Foundation sponsored FIP/FECV seminar. If your cat is already seropositive, there is not much evidence that the vaccine will help.
The vaccine will be more of a help when:
* the manufacturers demonstrate its effectiveness against the Type I strain of FIP
* it is shown to be effective in seropositive cats, and
* it could be shown to be safe and effective for kittens under the age of 16 weeks.
However, it appears that there is work being done to develop a FECV vaccine. Preventing FECV infections in the first place, and thus preventing FECV from mutating into FIP, might turn out to be another technique in trying to protect against FIP.
My vet believes that my cat has FIP, what is the best thing to do? Usually by the time the vet is able to pinpoint FIP as the cause of your cat's condition, the cat is pretty far along. So long as your cat is in pretty good shape, not in any pain or discomfort, there is no reason to euthanize it. Even if your cat is happy and healthy, however, you MUST make sure you keep it indoors and away from other cats. If you feel that this will be too great a compromise on its quality of life, it is better to euthanize it.
Since the cause of transmission is not known, by allowing your FIP+ cat outside, you could cause numerous other cats to become ill, and even further spread the disease. But please keep your cat's welfare foremost in your mind. When its systems begin to fail, when it is in obvious discomfort, you are only making things worse by delaying the inevitable. Keep him or her as happy and as comfortable for as long as possible, that is unfortunately the only solution at this point.