This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
There is a vaccine available, but it is controversial and some vets do not recommend it, although others highly encourage it. The manufacturer's tests state that it has an efficacy rate (protection rate in this case) of 69%. Cornell Feline Health Center then did a study which said the vaccine failed to show any protection, and that it accelerated the disease in 52.5% of exposed cats. However, this study used a different challenge virus strain and the route of administration was different than the manufacturer's tests. The 'real-life' significance of this has not yet been determined, neither Cornell nor the manufacture has received reports from the field of abnormally high numbers of cats which get the disease as a result of the vaccine. However, this study has caused a lot of people to swear-off the vaccine. It really is between an individual cat owner and their vet to determine the whether or not to vaccinate based on the best information available at the time.