This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Some cats do have bad reactions to vaccines. However, it is better to have a cat sick for one day per year from being vaccinated than to have it die a miserable death from an FeLV-related disease. If your cat has a bad reaction to a shot, ANY shot, and the reaction lasts more than 12-24 hours, you should immediately bring your cat to the vet. Even if your cat has a mild reaction, you should at least discuss the matter to see what are the best options for next year's vaccinations. It may be best to spread your cat's annual vaccinations out over a few months, or have them all administered at once. If your cat has had a *very* bad reaction, it very well may be best to discontinue vaccinating for FeLV. Definitely discuss this with your vet.