Before the days of effective vaccines, cats routinely fell victim to a grim menu of diseases. Millions of cats died from panleukopenia ("feline distemper"), feline leukemia, complications from upper respiratory infections (herpesvirus, calicivirus) and other infections. Current vaccination programs protect our cats (and us) from the threat of rabies. Traditional vaccines are administered by "shots," but nasal drops have also been developed to protect against a variety of infections.
Some veterinarians believe that annual revaccination is an important and critical part of preventative health care. Others, however, claim that there is little scientific information to suggest that annual revaccination of older, indoor-only cats is necessary for some diseases. Immunity to many viruses probably persists for the life of the animal. The major concern about repeated vaccinations in cats is the issue of feline vaccine-associated sarcoma, a cancer that develops near the site of vaccination in a small percentage of cats.
Certainly, routine vaccinations are essential for prevention of infectious diseases in kittens. Of course, some vaccines, such as rabies, are required by law and must be administered regularly.
Kittens receive immunity against infectious disease in the mother's milk; however, this protection begins to disappear in the first few months of life. To protect them during this critical time, a series of vaccines is given every 3-4 weeks until the chance of contracting an infectious disease is decreased. The typical vaccine is a "combination" that protects against feline distemper virus and respiratory illnesses (feline calicivirus and feline herpesvirus.) Rabies vaccines are given between 16 and 26 weeks of age in most states.
Many cats are immunized against feline leukemia virus. The usual approach is to test the kitten for leukemia at the time of initial vaccination to ensure the cat is not harboring the virus; current recommendations also include retesting after six months of age. The use of other vaccinations is on a case-by-case basis. Booster immunizations are given during the first one or two years of "adult" life.
After that, the issue gets cloudier. Don't be surprised if you encounter different views about booster shots. No one yet knows enough about long-term protection vs. risk of vaccine-associated sarcoma (tumors) to give a definitive answer.