Vaccination against disease is a medical procedure and, like all medical procedures, carries some inherent risk. As in any medical procedure or decision, the advantages must be balanced against the risks. Veterinarians recommend that no needless risks should be taken and that the best way to accomplish that is to reduce the number and frequency of administration of unnecessary vaccines. As is the case with any medical decision, you and your veterinarian should make vaccination decisions after considering your pet's age, lifestyle, and potential exposure to infectious diseases. Again, severe reactions are uncommon, but any needless risk is unacceptable.
Allergic reactions are less common, but if untreated can be fatal.
In a small number of patients, vaccines can stimulate the patient's immune system against his or her own tissues, resulting in diseases that affect the blood, the skin, the joints, or the nervous system. Again, such reactions are infrequent. In a tiny percentage of cats, there has been an increase in a particular form of tumor that is strongly associated with vaccine administration. The reported incidence of this side effect is one in 10,000. Researchers are currently studying this phenomenon to learn what causes the problem so that vaccines can be redesigned to avoid this unacceptable side effect.