Vaccine Reactions - Even though vaccines are highly effective against major feline diseases, they may not be completely foolproof. Sometimes they may not provide the necessary protection though this is very uncommon. Some side effects may also occur, but these are minor in most cases. Some vaccines may cause many cats to become slightly lethargic for a day or two after receiving their shots. This is generally no cause for concern. However some vaccines may cause allergic reactions that can be serious. These symptoms include convulsions, vomiting, diarrhea, wheezing, restlessness, itching, and labored breathing, swelling of the head and face, and even death. This type of allergic reaction usually occurs within 15 minutes to several hours after injection. These types of reactions require an emergency veterinarian treatment.
Some cats may be more sensitive to vaccinations than others. One particular type of problem associated with vaccines especially that of feline leukemia and rabies vaccination are vaccine-induced tumors. These cancerous tumors, called fibrosarcomas, may develop at the injection sites of these vaccinations. These tumors are fatal most of the time. It is suspected that the cause is not in the vaccine itself but the vaccine suspension. It can take as short of a time as 2 months and as long as 10 years to develop a vaccine site tumor.