Vaccines are designed to induce immunity and prevent disease development in the vaccinated party. They have made some diseases rare and others almost nonexistent. Through the use of vaccines we have saved an untold number of lives, both human and animal. Unfortunately, no vaccine is totally effective or completely free of adverse reactions.

Clinical signs of vaccine reactions may vary with the type and intensity of the reaction. Four types of vaccine reactions may occur: Type I is known as an "immediate hypersensitivity reaction" or "anaphylaxis" (exaggerated allergic response). Type II is called a "cytotoxic reaction" that may exhibit as hemolysis (the destruction of red blood cells) or thrombocytopenia (lack of platelets), both resulting in bleeding or bruising of the affected animal. Type III hypersensitivity occurs by immune complex formation, a typical example of which would be an anterior uveitis (inflammation in the anterior eye chamber). Reactions of this type were once seen after vaccination with canine hepatitis as clouding of the cornea or uveitis. This reaction has largely been eliminated with the newer vaccines. Type IV hypersensitivity is a delayed reaction leading to granulomas (lump) formations at the site of the injection.

Several studies have tracked vaccine reactions in both the dog and the cat. In a study published in the AVMA journal adverse vaccine reactions occurred at a rate of 38 per 10,000 dogs vaccinated. According to this and other studies, the risk of reaction decreased significantly as the body weight of the pet increased. Vaccines in the dog are given at a one-dose-fits-all basis. This means a Great Dane would receive the same dose of vaccine given to a 5 pound toy poodle. Not surprisingly, the 5 pound poodle would be more likely to incur a reaction to vaccination. The risk of vaccine reactions were also shown to increase significantly with the number of vaccine doses given concurrently during any one office visit. Each additional vaccine given to a dog weighing less than 22 pounds increased the risk of a reaction by 27%, and for those over 22 pounds, a 12% increase in risk was incurred. A genetic predisposition to vaccine reactions has been documented for some particular breeds of dogs. Neutering a pet also appeared to increase the number of adverse vaccine events and was thought to be due to the protective effect of androgen hormones.

In cats, a vaccine adverse event rate was shown to be 51.6 reactions for each 10,000 cats vaccinated. The risk was also shown to increase with the number of vaccines given per visit. Once again, cats with low body weight or neutered cats appeared to be more susceptible to the possibility of a reaction. The risk of reaction was greater for those cats over 1 year of age. In this study almost a half million cats were tracked for up to 2 years with none of these cats developing a neoplastic lesion.

The overall rate of vaccine adverse events occurring within three days of vaccination was determined to be 0.48% in cats and 0.38% in the dog. The most common reactions were rather nonspecific reactions including anorexia (not-eating), lethargy, fever, or local soreness.

Local vaccine reactions are common but are not true hypersensitivity reactions. Local reactions include discomfort upon injection, irritation, pain, and swelling at the injection site. These types of reactions will usually resolve on their own.

The most severe type of adverse event is usually considered to be a type I hypersensitivity reactions. Clinical signs with a type I hypersensitivity may be as mild as an elevation in temperature, to severe, which includes shock and subsequent death. Additional clinical signs of a type I hypersensitivity that may occur include facial swelling or edema, pruritus, salivation, vomiting, diarrhea, difficulty breathing, or collapse and death. Vaccine reactions of this type are less commonly seen in the cat verses the dog. These types of vaccine adverse events are also commonly seen following vaccination in other species. Ferrets in particular have a high rate of adverse events following vaccination.

Mild reactions usually respond to antihistamines and corticosteroids. Severe reactions will require hospitalization and aggressive therapy including the use of epinephrine.

Delayed hypersensitivity reactions are not uncommon and are typically exhibited as a lump that forms near the injection site. Most will resolve within weeks of vaccination, especially when moist heat or a warm compress is applied. Moist heat may be applied simply by placing a washrag under the warm water faucet, wringing out the rag, and applying it to the site of the injection, making sure it is not too hot to handle as to cause discomfort.

Vaccine-associated fibrosarcomas (VAS) would be considered a type IV reaction and are a severe complication of vaccination. Fibrosarcomas may occur in approximately 1 in 10,000 vaccinated cats. VAS may also occur in the dog but at a much less frequent rate than seen in the cat. The cause of fibrosarcoma development is still a matter of debate. Various vaccine components have been implicated but the research as to the cause has been inconclusive. There is a genetic mutation involving the p53 gene or the sis oncogene in cats that may predispose these animals to the formation of fibrosarcomas.

Lumps occurring following vaccination should be subjected to the 3-2-1 rule according to the representatives at Merial. If a lump meets any of these three criteria then a biopsy is warranted: