The use of vaccines is not free of risk, and an owner has reason to be upset if their healthy animal is sickened by the administration of a vaccine. Residual virulence and toxicity, allergic responses, disease in immunodeficient hosts, neurological complications, and harmful effects on the fetus are potential risks associated with the use of vaccines (Table 10.1). Veterinarians should use only licensed vaccines, and the manufacturer’s recommendations must be carefully followed. Before using a vaccine, the veterinarian should consider the likelihood that an adverse event will happen, and also the possible consequences or severity of this event. These factors must be weighed against the benefits to the animal. A common but mild complication requires a very different consideration than a rare, severe complication (Table 10.2).

The issue of the risk associated with vaccination remains in large part a philosophical one because the advantages of vaccination are well documented and extensive, whereas the risk for adverse effects is poorly documented, and in many cases, largely speculative. Nevertheless, established facts should be recognized, unsubstantiated allegations rebutted by sound data, and uncertainties acknowledged. For example, there is absolutely no evidence that vaccination itself leads to ill health. Although difficult to prove, a negative, competent statistical analysis has consistently failed to demonstrate any general adverse effect of vaccination.

Identification of an adverse event is based on the clinical judgment of the attending veterinarian and is therefore subject to bias. Standard case definitions of a vaccine-associated adverse event are not yet available. It still is often difficult to distinguish association from causality (Box 10.1).

Traditionally, adverse events resulting from vaccine administration have been reported by veterinarians to manufacturers or government agencies. The resulting numbers have been difficult to analyze satisfactorily for two major reasons. First, reporting is voluntary, so significant underreporting occurs. Adverse events are often regarded as insignificant, or it may be inconvenient to report them. Second, very little data has been available on the number of animals vaccinated. Although manufacturers know the number of doses of vaccine sold, they are unable to measure the number of animals vaccinated.

It has, however, proved possible by examining the electronic medical records of a very large small animal general practice, to determine the prevalence of vaccine-associated adverse events in over a million dogs. The use of a standardized reporting system within a very large population has permitted objective analysis of the prevalence of adverse events occurring within three days of vaccine administration. Out of 1,226,159 dogs receiving 3,439,576 vaccine doses, 4678 adverse events were recorded (38.2/10,000 dogs); 72.8% of these events occurred on the same day the vaccine was administered, 31.7% were considered to be allergic reactions, 1.7% were classified as anaphylaxis, and 65.8% were considered “vaccine reactions” and were likely caused by innate immune responses. Three dogs died. The lowest rate of such events was associated with Bordetella vaccination and the highest rate with Lyme disease vaccine. Additional analysis indicated that the risk of adverse events was significantly greater for small dogs than for large dogs (Fig. 10.1); for neutered than for sexually intact dogs; and for dogs that received multiple vaccines on one occasion. Each additional vaccine dose administered increased the risk of an adverse event occurring by 27% in dogs under 10 kg and by 12% in dogs heavier than 12 kg (Fig. 10.2). High-risk breeds included dachshunds, pugs, Boston terriers, miniature pinschers, and Chihuahuas. Overall, the increased prevalence of adverse events in young adult, small-breed, neutered dogs and their relationship to multiple dosing suggests that veterinarians should look carefully at the practice of giving the same vaccine dose to all dogs irrespective of their size.