Most animals are vaccinated at least twice between 8 and 16 weeks of age. The international "gold standard" for veterinary pharmacologic and biologic surveillance is the United Kingdom Suspected Adverse Reactions Reporting Scheme (SARRS), which has been recording adverse effects in a computerized data bank since 1984. Adverse reactions vary widely and include lack of efficacy, induction of immunemediated reactions (eg, immediate hypersensitivity reactions), and neoplastic reactions (eg, sarcomas associated with feline vaccine). Between 1985 and 1999, 47% of 1137 vaccine-related suspect adverse reactions occurred in dogs less than 6 months of age compared with 17% of 1468 non-vaccine-related reactions. For cats, 45% of 1355 reactions occurred in cats less than 6 months of age compared with 19% of 1361 of non-vaccine-related reactions. The most common adverse effect was a hypersensitivity reaction characterized by facial edema and pruritus shortly after vaccination. In the United States, data from the Banfield Hospital Group have shown that young dogs are particularly susceptible to vaccine-associated adverse reactions, with a peak incidence at 2 years of age. In addition, this group found a clear breed predisposition, with greater frequency in small breeds of dogs. These findings help support use of reduced dose vaccines for smaller dogs. The Banfield Hospital Group found that vaccine-associated reactions in cats peak at about 1 year of age. The immunologic trigger for the immediate type 1 hypersensitivity reaction is probably extraneous agents in vaccines (in particular bovine serum albumin, of which canine vaccines have a much higher content than do human vaccines). In a recent study that compared tissue changes after administration of a single dose of a multicomponent adjuvanted or nonadjuvanted vaccine in 14- to 16-week-old kittens, nonadjuvanted vaccines induced significantly less local vaccine inflammation than adjuvanted vaccines. The authors of this review concluded that further reduction in adverse vaccine reactions is possible by formulation of nonadjuvanted vaccines, reduction of extraneous proteins, investigations into reduced vaccine volumes for certain breeds, and consideration of optimal vaccine protocols.

COMMENTARY: The author emphasizes that currently available vaccines for cats and dogs are essential for prevention of deadly diseases in young animals and that suspected adverse reactions (SARs) are rare. However, it may be possible to reduce the incidence of adverse reaction even further. He suggests 2 areas of research which may lead to vaccines with equivalent efficacy and reduced SARs in neonatal animals. The first is the development of reduced dose vaccines for younger animals and small breed dogs, which are known to have a higher incidence of SARs in the first days after vaccination. This makes empirical sense for killed vaccines with adjuvants, but it will be important to show that a reduced dose does not lead to reduced efficacy or duration of immunity. Most modified live vaccines replicate in the animal and do not contain adjuvants; therefore a reduced dose of MLV vaccines is less likely to reduce the incidence of SARs. The second area of research is to develop vaccines without adjuvants and with fewer extraneous proteins. This also makes empirical sense, as long as efficacy is not compromised. Administering multiple vaccines concurrently may be a more important factor leading to SARs than either using a full dosage of a single vaccine, or the presence of adjuvants and extraneous proteins in a single vaccine. Any safety advantages of reduced dosage vaccines and vaccines without adjuvants and extraneous proteins may be lost if multiple vaccines are administered concurrently. Until further research is conducted, optimal safety and efficacy may be expected by administering only vaccines that are indicated for each patient's individual situation and by spacing administration of individual vaccines by at least 3 weeks. It is important to remember that current multiple- or single-component vaccines are tested for safety and efficacy when administered alone. No one is likely to have tested the safety and efficacy of 2 or more vaccines administered concurrently. -- James A. Roth, DVM, PhD, Diplomate ACVM

Vaccine safety in the neonatal period. Day MJ. J COMP PATHOL 137:S51-S56, 2007.