The veterinary profession has for years recommended annual vaccination boosters for most vaccines. By tradition, manufacturers of veterinary biologics have recommended annual revaccinations and veterinarians have usually judiciously followed this recommendation. Until recently, only manufacturers of rabies vaccines were routinely required by the USDA to evaluate DOI after vaccination. The majority of other vaccines are evaluated for efficacy only for a duration of a few weeks or months, but not necessarily the full vaccine interval listed on the vaccine label. Recently, USDA rules have expanded to require newly approved novel veterinary biologics (vaccines for diseases that have not previously had an approved product) to demonstrate the minimum DOI stated in the product label. However, there is no requirement to establish maximum, or actual, DOI provided by the vaccine. Therefore, if a novel vaccine label states that annual vaccination is recommended, that manufacturer has demonstrated that animals held in isolation for one year after vaccination have equal titers or respond to challenge in the same manner as animals held for only a few weeks after vaccination. These rules have governed rabies vaccine for many years. For a manufacturer of a rabies vaccine to claim either 1 or 3 years DOI, that vaccine must be tested under strict guidelines from the USDA.

In recent years, many investigators and practitioners have expressed concerns that cats are routinely over-vaccinated. Based on human medicine and our understanding of how the immune system functions, we would expect longer DOI's. Yet scientific information to substantiate this conviction has not been available for vaccines other than rabies.

The results of a recent study (Scott and Geissinger, see Appendix F), confirm that vaccination of kittens with an inactivated, parenterally administered, adjuvanted FPV, FHV-1, and FCV vaccine can produce long lasting antibody titers against these three viruses. It is important to note that this data does not include observations of protection after challenge. Despite this shortcoming, the results of this study, as well as the results of rabies studies, information from human medicine, and knowledge of how the immune system functions, are the basis for the panel's recommendation for a revaccination interval of up to 3 years for FPV, FHV-1, and FCV.

Various antibody titers may be determined, including virus neutralizing (VN) titers and ELISA antibody titers, as a measure of immunity of cats against those infectious agents. VN titers are often referred to as "serum neutralizing" (SN) antibody titers.

Some practitioners have begun to assay VN antibody titers against feline panleukopenia virus (FPV), feline herpesvirus, (FHV-1), and feline calicivirus, (FCV) in the sera of cats in lieu of booster vaccination. This procedure may be especially appropriate to extend the revaccination interval for a cat that has had an adverse reaction to a previous vaccination. However, the correlation between serologic titer to a specific infectious agent and protection against that infectious agent has not been thoroughly established for FHV-1 and FCV. A negative titer does NOT necessarily correlate with a lack of protection to subsequent exposure.

FPV antibody titers of 1:8 or greater are sufficient to prevent infection and provide protection against clinical disease. FHV-1 and FCV antibody titers, however, correlate with substantial but not complete protection. A cat with a positive FHV-1 antibody titer (1:2 or greater), or a positive FCV antibody titer (1:4 or higher) may still become infected after exposure to virulent virus (see Appendix F). The vaccines do not protect against infection - only against clinical disease. Small amounts of virus may be shed following replication of the virus in superficial epithelial cells. Cats with positive antibody titers have memory cells. Under normal circumstances, these cells are called into immediate action following exposure to the antigen which originally triggered them. Rapid anamnestic responses occur with increased humoral and cellular immunity.

Quality control issues make interpreting titers even more complicated. Only a few diagnostic laboratories run FPV, FHV-1, and FCV antibody titers routinely and practitioners need to be aware that there is no quality control to assure that private laboratories run antibody titers accurately. Inter-laboratory accuracy can be inconsistent. The practitioner is cautioned to assess the reliability of serologic results before using titers to determine vaccination protocols which deviate from established recommendations.