Similar data have been published recently for cats (AJVR, May 1999; JAVMA, January 1, 2002). An earlier study of 15 vaccinated and 17 unvaccinated specific-pathogen-free kittens given 2 doses of inactivated triple vaccine showed persistent high antibody titers against feline panleukopenia virus (FPV) for at least 6 years, and measurable titers against feline herpesvirus (FHV) and feline calicivirus (FCV) for at least 3 and 4 years, respectively. A followup challenge study of 9 vaccinated and 8 unvaccinated cats from the initial groups showed high FPV titers after 7.5 years and solid protection against challenge. For FHV and FCV, titers declined gradually and the cats were partially protected against viral challenge (52% relative vaccine efficacy for FHV, 63% relative vaccine efficacy for FCV).

A large study of these same viruses in 72 laboratory reared and 276 client owned cats found similar predictive vaccine efficacy based on serologic titer testing. The laboratory reared cats were vaccinated with a triple valent vaccine and then challenged with virulent viruses 9-36 months later. Serologic titer testing was 100% predictive of protection for FPV and FCV, and 90% for FHV. In the client owned cats, 92.4% were titer positive for FCV, 70.7% for FHV, and 68.5% for FPV. The authors concluded that detection of FPV-, FHV- and FCV-specific antibodies is predictive of whether cats are susceptible to these diseases regardless of the vaccine type or vaccination interval. Thus, titer testing is reliably predictive of resistance to infection and can be used to avoid unnecessary vaccination.

When an adequate immune memory has already been established, there is little reason to introduce unnecessary antigen, adjuvant, and preservatives by administering booster vaccines. By titering annually, one can assess whether a given animal's humoral immune response has fallen below adequate levels. In that event, an appropriate vaccine booster can be administered.