Even though rabies vaccination certificates can only be issued for one year after the first vaccine is given, use of a product labeled for multi-year efficacy does indeed protect cats for years after the first dose.
Any product labelled for multi-year use has to be tested by challenge for their duration of immunity. There is an older paper from Cornell University showing 3-year duration of immunity.
In a more recent USDA challenge study of a commercial vaccine, kittens were vaccinated only once, and then challenged with rabies virus four years later. All 25 of the vaccinated kittens remained healthy, whereas 14 of the 15 unvaccinated kittens developed rabies. The kittens also maintained adequate antibody titers (> 5) the entire time.
In another study that examined the immune responses of feral cats in TNR programs, giving a single dose of vaccine at the time of surgery was shown to induce protective antibody titers a few months later.
Together, these studies show that the vaccine cats get in TNR programs is effective for multiple years, even if the certificate is not.
Duration of immunity aside, public health is better served when the niche is filled by a stable population of neutered, vaccinated community cats than by unfettered reproduction leading to high turnover of susceptible cats. That’s a fact not lost on the American Public Health Association, which proposed TNR programs as a methods for protecting public health.
Dr. Julie Levy