At one of the many vet offices I took Pookie's sister to, the vet tech mentioned that if a pet gets out, once they're reunited with their family, one of the first things the vet recommends it to give the rabies vaccine in case the pet was exposed, whether they're up-to-date on the vaccine or not.
1: Clearly they think there's no harm in over-vaccinating (and this was a holistic vet's office, with literature about over-vaccination) and
2: What is the point of giving regular "boosters" if you're going to give one after a lost pet comes home? That tells me you (the vet) don't think the boosters are effective, or you wouldn't give one to a returned pet that's current on their shots.
UGH.
While conducting trials, researches also found that some manufacturers' formulations for their one-year and three-year rabies vaccines were identical.
I'm pretty sure the Merial Purevax, which is only approved for 1 year, is actually effective for 3 years. It wasn't approved for 3-year use because, if I remember correctly, not enough cats died during the clinical trial. (I think I read this on Dr. Hovfe's site).
I think (but not sure) some of the vaccines were approved for 1 year use, and they found over time that they were effective for at least 3 years, so the vaccine companies got FDA approval for 3 years.