I think it will vary with the manufacturer, and it will be interpreted differently by different vets. If a manufacturer states that yearly boosters are required, then the vet is bound to vaccinate to those guidelines but it isnt' going to offer immunity one day and then not the next because it is one year and one day since the vaccination. So how long can you go before starting again? I guess that's open to interpretation and many people (inc vets) consider vaccinations to offer immunity for longer than a year. But if a cat needs to be up to date with vaccinations for a cattery or a cat show, then the manufacturers instructions are going to have to be followed and there has to be a cut off point. But exactly when that cut off point shouldl be is not clear cut. Mosi is now nearly 5 months late for his vaccinations. I do think he probably still has some immunity from his previous vaccinations, but I'm going to start him again with the course of 2 (vet would say I have to anyway, but even if they didn't I would still do that) because I don't want the cattery looking at his certificate and saying that they don't consider his vaccinations up to date.