Most vaccination courses start with two separate injections about three weeks apart. This course must be completed before your puppy is fully protected by the vaccine. The protection given by most vaccines wears off in time and at different rates for each particular vaccine. The level of environmental infection of many of the diseases against which we vaccinate (notably distemper and infectious hepatitis) is low. This means that it is unlikely that a vaccinated animal will come into contact with the wild strain sufficiently frequently to receive natural boosts to its immunity. Repeated vaccination is necessary to maintain adequate antibody titres in these cases. If your dog has missed the date of its regular booster vaccination by a year or two, your vet may think it is safer to start again with a new course of injections. Most kennels will insist on seeing proof of regular vaccinations before looking after your dog.