One of the more frequent comment we hear from clients with new puppies:
She only needs one more puppy vaccine. The breeder already gave her three 4-in-1 shots.
Even assuming those three 4-in-1 shots or DA2PP (DHPP) vaccines --- as the combination vaccine for distemper, adenovirus-2 (infectious hepatitis), parainfluenza, and parvovirus is commonly known --- were given every 3 weeks, the above comment is true only if the puppy is currently at least 14-16 weeks old. If the breeder gave the vaccine at 6 weeks old then again at 8 weeks and 10 weeks (we usually recommend and follow a 3-4 weeks interval but it is not uncommon to see puppies coming in with 2 weeks intervals), that puppy would still need at least 2 more vaccines to complete the puppy series. In this example, we would have postponed the vaccine until 3 weeks after the last one (puppy would be 13 weeks old then) and give the last of series 3 to 4 weeks after that (puppy would be 16-17 weeks old). If there is a question about the puppy's immune status, we sometimes would give another one so that the puppy's last of series vaccine would be at 5 months of age. Why? Because even though we know that maternal antibodies are gone from most puppies immune system by 4 months of age, some puppies may take up to 5 months of age.
Maternal antibodies protect puppies (and kittens) from diseases that their immature immune system are not able to handle. However, the presence of maternal antibodies also prevent puppies from producing their own antibodies. Maternal antibodies neutralize or inactivate the vaccines (virus) before the puppy's immune system has a chance to "see" the virus to produce his own antibodies. If we stop the vaccine series when maternal antibodies are still present and the puppy is later exposed to the virus (most commonly parvovirus) after maternal antibodies are gone, the puppy, not having adequate appropriate antibodies of his own, will succumb to the infection. It is no surprise then that parvo infection is often seen in unvaccinated (or incompletely vaccinated) puppies around 14 to 24 weeks old.
An 8-weeks old puppy that will receive three DA2PP (DHPP) vaccines, one given every 3-4 weeks (ending at 15-16 weeks old), will likely have adequate protection. So would a 15-weeks old puppy with two DA2PP (DHPP) vaccines, one given every 3-4 weeks (ending at 18-19 weeks of age). However, not that we recommend this, a 6-weeks old puppy that will receive four DA2PP (DHPP) vaccines, one given every 2 weeks (ending at 12 weeks old) may not have adequate protection.
Bottom line, it does not matter if the puppy received two, three, or even four DA2PP vaccines already. What matters is the time interval and when that last one was given.
Timing of vaccines is only one aspect of the vaccine protocol. Add to that, the different vaccines, the pet's lifestyle, health status and risk exposure and you can see why the best vaccine protocol is one that is tailored to your pet's needs.