The old puppy shots schedule – taught to me and other aspiring vets at vet school decades ago – went like this:
We need to protect puppies from as young an age as possible. So let’s give them their first vaccination at six weeks of age. However, every puppy gets a big dose of protective antibodies in the first suckle (of colostrum) from its mother. These antibodies wane over time but are still relatively high in the puppy’s body at six weeks of age and tend to neutralize vaccinations given at that time.
So, in short, the first vaccination given at six weeks of age, is very often not effective.
Because we can’t rely on it, we then recommended another two rounds of vaccine at around 10 and again at 14 weeks of age.