When puppies are born, their immune system is not
fully developed. This would normally leave the puppy open to many
forms of infection. But, thanks to maternal antibodies, these
puppies get immunity from their mother while in the uterus and also from
the first couple days of her milk (colostrum). This is crucial for their
How long those antibodies last in any given puppy
varies from individual to individual, even within the same litter.
Maternal antibodies against different diseases even wear off at
different times within the same puppy.
While maternal antibodies are present in the puppy,
any vaccines that are given to that puppy will be inactivated by Mom's
antibodies without ever stimulating the puppy's own immune system.
Because of this fact, we, as veterinarians, give a series of vaccines in
order to stimulates that puppy's own antibodies. The problem is
that we don't know when each puppy will become susceptible to the
vaccine. Unfortunately, they must be susceptible to the virus in
order for the vaccine to be effective (because that is when Mom's
immunity wears off). The goal then becomes to get the vaccine in
the puppy before he/she encounters the virus.
We do know that at 16 weeks of age, maternal
antibodies in 95% of all puppies are gone. That means that if you
vaccinate your puppy at 16 weeks of age, there is a 95% chance that he
is covered for those viruses you vaccinated against. However, when
you vaccinate against a specific disease for the first time, even in an
adult animal, it is best to give at least two vaccinations. This
is because the second vaccination will produce a much greater response
if it is following a vaccine given 2-4 weeks earlier. (The first
one sort of "primes" the system.)
So you ask, "Why don't we pull blood and check the
puppy's antibody titers, to see when they need to be vaccinated?"
Answer: Economics! It actually cost more to pull blood, send
it off to the lab, wait on results, continue to re-check levels every
couple of weeks, and then vaccinate when the titers drop. Other
than that -- there is nothing wrong with that approach.