Why puppy vaccines continue until 16 weeks

Puppy vaccines continue until 16 weeks because antibodies from the mother can block them. Those antibodies protect the puppy in its first weeks, but they fade at a different age in each puppy, and nobody can predict when without blood tests. The 2024 guideline of the World Small Animal Veterinary Association (WSAVA) therefore advises several doses, 2 to 4 weeks apart, with the last at 16 weeks of age or older, when the mother’s antibodies have waned substantially in a large majority of puppies.

If a puppy is vomiting, has bloody diarrhoea, or is dull and off its food, see a vet the same day. The Merck Veterinary Manual lists these as early signs of parvovirus, a potentially fatal disease that most often affects puppies and unvaccinated adult dogs.

Why do puppies need vaccines until 16 weeks?

WSAVA says puppies and kittens acquire maternally derived antibodies, known as MDA, mostly by drinking colostrum, the mother’s first milk, in the first hours after birth. MDA gives passive immunity, which is important for the first weeks of life. The same antibodies can also stop the young animal from making its own immune response to most vaccines. In most puppies, MDA declines to a level that allows an active response to vaccination by about 8 to 12 weeks of age. Puppies with low MDA may respond earlier, and others carry so much that they cannot respond until 12 weeks or later.

WSAVA says MDA can be expected to have waned substantially by 16 weeks in a large majority of puppies, so almost all puppies should be able to respond to a vaccine then, if not before. That is why it calls the dose at 16 weeks or older the most important of the early doses, and advises that the last dose in the series is given at 16 weeks of age or older whatever the number of earlier doses. If a puppy can have only a single vaccination, for example because of cost, WSAVA says it should be a core vaccine at 16 weeks or older. WSAVA adds that 18 to 20 weeks may be more appropriate where the risk of exposure is particularly high, and its table says to continue to 20 weeks in especially high-risk situations.

How do the mother’s antibodies block the vaccine?

WSAVA explains that antibody from the mother in the young animal’s blood inhibits its own production of antibody and prevents the vaccine from stimulating an active immune response. This applies to both modified live and inactivated vaccines. For an inactivated vaccine, if the first dose is blocked by MDA, the immune system is not primed, and a second dose will then fail to immunise. For a modified live vaccine, a single dose given after MDA has waned enough is usually sufficient.

WSAVA illustrates the problem with a graph for one puppy, using antibody to parvovirus as the example. The puppy’s first vaccine was given at 6 weeks and did not work, because the mother’s antibodies neutralised it, and the same was true of the next two. By 8 weeks, this puppy’s antibody level had fallen below the level needed to protect it from a moderate challenge with parvovirus, but it was still high enough to prevent a vaccine from working. By about 13.5 weeks, the level had fallen enough to allow immunisation. The puppy was vaccinated again at 16 weeks and promptly made its own immune response. WSAVA says the antibody declines exponentially with a half-life of approximately 9 to 10 days.

The stretch in between is what WSAVA calls the “window of susceptibility”: the period when MDA is too low to give complete protection but still high enough to interfere with the puppy’s own response. During this window, a puppy cannot be immunised by conventional vaccines but is susceptible to disease if it comes into contact with “street” or virulent pathogen. WSAVA says it is not possible, without blood tests, to predict when the window will open or close, because the amount of MDA passed on varies between litters and within litters. It does not recommend routinely measuring MDA in very young puppies.

Why is the vaccine repeated every 2 to 4 weeks, and why not stop at 10 or 12 weeks?

Because no one can predict when MDA will fall enough, WSAVA says the first vaccination series involves several doses in sequence. It stresses that the repeated doses are not booster doses: each is another attempt to trigger the puppy’s own immune response as soon as possible after MDA has dropped. The more often the doses are given, the narrower the window of susceptibility becomes, but WSAVA advises against vaccinating more often than every 2 weeks. It also says vaccines differ: some modified live and recombinant vaccines are more immunogenic and can break through MDA earlier than others, and one recombinant product is given as a single dose from 4 weeks of age before the routine series, aimed at young puppies likely to have interfering MDA.

Some licensed vaccines recommend finishing the series at 10 or 12 weeks. WSAVA does not advise this. It says small experimental studies supported the early finish, but other experimental and field studies gave contrary results, and some of the supportive evidence was compromised by the “pen effect”. Group-housed experimental puppies can share the vaccine virus they shed, which artificially increases their chances of becoming immunised and overestimates the benefit of the vaccine. WSAVA notes that the idea behind early finishes was to allow early socialisation, and says socialisation can be done while following its guideline; the article on when a puppy can go out for walks covers what the guideline says on that.

What is the dose at 26 weeks, and does the same apply to kittens?

Even when the last dose is given at 16 weeks or slightly later, WSAVA says a small percentage of puppies may not respond because MDA persists. It therefore recommends either a blood test at least 4 weeks after the last puppy dose (at or after 20 weeks of age) or an extra vaccination at or shortly after 26 weeks. This replaces an earlier recommendation for a first booster at 12 to 16 months. WSAVA says that giving this dose at 26 weeks or later, rather than waiting until 52 weeks or later, does not increase the number of core doses an animal receives but substantially shortens the period of susceptibility in the few that have not yet responded. Puppies that test as protected at 20 weeks or later do not need it.

If a blood test finds no antibodies, WSAVA says the animal, which is probably only a small percentage, should be revaccinated and tested again several weeks later; a second negative result suggests a non-responder. It also says in-practice test kits vary in accuracy compared with reference laboratory tests. In German comparisons some kits did very well for parvovirus antibody, while kits for distemper antibody, and one for adenovirus antibody, performed much less well.

For kittens, WSAVA recommends an initial core vaccine at 6 to 8 weeks of age, repeated every 2 to 4 weeks until 16 weeks or older, with a further dose at 26 weeks or older, because MDA interference is important and long-lasting in some kittens.

Frequently asked questions

Why did my vaccinated puppy still get parvovirus?

WSAVA gives two explanations. The most common is that the puppy was already incubating the disease before it was vaccinated. The other is that the puppy was infected during its window of susceptibility, when it no longer had enough MDA to protect it but still had enough to interfere with the new vaccine.

Can a puppy be vaccinated before 6 weeks?

WSAVA’s table says to start core vaccines no earlier than 6 weeks. Its FAQ says puppies under 4 weeks are in general not vaccinated, because most have MDA that can block the vaccine, product leaflets do not support the practice, and injected live vaccines may cause harm at that age.

What if a puppy did not get colostrum?

WSAVA says about 95% or more of the maternal antibody in the blood of a newborn puppy that suckled normally comes from colostrum, absorbed through the gut for up to 24 hours after birth and especially in the first 4 hours. A puppy that gets none will have little or, more likely, no protection from its mother. WSAVA also says outbreaks might occur among puppies that received too little MDA because the mother was not properly vaccinated.

Can a blood test replace the 26-week dose?

WSAVA says puppies in which testing at 20 weeks or later shows protection against parvovirus, distemper and adenovirus do not need the dose at 26 weeks or later. Test results can only be read alongside the kit’s reliability, so this is a decision for your vet.

Sources

• WSAVA Vaccination Guidelines Group (2024). 2024 WSAVA guidelines for the vaccination of dogs and cats. J Small Anim Pract 65(5):277-316. https://wsava.org/wp-content/uploads/2024/04/WSAVA-Vaccination-guidelines-2024.pdf
• Merck Veterinary Manual, dog owners: disorders of the stomach and intestines in dogs (canine parvovirus). https://www.merckvetmanual.com/dog-owners/digestive-disorders-of-dogs/disorders-of-the-stomach-and-intestines-in-dogs