I’ve been dealing with questions (and some threats) about this issue for many months.

How to handle overdue vaccines because of COVID delays?

It’s a complex issue and one that’s hard to navigate because of limited data. We know how most vaccines work when used according to the label instructions. We don’t know much about things when use of those vaccines differs from the label instructions.

Dr. Michelle Evason and I did a talk on this topic the other day, and it made me think a lot more about vaccination.

The main questions are….

  • How long do vaccines actually protect?
  • Does being overdue for a vaccine mean you have restart the vaccine series or just give the single booster later than normal?

Manufacturers can’t give too much guidance because they are legally bound to stick with the licensing (label) claims. They also don’t have much data about uses that differ from the typical approaches. No company is going to test various combinations of delays and boosters.

Owners want their animal protected, but also don’t really want to come back in for another vaccine.

Vets want to make sure they patients are protected, that their owners are happy (or at least not overly unhappy) and that they can provide veterinary care during a time when that might be complicated and when most clinics (around here at least) are completely swamped.

Our two main vaccination guidance providers (AAHA and WSAVA) want to help but are restricted by the limitations above…there’s not much hard evidence.

This could become a monster post so I’ll break it down. Today’s will start with the ‘core’ vaccines. These are the diseases we vaccinate pretty much all dogs/cats against, using single dose combination vaccines. For cats, it’s against panleukopenia,  feline viral rhinotracheitis  (feline herpesvirus type 1) and calicivirus. For dogs, it’s distemper, parvovirus and adenovirus-2

[Comments below refer to vaccine and vaccination strategies used in Canada, which are pretty much the same as many other countries. However, there can be some variation in vaccines and labels.]

The discussion below will refer mainly to modified live virus vaccines. That’s mainly what we have and those are highly effective vaccines that result in excellent immunity.

Puppy or kitten with a delayed initial series

With modified live vaccines, it’s not a big deal. These are highly effective vaccines and they should provide long-lasting immunity with a single dose. There’s an ‘if’ here, though. The ‘if’ is ‘if the puppy/kitten didn’t have a lot of maternal (mother-derived) antibodies at the time of vaccination”.

Puppies and kittens get antibodies from mom, and those circulate for a while, providing protection while they are very young. They also decrease the ability to respond to a vaccine.

We usually give a series of 3-4 doses of modified live virus vaccines to puppies and kittens, not because they need an initial dose and a series of boosters. Rather, it’s because we want to protect them as early as possible, but make sure they get long-lasting immunity. So, we start early, and if they are able to respond, great. They’re protected early. If not, they get another dose….and another dose. By 16 week of age, we assume they are able to fully respond to vaccination.

So, the key is we want a dose of vaccine into a kitten or puppy 16 weeks of age of later. If they were delayed on only got their first dose at 16+ weeks of age, they’re usually good to go. They should only need that one dose. (In some situations where it’s higher risk, such as a dog that’s going to encounter a lot of dogs or be in a high risk environment like a shelter, we consider another dose if they got their only shot at 16-20 weeks, but that’s case-by-case.)

The take home for this group….get them vaccinated if at all possible. If they’re 16 weeks of age or greater, they may just need a single dose.

Also, if there are delays getting vaccinated, care must be taken to reduce the risk of exposure as maternal antibodies drop. That means limiting contact with other dogs/cats and high risk environments (e.g. parks, puppy classes, kennels) until vaccinated.


Puppy or kitten with delayed 6 month/one year booster

With killed vaccines, the booster is important to get the full vaccine response. With modified live vaccines, there aren’t the same concerns. Boosters can be given any time and probably results in a similar response. After the initial vaccination series, dogs/cats are usually vaccinated again 6 month or 1 year because we are less confident about a longterm  (multiple years) response from the initial vaccine.

The risk of being a bit overdue is probably low. The vaccines are very good and immunity is probably fairly long-lasting.  We just don’t have as much confidence in it. I’d prioritize these below a younger animal needing its 16 week dose, for sure, but above an older dog/cat waiting for its 3 year booster.

The take home….get them in when possible, but don’t worry about a delay. If there was a delay, they just need the single dose they were going to get.


Adult with delayed 3 year booster

After the initial series (or single dose), and the subsequent dose, we usually re-vaccinate no more than every 3 years. Being late for the 3yr dose isn’t a big deal. The vaccines are highly effective. Some are actually labelled for 4 years now anyway. Regardless, they produce an excellent response with a single dose, regardless of vaccination history. We rarely see the core vaccine diseases in vaccinated adults.

These patient are lower priority for me. If there’s a need to triage who gets in for vaccines, these are low on the list. They’re unlikely to get these diseases at this point in their life and a delay will not impact how well they respond to the next vaccine. I want to get them done, and the need for a rabies booster (more on that in a later post) usually bumps them up the priority list. However, just in terms of the core vaccines, the response is vaccinate them when possible, but don’t stress about it. They just need the single dose they were going to get, whenever they can get it.


Modified live core vaccines are the easiest part of the decision process. It’s basically just a matter of getting them in when possible, but not changing anything (beyond the potential need for fewer puppy/kitten doses).

I didn’t talk about killed vaccines but I’ll mention them quickly. The story is completely different there. With killed vaccines, we’re more dependent on properly timed boosters. If those aren’t given, the default is to re-start the initial series. So, if a puppy or kitten series is started then stalled, the whole series may need to be re-started.

More on other vaccines soon.