I’ve been dealing with questions (and some threats) about this issue for many months:
How to handle overdue vaccines because of COVID-19 delays
It’s a complex issue 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 what happens when use of those vaccines differs from the label instructions.
Dr. Michelle Evason and I did a webinar for CommuniVET on this topic the other day, and it made me think a lot more about it.
The main questions are:
- How long do vaccines actually protect an individual animal?
- Does being overdue for a vaccine mean you have restart the primary vaccination series, or can you just give a single booster later than normal and continue with the regular vaccination schedule?
Manufacturers can’t give too much guidance because they are legally bound to the licensing (label) claims. They also don’t have much data about uses that differ from the instructions on the label, because no company is going to pay for expensive vaccine trials to test various combinations of delays and boosters once the product is licensed.
Owners want their animal protected, but they also don’t really want to come back in / pay for another vaccine, and many want to avoid giving any more vaccines than are absolutely necessary.
Veterinarians want to make sure that their 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 (the American Animal Hospital Association and World Small Animal Veterinary Association) want to help, but are restricted by the limitations above, i.e. there’s not much hard evidence on which to base off-label recommendations.
This could become a monstrously long post, so I’ll break it down. Today’s post will cover scenarios for “core” vaccines (other than rabies). These are vaccines for diseases against which we vaccinate pretty much all dogs and cats, often using combination vaccines. Core vaccines for cats are for panleukopenia, feline viral rhinotracheitis (aka feline herpesvirus type 1) and calicivirus. Core vaccines for dogs are for distemper, parvovirus and canine adenovirus-2
Comments below refer to vaccines and vaccination strategies used in Canada, which are similar to many other countries, but there can be some variation in vaccines and label instructions in other jurisdictions. The discussion will also refer primarily to modified live virus (MLV) vaccines, which are the most common type of vaccine used in dogs and cats, as they are highly effective and generally result in an excellent immune response.
1. Puppy or kitten with a delayed initial series
With MLV vaccines, a delay in the initial series is not a big deal. Core MLV vaccines are highly effective and should provide long-lasting immunity even with a single dose, but only IF the puppy/kitten didn’t have a lot of maternal (mother-derived) antibodies at the time of vaccination.
Puppies and kittens get these antibodies from their dam, which provide protection while the animal is very young and its own immune system gears up to produce its own antibodies. Unfortunately, the maternal antibodies also decrease the ability of the animal to respond to a vaccine. We usually give a series of 3-4 doses of MLV vaccine to puppies and kittens; this is not because they need an initial dose and a series of boosters, but rather, it’s because we want to make sure they’re protected as soon as those maternal antibodies run out, and that they get long-lasting immunity. So, we start the vaccination series early, and if their immune system responds that’s great, they’re protected early. If not, they get another dose in a few weeks, and then another dose a few weeks later… By 16 weeks of age, we assume they are able to fully respond to vaccination because their maternal antibodies have disappeared (please note that puppies and kittens can be vaccinated for rabies at 12 weeks – you do not (and ideally should not) wait until 16 weeks for rabies vaccination, in order to get them protected as soon as possible – more on that in Part 2 tmrw).
So, the key is we want to get a dose of MLV vaccine into a kitten or puppy at 16 weeks of age or older. If they missed one or more shots in the series, as long as they get a dose at 16+ weeks of age (even if their very first one), they should be good to go because that’s the one that gives them lasting immunity. In some high-risk situations, such as a dog that’s going to encounter a lot of dogs or be in a high risk environment like a shelter, we’d still consider giving 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 when they’re young if at all possible, but once they’re 16 weeks of age or older, they may just need a single dose. If there are delays getting a young animal vaccinated, care must be taken to reduce the risk of exposure as maternal antibodies drop, as there is a risk the animal will be unprotected for some time. That means limiting contact with other dogs/cats and high risk environments (e.g. parks, puppy classes, kennels) until vaccinated.
2. Puppy or kitten with a delayed 6 month/one year booster
Unlike killed vaccines for which the timing of the booster is important to get the full vaccine response, boosters of MLV vaccines can be given almost any time and probably results in a similar response. After the initial puppy/kitten vaccination series with a killed vaccine, these animals are usually vaccinated again 6 months or 1 year later, only because we are less confident about long-term (multi-year) protection from the single initial vaccine.
The risk of being a bit overdue is probably low. The MLV vaccines we use are very good and immunity is probably fairly long-lasting. We just don’t have as much confidence in it (because we don’t have the data to back it up). I’d prioritize these boosters below a younger animal needing its 16-week vaccination, but above an older dog/cat waiting for its 3 year booster.
The take home for this group: Get them boostered when you can, but don’t worry about a delay. If there is a delay, they still likely just need the single booster they were going to get.
3. Adult dog or cat with a delayed 3 year booster
After the initial vaccination series (or single dose), and the subsequent dose 6-12 months later, we now typically give dogs and cats core booster vaccinations approximately every 3 years. Being late for the 3 year booster isn’t a big deal. These vaccines are highly effective and produce an excellent response with a single dose, regardless of vaccination history. We rarely see the core vaccine diseases in vaccinated adult dogs and cats.
If there’s a need to triage who gets in for vaccines, these animals are lowest on my priority list (but remember we’re not talking about rabies vaccination – see Part 2). They’re unlikely to get these diseases as adults, and a delay will not impact how well they respond to the next vaccine. I want to get them done, though, and the need for a rabies booster may bump them up the priority list.
The take home for this group: When it comes to just the non-rabies core vaccines, get them boostered when when you can, but don’t worry about a delay. They just need the single dose they were going to get, whenever they can get it.
MLV core vaccinations are the easiest part of the decision process. It’s basically just a matter of getting them done when possible, but not changing anything else (beyond the potential for needing fewer puppy/kitten doses).
I didn’t talk about killed vaccines (yet) but I’ll mention them quickly because the story is completely different for them. With killed vaccines, we’re much more dependent on properly timed boosters. If those aren’t given, the default is to re-start the initial series to ensure there’s a robust immune response. So, if a vaccination series is started and then delayed, the whole series may need to be re-started.
More on other vaccines soon.