The first two parts of this series covered our approaches to vaccination of ‘healthy’ animals. We focus healthy pets since they comprise the majority of the pet population. Also, vaccines are typically labelled for use in healthy pets:  “e.g. “This product is recommended for the vaccination of healthy dogs…”. It doesn’t say not to use it in ‘unhealthy’ pets, but it doesn’t give guidance.

Why not?

Manufacturers aren’t going to test vaccine effectiveness and safety in animals with a wide range of different health problems. That’s complex and very expensive (and might require use and euthanasia of lots of animals if an experimental study is done).

That means we’re left in a grey area and with little guidance, and often, that means we default to being conservative….’let’s only vaccinate healthy animals’.

However, there are lots of unhealthy animals and some of these need to be vaccinated. So, what do we do if an animal isn’t the picture of health? {Note….that picture is an update on Ozzie. He’s not sick (as far as we know) but is definitely a drama queen}.

“It depends” is the answer, unfortunately.

While veterinarians often shy away from vaccinating sick animals, it’s important to look to human medicine, where there’s much more information. Let’s think about COVID for a minute. Did we say “let’s not vaccinate people that are sick”? No. We said “let’s prioritize vaccinating people that are sick”. It’s done with the recognition that some people may not respond well to the vaccine, but a poor response to a vaccine is still better than no vaccine, and people that are sick are at greater chance of severe disease.

Back to pets….what are the concerns?

There are two main issues.

  • One is the potential impact of illness on vaccine response. If the animal’s immune system can’t respond adequately, the vaccine may not work or may not work as well as desired.
  • The other is the potential for adverse events. There’s probably not a much greater likelihood of most sick animals to have an adverse response to a vaccine. However, the impact of an adverse event in an animal that’s already in a tenuous state of health could beworse than one in a normal animal.

We have no data about this, though. We don’t want to cause more damage to a sick animal but we also don’t want to skip a useful vaccine because of unfounded fears. That means we have to do a cost-benefit assessment, thinking about things like:

  • Risk of an adverse vaccine response
  • Impact of an adverse vaccine response
  • Likelihood of exposure to the disease
  • Impact of the disease
  • Severity of underlying disease
  • Underlying disease course and duration

Let’s consider a couple scenarios.

  • Dog with bacterial pneumonia. This dog could be pretty sick and maybe wouldn’t respond to a vaccine as well as normal or, if nothing else, we’d rather not add the potential for vaccine complications on top of the existing disease. It also should be a temporary state. Presumably the dog is being treated and will get better. Unless the risk of exposure to vaccine-preventable diseases in the short term is particularly high, I’d postpone vaccination until the dog has recovered. How long? That’s a pure guess, but if vaccines are considered important, I’d be happy giving them 2-4 weeks after recovery.
  • Cat with chronic kidney disease. While we have management strategies for kidney disease, this cat will never be ‘normal’. It will always have some degree of disease and it’s likely that it will progress over time. So, we’re never going to get this cat to the‘normal’ state. Therefore, we need to think about whether vaccines are needed, considering exposure risk, vaccination history, age and other factors. Odds are probably high we can skip “core” vaccines. However, we wouldn’t want to skip rabies. So, we might decrease the number of vaccines but still want to give some. Unless the cat’s in an acute crisis that’s being managed and its condition will likely be better in the near future, I’d vaccinate it any time.
  • Dog that is being heavily immunosuppressed because of immune mediated disease. A modified live vaccine (common for our core vaccines) would be best avoided since it’s possible that the attenuated viruses in these vaccines could cause disease in a highly compromised animal. Beyond that, response to other vaccines might be poor. Hopefully, the dog’s immunosuppressive treatment will be tapered over the short term, so the risks might decrease over the next few weeks or months. So, I’d rather avoid vaccinating for now, and catch up when the dog’s less immunosuppressed. However, I’d consider the risk of exposure and overall vaccine history. If this was a young dog that didn’t have good initial parvo and distemper vaccination, and its lifestyle is such that exposure is a realistic concern (e.g. meets lots of dogs at off-leash dog parks, contact with imported dogs…), I’d be more inclined to use a non-modified live vaccine to try to get some added protection.

Ultimately, decisions need to be made based on a discussion between the vet and owner, considering the health issues, management strategies and risk aversion. Risk management strategies (e.g. can/will owners do things to reduce the risk of exposure in lieu of vaccination or is that not possible?). Like lots of things with infectious diseases (and vaccination in particular), we’re flying blind with limited data, but we can typically come up with a good plan thinking about the issues outlined above.

Next up….vaccinating animals at the time of surgery, and, how (or if) to use titres to make vaccination decisions.

[No, the picture isn’t a sick dog…at least I hope. It’s an Ozzie update, following up on my recent introduction to him.]