The first two parts of this series covered our approaches to vaccination of “healthy” animals. We focus on healthy pets because they comprise the majority of the pet population, and because vaccines are typically labelled for use in healthy pets, e.g. “This product is recommended for the vaccination of healthy dogs…”.

Vaccine labels don’t say not to use them in “unhealthy” pets, but they also don’t give any guidance for vaccine use in such animals.  That’s because manufacturers aren’t going to test vaccine effectiveness and safety in animals with a wide range of different health problems, which would be vastly complex and very expensive (and might require use and euthanasia of lots of animals if an experimental study was done).

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

However, there are lots of unhealthy animals at any given time and some of these need to be vaccinated. So, what do we do if an animal isn’t the picture of health? (Note that the 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, we can look to human medicine, where there’s much more information. Let’s think about COVID-19 for a minute. Did we say “let’s not vaccinate people who are sick (for other reasons)”? No. We said “let’s prioritize vaccinating people who are sick.” (e.g. those who have other illnesses and may be more vulnerable to complications from COVID-19). This is 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 who are otherwise compromised already are at greater chance of severe disease.

Back to pets… what are the concerns with vaccinating sick animals?

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. The risk of an adverse response to a vaccine is probably not much greater in most sick animals compared to healthy animals, however, the impact of an adverse event in an animal that’s already ill could be worse than the impact in a healthy animal.

We have no data about this, though. While we don’t want to cause more harm to a sick animal, 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 of scenarios:

• Dog with bacterial pneumonia. This dog could be pretty sick and maybe wouldn’t respond to a vaccine as well as a healthy dog, and we’d rather not add the potential for a vaccine complications on top of the existing disease. Also pneumonia should be a temporary condition. 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 after recovery? That’s a guess, but if the 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. 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 most “core” vaccines, but we still wouldn’t want to skip rabies. So, we might decrease the number of vaccines but still 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 (a common format for most core vaccines) would best be avoided, since it’s possible that the attenuated viruses in these vaccines could cause disease in a highly immunocompromised 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 veterinarian and pet owner, considering all the health issues, management strategies and risk aversion, as well as feasibility of other 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 often flying blind with limited data, but we can typically still come up with a good and reasonable 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.