Vaccine hesitancy by pet owners has been in the news a lot recently, largely due to a recent study about the prevalence and consequences of vaccine hesitancy among dog owners in the US (Motta et al. 2023). The survey-based study reported 37% of respondents think vaccines can cause “cognitive issues, like canine/feline autism”, 22% think the risks of vaccination outweigh the benefits, and 30% think most dogs receive vaccines that are not necessary. A colleague and I wrote a commentary for the San Francisco Chronicle about how this kind of vaccine hesitancy among pet owners is a concern for both people and pet, especially if it results in reduced rabies vaccination coverage.

The survey by Motta et al. only scratched the surface of the issues. We need a lot more information about vaccine hesitancy to truly understand the problem, and to be able to properly address it, but it’s definitely a concern.

Vaccine hesitancy, in both people and animals, is a really complex area. I think we’ve tended to over-simplify the issue in the past by lumping people into really broad groups (e.g. “anti-vaxxer,” “cheap”) without properly investigating the reasons behind their actions. People who don’t want to use vaccines aren’t one homogenous group.

• Some are hardcore, true “anti-vaxxers.” This is often driven by mistrust in the system or science, and it’s hard to address this group. You can’t convince someone to trust, and trying to throw more facts at them doesn’t help.
• Most are “vaccine hesitant,” with concerns that are not unreasonable but may be misplaced, misunderstood or simply not adequately addressed. This is the group on which we should focus, because it’s the group with which we have the most potential to engage, address their concerns and hopefully alleviate those concerns.

A lot of people who are vaccine-hesitant are worried about adverse effects of vaccination. We have to be honest: adverse effects occur. There’s a basic level of risk that we accept when we use vaccines because of the broader benefits. I have no doubt that the core vaccines we currently use in pets do more good than harm. But, I also have no doubt that some harm can occur. I’ve seen it.

That’s tough messaging, because a lot of people don’t really care what happens to 99.9% of the population after vaccination. They care what happens to their individual pet. While we can never tell people that the risk is zero, we can explain what the risks are and try to put them into perspective, so they can make an informed cost-benefit decision.

To do that, we need to understand what the risks really are. What data do we have on vaccine adverse effects in animals?

A landmark study about adverse events in dogs within 3 days of vaccination was published in 2005 in the Journal of the American Veterinary Medical Assocaition (Moore et al, 2005). The authors studied medical records from over 1.2 million vaccinated dogs. Here are a few of the things they found:

• An adverse event rate of 38.2/10,000 dogs (0.38%).
• Adverse events were more common in smaller dogs and when more vaccine doses were administered. It’s important to note that this means doses of different vaccines (i.e. number of injections), not the number of antigens. A vaccine that contains 5 different antigens is still just one vaccine dose from the standpoint of adverse event risks.
• Each additional vaccine increased the risk of an adverse event by 27% in small (<10kg) dogs and by 12% in larger dogs.
• The highest adverse event rates were in dachshunds, pugs, Boston terriers, miniature pinschers and chihuahuas.
• 1.7% of the reactions were anaphylaxis (the most serious kind of reaction). That corresponds to about 0.006%.

By vaccine, the adverse event rates were:

• Injectable Bordetella: 15.4/10,000
• Rabies: 24.7/10,000
• DAPP (distemper, adenovirus, parvovirus, parainfluenza): 26.2/10,000
• Leptospirosis: 28.8/10,000
• Borrelia (Lyme disease): 43.7/10,000

The highest overall rate was when a combination of rabies and Borrelia vaccines was given (54/10,000). The very common combination of rabies and core vaccines (DAPP) resulted in a rate of 39.3/10,000.

A subset of these records were investigated further to look in more detail at the adverse effects.

• Facial swelling was the most commonly reported problem (31%), followed by wheals or welts (21%), general itchiness (15%) and vomiting (10%). 
• Collapse was noted in 1% of dogs with a reaction.

A similar study in cats (Moore et al, JAVMA 2007) looked at adverse reactions in 496,189 cats within 30 days of vaccination. The overall adverse event rate was 51.6/10,000 cats. As for dogs, the risk increased with the number of vaccine doses that were administered. Lethargy was the most commonly reported problem.

The canine study has recently been repeated. I haven’t seen the results published yet, but snippets have been reported, and some data were presented by Dr. George Moore at the 2022 ACVIM Forum.

• They evaluated the records of 4.9 million dogs that were vaccinated at Banfield Pet Hospitals in the US from 2016-2020.
• The incidence of adverse events linked to vaccination within 3 days of vaccination was 18.45 per 10,000 dogs (or 0.18% of dogs). That’s less than half the rate of the older study.
• Dachshunds, Boston terriers, miniature pinschers, French bulldogs and havanese were over-represented, continuing to show the increased risk in small breeds. The higher risk for certain breeds (and consistency between studies over a 20 year timespan) suggests that there are possibly genetic factors that drive the risk.
• Increased number of vaccine doses given at the same time increased the risk in dogs les than 20kg but not in dogs over 20 kg.
• There were no significant differences between adverse event rates for DAPP, leptospirosis, rabies or Lyme disease vaccines, with rates ranging between 19.2-21.3/10,000.

With these numbers, we can pretty confidently say that adverse event rates are low in dogs, and the most recent study suggests that they’ve actually dropped. A reason for that is unclear, but it could relate to newer, more refined, vaccines.

So, while I’d never guarantee that someone’s dog or cat won’t have a vaccine reaction, we know the rates are low and we have ways to reduce the risk even further. If we had lower vaccination rates we’d have fewer animals with vaccine reactions (that are almost always transient) but a lot more animals with severe and potentially fatal disease (which can have permanent long-term consequences, even if the animal survives.

The cost:benefit calculation is clear to me.