A large and eagerly-awaited follow up study on adverse post-vaccination events in dogs was recently published recently, and it provides a lot of solid – but unsurprising – data.

The study (Moore et al. 2023), published in the Journal of the American Veterinary Medical Association and led by Dr. George Moore from Purdue, used medical records from a large corporate practice network in the US (Banfield) to study adverse events that occurred within 3 days of vaccination of dogs. In total, they had data from 4,654,187 dogs (quite impressive) from 1119 veterinary clinics.

Here are some of the study highlights:

• A total of 31,197 adverse events were identified. That corresponds to a rate of 19.4 events per 10,000 vaccinations, or 0.19%.

• Forty-five percent (45%) of vaccine reactions were classified as mild, while 15% were considered severe.

• As expected, adverse events were more common in small dogs. The figure below shows the decrease in adverse events with increasing dog size.

We’ve known about this association for a while, and it still holds true. The highest rates of adverse events were in French bulldogs (55.9/10,000), dachshunds (49.4/10,000) and Boston terriers (44.9/10,000). The lowest risk breeds were mixed breed dogs (14.0/10,000), golden retrievers (12.6/10,000), Labrador retrievers (11.1/10,000… Ozzie and Merlin will be happy about that), German shepherds (9.2/10,000) and border collies (8.6/10,000).

Adverse events were also more common in younger dogs. Rates were 24.6/10,000 for 2-9 month-old dogs and 25.6/10,000 for 9-18 month-old dogs.

There was some variation in adverse event rates between different vaccine types, but nothing dramatic. Rabies vaccines had the highest incidence of adverse reactions, coming in at 24.8/10,000, just a smidge ahead of other core vaccines (i.e. distemper virus, parvovirus, adenovirus) for which the rate was 24.6/10,000 (see table below)

There are often unwarranted fears about (current) leptospirosis vaccines, likely based on historical issues since much older lepto vaccines seemed to cause more reactions. However, the incidence of adverse events with our current vaccines was lower than that for core and rabies vaccines (21.4/10,000).

When the researchers looked at moderate and severe reactions, rabies and other core vaccines had the highest risk. Lepto vaccines were associated with highest risk of mild reactions.

• This shouldn’t be taken as indicating core and rabies vaccines are dangerous. Rather, it highlights that lepto vaccines are low risk.

As expected, adverse events increased with more vaccines administered at a single visit. Note that this refers to more vaccines, not more antigens (a core vaccine that covers parvo, distemper, adenovirus and paraflu is one vaccine). The figure below shows that. The increase for large dogs was pretty unremarkable but it was pretty clear for medium and small dogs.

Overall, none of the results are surprising. They fit with what we’ve known and observed for a while, but it’s great to have very solid data to back it up.

Vaccines save lives. There’s no denying that.

Vaccines can cause adverse effects. There’s also no denying that.

The low risk of adverse effects and the high risk (and implication) of these infectious diseases make the cost:benefit ratio very clear to me.