Vaccine Reactions: How Common, and Which Dogs Are at Higher Risk
Most vaccine reactions in dogs are mild. What two studies of millions of dogs found, which dogs are at higher risk, and the signs that can't wait.
Written by thecanineledger
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If your dog’s face swells, it breaks out in hives, vomits repeatedly, has trouble breathing, or collapses after a vaccine: go back to the vet now. These can be signs of a severe allergic reaction. Severe reactions usually start within an hour, but some come later.
In a study of 4.6 million dogs at more than 1,100 US hospitals, adverse events were recorded after 0.19% of vaccination visits, about 19 in every 10,000 (Moore et al., Journal of the American Veterinary Medical Association 2023, abstract). The rate was highest in the smallest dogs, and in French Bulldogs and Dachshunds.
While serious reactions are rare, they are not evenly spread. Some dogs are at higher risk, and owners of those dogs can plan for it with their vet. This page sets out what counts as a reaction, how common they are, and which signs need a vet straight away.
1. What counts as a reaction
The 2024 vaccination guidelines from the World Small Animal Veterinary Association (WSAVA) define an adverse event as “any detrimental, unintended consequence that follows vaccine administration (including lack of protection).” (Squires et al., Journal of Small Animal Practice 2024)
Common and mild. “Local reactions such as pain and swelling at the injection site and minor systemic reactions such as lethargy, anorexia, fever and episodes of vomiting are commonly observed”. Anorexia here means going off food.
Less common. “Urticaria and anaphylaxis are less common”. Urticaria is hives. Anaphylaxis is a severe, whole-body allergic reaction.
How quickly. In a Japanese study described in the guidelines, “A total of 83.3% of AEFVs were noticed within 12 hours of vaccination.” AEFV stands for adverse event following vaccination. In dogs with anaphylaxis, it began “within 60 minutes of vaccination in all of them and within 5 minutes in nearly half of them”.
Our note: The guidelines say anaphylaxis began within an hour in every dog in the Japanese study. If your dog is at higher risk, ask whether you can wait at the clinic for a while after the vaccine.
2. How common reactions are
Two large US studies. Both searched electronic records for reactions diagnosed within 3 days of vaccination.
- 2005: 1.2 million dogs at 360 hospitals in 2002 to 2003. The rate was “38.2/10,000 dogs vaccinated” (Moore et al., JAVMA 2005, abstract).
- 2023: 4.6 million dogs at 1,119 hospitals in 2016 to 2020. The rate was 19.4 per 10,000 visits.
The two figures aren’t directly comparable. The first counts dogs, the second counts visits.
Probably undercounted. The WSAVA guidelines say adverse events “are uncommon but undoubtedly underreported, so it is difficult to be sure.”
3. Which dogs are at higher risk
Small dogs. The 2005 study found that “The VAAE rate decreased significantly as body weight increased.” In 2023, the rate was “generally inversely related to body weight”, and highest in dogs weighing 5 kg or less.
More vaccines at one visit. In 2005, “The risk of a VAAE significantly increased as the number of vaccine doses administered per office visit increased.” In 2023, rates “increased from 1 to 4 vaccines administered.”
Some breeds. In 2023, “The largest AE rates were noted in French Bulldogs and Dachshunds (ORs > 4 compared to mixed-breed dogs).” The authors suggest “Breed differences may indicate genetics as the primary risk factor.”
Which vaccine. In 2023, among individual vaccines, rates “were greatest for rabies vaccine.”
Age and neutering. The 2005 study found risk was “27% to 38% greater for neutered versus sexually intact dogs” and “35% to 64% greater for dogs approximately 1 to 3 years old versus 2 to 9 months old.”
Its conclusion. “Young adult small-breed neutered dogs that received multiple vaccines per office visit were at greatest risk.”
Our note: We read only the abstracts of both Moore studies, which aren’t open access.
4. What owners of higher-risk dogs can ask
Spacing vaccines out. Because risk rose with the number of vaccines per visit, owners of small dogs may ask whether some vaccines can be given at separate visits.
Don’t ask for a smaller dose. The WSAVA guidelines answer this directly. Giving small dogs part of a dose “is not recommended”. They add: “Severe, adverse reactions are uncommon, even in small and tiny dogs”.
After a previous reaction. The guidelines say that for a dog that reacted to a core vaccine and was previously vaccinated as they recommend, “the dog is very likely to remain protected and revaccination is probably not needed.” A titre test may help decide.
Report it. The guidelines say “there is gross under-reporting of AEFVs” and encourage vets to report suspected reactions. Ask your vet to record and report any reaction.
Our note: “Spacing vaccines out” is our inference from the risk-per-visit finding. Neither abstract tests it, and it means more visits and possibly delayed protection.
→ Titre Testing Instead of Boosters: What It Can and Can’t Replace · → Core and Non-Core Vaccines: What the 2024 WSAVA Guidelines Say
5. Rarer, slower questions
Some immune diseases have been studied for a link to recent vaccination. A 2025 study of 172 dogs with meningoencephalitis of unknown origin, an inflammatory brain disease, found it was more common in the weeks after vaccination. The authors concluded that “the highest risk to develop MUO may be in the first 45 days post-vaccination” (Johnson et al., Journal of Veterinary Internal Medicine 2025, abstract).
This was a small study and it shows a timing link, not proof that vaccines cause the disease. The WSAVA guidelines warn, about relapses after a booster, that “temporal association does not prove causality”.
Our note: This finding comes from a 172-dog study. Read it with that in mind.
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “Vaccine reactions are common” | Recorded reactions are uncommon. About 19 to 38 in every 10,000 in two US studies, though probably underreported. |
| “Small dogs should get a half dose” | No. The WSAVA guidelines say this is not recommended. |
| “Big dogs react more” | The opposite. Reaction rates fell as body weight rose. |
| “A dog that reacted once must never be vaccinated again” | Not necessarily. A previously vaccinated dog is very likely still protected. Ask your vet about titre testing. |
| “Vaccines cause brain inflammation” | Not shown. One small study found a timing link. It doesn’t prove cause. |
Sources
- Moore GE, Morrison J, Saito EK, Spofford N, Yang M. Breed, smaller weight, and multiple injections are associated with increased adverse event reports within three days following canine vaccine administration. J Am Vet Med Assoc 2023. ⚠️ abstract read. 4,654,187 dogs. The corporate practice’s records were used
- Moore GE, Guptill LF, Ward MP, et al. Adverse events diagnosed within three days of vaccine administration in dogs. J Am Vet Med Assoc 2005. ⚠️ abstract read. 1,226,159 dogs
- Squires RA, Crawford C, Marcondes M, Whitley N. 2024 guidelines for the vaccination of dogs and cats (WSAVA VGG). J Small Anim Pract 2024: expert guidelines; free access; read in full. Supported by a vaccine manufacturer, which had no right of veto
- Johnson MK, Windsor RC, Schmidt JL, Raczek DJ, Moore GE. Vaccination and Seasonality as Risk Factors for Development of Meningoencephalitis of Unknown Origin in 172 Dogs. J Vet Intern Med 2025. ⚠️ abstract read. 172 dogs
Always confirm with your vet before acting on anything on this page.