Vaccine Reactions in Dogs: Normal Side Effects vs Emergencies

Most dogs have no reaction to vaccines, or only mild ones: a sore spot, a small lump, tiredness, a low fever or a smaller appetite that starts within hours and usually goes away within 24 hours.^1,2 Serious allergic reactions are rare. In the largest recent study of more than 4.5 million dogs, adverse events of any kind were reported after about 19 of every 10,000 vaccination visits.^3,4 An allergic reaction usually starts within minutes to hours, so watch your dog closely for the rest of the day and get help right away for facial swelling, hives, repeated vomiting, trouble breathing or collapse.¹

Normal Mild Side Effects

Call your vet if mild side effects last more than a day or two, or your dog seems more unwell than expected.¹

How Common Are Vaccine Reactions?

• 2005 study: of 1.2 million dogs vaccinated at 360 hospitals, 38.2 per 10,000 dogs (about 1 in 260) had an adverse event within three days. Anaphylaxis made up only 1.7% of those events.^5,2
• 2023 study: of more than 4.5 million dogs at 1,119 clinics, adverse events followed 19.4 per 10,000 vaccination visits (0.19%).^3,4
• The two studies counted differently (per dog versus per visit), so the numbers can't be compared directly.
• Timing: in the 2005 study, nearly three-quarters of reactions were diagnosed on the day of vaccination, and almost all within three days.⁵

Which Dogs Are at Higher Risk?

• Small dogs: risk rises as body weight falls. Dogs under about 11 pounds had the highest rates in both large studies.^5,3
• Several vaccines at once: each extra vaccine given in the same visit raised the odds of a reaction by about a quarter.^5,3
• Breed: Dachshunds, French Bulldogs, Pugs, Boston Terriers, Chihuahuas and Miniature Pinschers had some of the highest rates.^5,4
• Age: young adult dogs had higher rates than young puppies in the 2005 study.⁵
• A previous reaction is an important risk factor to tell your vet about.⁶

Allergic Reactions and Anaphylaxis

Allergic reactions can begin within a few minutes to a few hours after a vaccine.¹ Anaphylaxis, the most severe form, usually starts within minutes and is life-threatening: signs include sudden vomiting or diarrhea, pale gums, heavy or difficult breathing, weakness and collapse.^7,1,8 Facial swelling and hives are more often a milder allergic reaction, but they still need a vet the same day, because some dogs get worse.^1,2 For a dog's first vaccines, or a dog who has reacted before, ask whether you can wait at the clinic for 30 to 60 minutes afterward.^1,2

Lumps at the Injection Site

A small, firm, painless lump where the vaccine went in is common and should start shrinking within a couple of weeks. Contact your vet if it lasts more than three weeks, seems painful or keeps growing.¹

You may hear about a "3-2-1 rule": biopsy a lump that is still there 3 months after vaccination, is larger than 2 centimeters, or is growing 1 month after vaccination. That rule comes from the feline vaccination guidelines, because cats can develop rare tumors at injection sites. It wasn't written for dogs, though some vets use it as a guide.⁹

Is the Leptospirosis Vaccine More Reactive?

The lepto vaccine has a reputation for causing more reactions. A 2015 study of more than 130,000 dogs found more mild, nonspecific reactions after lepto vaccines, but no difference in allergic reactions.¹⁰ The larger 2023 study found lepto vaccines no more likely to cause reactions overall than rabies or core vaccines, though they were linked to more of the mild ones.⁴ The American Animal Hospital Association says reactions to lepto vaccines seem to be rare and serious anaphylaxis is no more frequent than with other vaccines.^6,8 See the leptospirosis vaccine for dogs.

How Vets Reduce the Risk

• Spreading vaccines out over more than one visit, especially for small dogs, with at least 2 weeks between visits.⁶
• Not splitting doses: giving a smaller amount than the label says isn't approved and isn't recommended, even for tiny dogs.⁶
• Pretreatment with an antihistamine for dogs who have reacted before. It isn't recommended for dogs with no history of reactions.^6,8
• Choosing a different route (such as a nose or mouth vaccine) where one is available.⁶
• Watching the dog at the clinic after vaccination.^1,2
• Reviewing which vaccines are needed for your dog's lifestyle. See the dog vaccination schedule.

Can Titer Testing Replace Vaccines?

Titer tests measure antibodies and are most meaningful for distemper and parvovirus. The 2022 AAHA guidelines say a titer result doesn't necessarily show whether a dog is protected, and routine titer testing to decide on revaccination isn't usually advised. They can help in dogs with a past serious reaction or an immune condition. A rabies titer can't replace a rabies vaccine, which is legally required in most places.⁶

Reporting a Reaction

Get your dog treated first; reporting can come afterward.¹¹ Your vet will usually report the reaction to the vaccine maker, which must pass it on to the USDA Center for Veterinary Biologics. If you need to report it yourself, contact the manufacturer, or use the USDA's online adverse event form or call 800-752-6255.^12,11 Keep a note of the vaccine and reaction in your dog's records, so future visits can be planned.⁶

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