Vaccine Reactions in Pets: What's Normal & When to Call Your Vet

Many pets have no side effects at all. When mild reactions happen, they usually start within a few hours of vaccination and improve within 24 to 48 hours. Common examples include sleepiness, mild soreness, a brief decrease in appetite, or a small amount of swelling where the injection was given.

A small, firm nodule at the vaccine site can also happen. In dogs and cats, this kind of lump often begins shrinking within about 2 weeks. If it lasts longer than 3 weeks, gets bigger, or seems painful, call your vet for guidance.

Call your vet the same day if your pet is much more tired than expected, will not eat, seems painful, or has vomiting or diarrhea after vaccination. These signs do not always mean a severe emergency, but they are more than the usual mild post-shot slump and deserve advice.

Also call if your pet has a growing injection-site lump, ongoing limping after a leg vaccine, or symptoms that are still present after 48 hours. Pets with a history of prior vaccine reactions should be discussed with your vet even if the current signs seem mild.

See your vet immediately if your pet develops facial swelling, hives, trouble breathing, repeated vomiting, repeated diarrhea, pale gums, weakness, collapse, or sudden severe lethargy after a vaccine. These signs can happen within minutes to hours and may signal an allergic reaction that needs urgent treatment.

If your clinic is still open, call while you are on the way. If it is after hours, go to the nearest emergency hospital. If your pet has had a previous serious reaction, ask your vet whether staying at the clinic for 30 to 60 minutes after future vaccines makes sense.

Cats can develop small vaccine-site lumps that go away on their own, but persistent masses deserve more attention. Many vets use the feline 3-2-1 rule as a guide: a lump should be checked if it lasts longer than 3 months, is larger than 2 centimeters, or is still growing 1 month after the injection.

This does not mean every lump is cancer. Most are not. It does mean your vet should decide whether monitoring, recheck measurements, or sampling is the safest next step.

Treatment depends on the type of reaction. Mild cases may only need monitoring, rest, and a recheck plan. Pets with hives, facial swelling, vomiting, or more significant discomfort may need an exam and medications such as injectable antihistamines, anti-nausea medication, corticosteroids, or fluids, depending on the situation.

For future visits, your vet may recommend giving fewer vaccines at one appointment, spacing them out over separate visits, or observing your pet in the hospital after vaccination. The right plan depends on your pet's vaccine history, disease risk, and overall health.

If your pet only needs a phone triage or brief recheck, the cost range is often about $0 to $95 depending on the clinic and whether the recheck is bundled. A same-day exam for mild to moderate signs commonly runs about $75 to $180, with medications adding roughly $20 to $120.

Urgent treatment for an allergic reaction can cost about $250 to $800 for an exam, injectable medications, and short monitoring. Emergency hospital care for severe reactions with IV fluids, oxygen support, or extended observation may range from about $800 to $2,500 or more. Ask your vet what options fit your pet's needs and your budget.