Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Most cats have no reaction to vaccines, or only mild ones: sleepiness, a slightly lower appetite, a mild fever, or soreness at the injection site for a day or two. Serious allergic reactions are rare and usually happen within minutes to a few hours: vomiting, facial swelling, hives, trouble breathing, or collapse. Those need a vet immediately. A small lump at the injection site is common, but a lump that lasts more than 3 months, is larger than 2 cm, or keeps growing a month after the vaccine should be checked.
Common, mild reactions (usually 1–2 days)
- Sleepiness or quieter behavior
- Mild fever
- Reduced appetite
- Soreness or a small, firm swelling where the shot was given
- Sneezing or mild eye or nose discharge after intranasal (nose drop) vaccines
Serious reactions: see a vet right away
- Vomiting or diarrhea, especially soon after the vaccine
- Swelling of the face, lips, or around the eyes
- Hives or intense itching
- Difficulty breathing
- Weakness, pale gums, or collapse
- Lethargy or no appetite lasting more than 24–48 hours
Injection-site lumps and sarcoma
A small lump at the injection site is a common, harmless inflammatory reaction that usually disappears within a few weeks. Rarely, cats develop an aggressive tumor called a feline injection-site sarcoma. Vets use the “3-2-1” rule: have a lump biopsied if it’s still there 3 months after the injection, is larger than 2 cm, or is still growing 1 month after the injection. To make any tumor easier to remove, vets now usually give vaccines low on the legs or tail.
Lowering the risk
- Tailor vaccines to your cat’s lifestyle: core vaccines for all cats, and others based on risk, following feline vaccination guidelines.
- Don’t over-vaccinate: many adult core vaccines are given every 3 years rather than yearly.
- Tell your vet about any past reaction. They may give medication beforehand, split vaccines across visits, or watch your cat longer after the shot.
- Keep a record of which vaccine was given, where, and when.
Home care after vaccines
- Let your cat rest somewhere quiet and warm.
- Offer food and water as usual.
- Avoid handling the injection site.
- Check your cat a few times over the first few hours.
What to avoid
- Human pain or fever medicines: acetaminophen is deadly to cats, and ibuprofen and aspirin are dangerous too.
- Antihistamines without your vet’s advice.
- Skipping core vaccines out of fear: serious reactions are rare, while diseases like panleukopenia and rabies are often fatal.
Frequently asked questions
How long do vaccine side effects last in cats?
Mild signs usually pass within 24–48 hours. Call your vet if they last longer.
Is it normal for my cat to sleep a lot after shots?
Yes, for a day or so. Being difficult to wake or weak isn’t normal.
Should my indoor cat be vaccinated?
Yes. Core vaccines are recommended for indoor cats too, since some viruses can come in on shoes and clothing, and rabies vaccination is legally required in many places.
Sources
- Stone AE, et al. 2020 AAHA/AAFP Feline Vaccination Guidelines. J Feline Med Surg. 2020
- Hartmann K, et al. Feline injection-site sarcoma: ABCD guidelines on prevention and management. J Feline Med Surg. 2015
- WSAVA Vaccination Guidelines Group: Guidelines for the vaccination of dogs and cats
- Cornell Feline Health Center: Feline vaccination guidelines
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.