Side Effects of a Vaccination in Kittens

Updated December 15, 2016

Vaccinating kittens will keep them from contracting serious diseases that can shorten their lives, so the benefits far outweigh the side effects, notes the Cornell University College of Veterinary Medicine. However, by understanding the possible side effects of vaccinations, you can determine whether the reactions your kittens experience are normal.


After receiving its shots, a kitten may seem lethargic and sleep more than usual, at least for the next couple of days. While many vaccines for kittens carry the risk of this side effect, it can be particularly pronounced following the vaccination for feline leukaemia, according to the Pet Informed website. The lethargy or depression also may be accompanied by a slight fever and lack of appetite.


Kittens may be sore at the injection site for a couple of days following a vaccination, so they may scratch or bite if touched there. If you look closely, you may notice the injection area to be red and swollen. This is the body's natural, inflammatory response to the injection, and will pass. If the injection was given in the hind legs, you may also notice some limping until the swelling goes down.

Lump at Injection Site

You may notice a pea-sized lump just under the skin at the injection site. This is a common side effect. The lump is comprised of inflammatory cells and scar tissue, and can stay there for years or gradually disappear in the weeks after the injection. Worry only if the lump grows very large during the first month after the injection and seems red and painful to the touch, as a bacterial infection may have developed there.

Allergic Reaction

Allergic reactions to vaccines can range from mild to severe in kittens. Mild allergy symptoms may include puffiness around the face and ears and can be treated with antihistamines, while severe symptoms can include an anaphylactic reaction--vomiting, diarrhoea, fluid in the lungs--which requires immediate, lifesaving veterinary attention.

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About the Author

Cynthia Gomez has been writing and editing professionally for more than a decade. She is currently an editor at a major publishing company, where she works on various trade journals. Gomez also spent many years working as a newspaper reporter. She holds a bachelor's degree in journalism from Northeastern University.