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Cat has lump after vaccine. Is it a reaction? When to see a vet?

Updated On September 23rd, 2025

Pet's info: Cat | Domestic Shorthair | Male | neutered | 1 year and 5 months old | 8 lbs

I have recently noticed a large lump on my cats back, where we think was the same spot as a vaccination he had two weeks ago. The lump doesn't seem to be irritating my cat and he doesn't have any other symptoms that we can tell. He is only 18 months old and an indoor cat, with another indoor cat which is his brother. Does this sound like something we should be worried about? And should it require an appointment with out vet?

2 Answers

Most Helpful Answer

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Answered By Jenny, DVM

Veterinarian

Published on November 11th, 2017

It does need to be noted with your vet to verify if it is a vaccine reaction. Cats can have injection induced sarcomas which are cancer and require surgery. The general rule is if it is there longer than 3 months or gets larger, then surgery is warranted, otherwise they go away on their own I would have them document the vaccine site and the size for proper monitoring in the future.

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Other Answers

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    Answered By Amber Da Silva, DVM

    Veterinarian

    Published on August 23rd, 2017

    Cats and dogs will occasionally get a firm lump under the skin where a vaccine was given. If it is 3 or 4 cm (not 3 or 4 mm) then I would definitely have it evaluated by a veterinarian. Although it is rare, cats can develop cancer at the site of an injection.

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