I am very sorry to hear that Molly has a firm lump between her shoulder blades where she was vaccinated 2 weeks ago.
This is not normal, but it does happen.
Given her history of vaccines being given this may be a fibrous injection reaction.
Some cats will have an inflammatory response to a particular vaccine, pain relief, or antibiotic injection as it is seen as foreign material by the body. Most commonly we see them with Rabies vaccines or the leukemia vaccine in cats, but it occur with any vaccine, or in some cases related to a microchip. We see firm swellings, usually a few days to a few weeks after the vaccine. Usually by warm compressing them they resolve on their own over time.
If the swelling becomes larger, is painful, she runs a fever or the lump ruptures open or isn't resolving then she should see her veterinarian.
If you want to be sure that is what this lump is or the lump seems at all painful she should see her veterinarian and have this lump aspirated to look for signs of infection. If infection is present the abscess should be lanced and drained and she should be on antibiotics. If this lump looks like an inflammatory granuloma on aspiration then an anti-inflammatory can be prescribed to hasten its resolution. Very rarely these can become cancerous so if it isn't resolving it should be removed surgically.
I would call your veterinarian and let them know about her lump even if you choose to take a watchful approach. If there are several complaints about a particular vaccine perhaps there was a manufacturing problem that caused the reaction you are seeing, and the only way the manufacturer will know is if reports are made.
Does she have animal housemates she plays with? If so the other possibility is that this is not related to the vaccine, but rather related to an accidental nip or scratch that is forming an abscess. Those tend to be hot and painful though.
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