Hi,
The Vaccine Side Effects and Risks That Can Occur When Immunizing Dogs and Cats. A firm, non-painful lump under the skin at the site of injection:
It is not uncommon, particularly with killed vaccines containing reactive adjuvants (immune system stimulatory additives), to get a lump at the site of vaccination. Generally this lump is under the skin and can be quite mobile. Often somewhere around the size of a pea, right up to the size of a large marble, these vaccine-site-reaction lumps are comprised of a mixture of inflammatory cells (the immune system cells that attacked the injected vaccine) and scar tissue. They may stay the same size for years or can, over time (generally 2-6 weeks), reduce in size. Some will even disappear. In some breeds (e.g. poodle, bichon), these reactions can be associated with dark skin pigmentation and loss of hair at the site of the vaccine. Many veterinarians will vaccinate show dogs in discrete areas (inside the groin, under the point of the shoulder) to avoid vaccine-associated lumps or hairloss being picked up by judges at shows.
When to worry:
If the lump grows very large within the first month of vaccination (particularly if the lump is red and painful or the animal is off colour), I would advise veterinary attention. Not all vaccine lumps are typical vaccines site reactions: abscesses can occur if the vaccine was contaminated with bacteria or if bacteria from the skin was inadvertently pushed under the skin by the vaccine needle. Abscesses need veterinary attention.
Also, if the lump stays one size for ages and then, inexplicably, starts growing in size, I would definitely advise checking this. It is possible for cancers (e.g. feline vaccine associated sarcoma) to form at sites of inflammation, including vaccination inflammation. For this reason, you should make a habit of regularly palpating and noting the size of any lump present on a pet.
Hope this is helpful ...