After Vaccination

What to expect after vaccination

Most pets, like people, experience a few mild side effects after they have been vaccinated. These side effects are normal and are part of the process of how the immune system develops the immunity, just like after you or a child gets a vaccination. For the vast majority of pets, the risk of not vaccinating far outweighs the minimal risks of vaccinating. Many vaccine-preventable diseases cause severe illness, lengthy and expensive in-clinic treatment, suffering and even death. It is very important to distinguish between normal and rare side effects.

Normal symptoms are mild and short-lived, and can include:

  • A mild fever
  • Decrease in social behavior
  • Diminished appetite or activity
  • Sneezing
  • Discomfort or mild swelling at the injection site

These symptoms typically start within a few hours after vaccination and end in a couple of days. They do not usually require any treatment or a trip to your veterinarian.

Rare symptoms can include:

  • Swelling to the face and legs
  • Repeated vomiting or diarrhea
  • Whole-body itching
  • Difficulty breathing
  • Collapse
  • Persistent injection-site swelling

These symptoms can be observed within minutes or hours after vaccination and require immediate veterinary attention. Serious reactions, though rare, can result in death. Do not hesitate to call your veterinarian if you observe any of these symptoms in your pet after vaccination. Download our helpful PDF, What to Expect After Your Pet’s Vaccination for more helpful tips.


Richard Goldstein, DVM discusses common and rare vaccination reactions.


Carrie R. White, DVM reports on rare vaccination reactions seen in her clinic.


The fear of sarcomas has stopped many pet owners from vaccinating their cats. Sarcomas are a rare form of cancer and are often fatal. But know the facts. If you remember one thing, remember this: Injection-site sarcomas are extremely rare. There is about a 1-in-10,000 to 1-in-30,000* chance that a cat will develop an injection-site sarcoma. This risk is far outweighed by the benefits a cat will have from receiving the vaccination. Vaccinated cats have a very slim chance of developing a sarcoma, but unvaccinated cats have a significant chance of developing a severe, potentially fatal disease such as rabies or panleukopenia. Injection-site sarcomas are not a good reason to stop vaccinating, but you should understand that it is a rare but possible side effect. Always be aware of lumps that remain more than a month at the site of injection, and notify your veterinarian if you have any concerns.

Source: Kass, PH, and Gobar, GM, JAVMA (220) 2002, 1477-1482.