Vaccination reactions in pets

isolated pets


  • A vaccination, like any other medication, can sometimes cause side effects - fortunately, they are often mild and pass within a few days.
  • Severe vaccination reactions are extremely rare.
  • Contact your vet if you think your pet is suffering any side effects from a vaccination.
  • Contact your vet immediately for an emergency appointment if you think your pet may be having an allergic reaction.
  • We believe the benefits of vaccinations far outweigh the risks. Vaccination protect our pets (and us) against potentially deadly diseases – some of which have been so well controlled by vaccination we rarely see them anymore.

Vaccine reactions explained

Vaccination is a very effective way of preventing disease. When we vaccinate, we give a tiny amount of a germ (in a safe form) so that the body can create defensive antibodies, which will fight the germ if it enters the body in the future. Millions of pets are vaccinated each year and most don’t suffer any side effects at all.

Vaccine reactions happen when your pet reacts to the germs in the vaccine or the substance that they are sitting in.

If your vet thinks that your pet is at a higher risk of having a vaccine reaction (for example if they are old and frail, unwell, underweight or suffering from an illness), they will discuss the risks with you so that you can make an informed decision about whether to vaccinate or not.

Common vaccine side effects

These slightly more common side effects often last a few days and disappear without treatment:

  • Low energy (lethargy)
  • Eating less
  • Sleeping more
  • Fever (high temperature)
  • Mild swelling around the vaccine site that disappears in 2-6 weeks.

Less common side effects

  • Twitching
  • Itchy skin
  • Vomiting
  • Diarrhoea
  • Limping.

Rare vaccine side effects

It’s extremely rare, but like any other drug, vaccines can sometimes cause a severe allergic reaction (anaphylaxis). Symptoms of a severe allergic reaction include:

  • Swelling anywhere on the body
  • Breathing difficulty
  • Cold ears, legs and feet
  • Seizures
  • A pounding heart beat
  • Collapse
  • Coma.

Contact your nearest vet for an emergency appointment if your pet is having a severe allergic reaction.

When to contact your vet

If your pet is suffering mild side effects after a vaccination, monitor them and call your vet for advice if you are concerned. If their side effects get worse or have continued for more than 24-48 hours, contact your vet for an appointment. Contact your vet immediately for an emergency appointment if you think your pet may be having an allergic reaction.

You know your pet best. Contact your vet if you’re concerned.

Why vaccinate?

Negative media coverage around vaccination side effects in recent years has raised concerns for many people. However, it’s important to remember that despite there being a very small risk when giving a vaccination, the risk of leaving them without is far greater.

Vaccinations have been so effective in recent times, that we hardly see some of the diseases we vaccinate against anymore. However, we do still see these diseases in areas where vaccination rates are low. If we all stopped vaccinating it would only be a matter of time before our pets were routinely catching the diseases we can easily protect them from.

Feline injection site sarcomas

There is a very small risk that your cat could develop an injection site sarcoma in the injection site of a vaccine, microchip or medicine. An injection site sarcoma is a type of cancer that forms underneath the skin – often in between the shoulder blades where vaccines and microchips are placed.

Published: June 2019

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Written by vets and vet nurses. This advice is for UK pets only.

Illustrations by Samantha Elmhurst