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What is a vaccine?

Vaccines typically use weakened, modified or inactivated versions of the causes of infectious disease to prime the immune system. Infectious diseases can be potentially fatal and difficult to treat. Vaccines are better than treating disease, which can be difficult or impossible to cure.

Veterinary vaccines are preventative medicines, which are licensed through a rigorous regulatory process in a similar way to other human and animal medicines to provide clear reassurance around their expected levels of efficacy, safety and quality. Furthermore strict batch quality procedures are enforced to ensure that only product of a sufficient consistent quality and safety are released for use.

How do vaccines work?

Headshot of a white cat

How do vaccines work?

Vaccinated pets produce cells and antibodies specifically targeted against the infections that vaccines are designed to be effective against. Some of this response is stored in the form of long-lived memory cells which are able to respond rapidly if the body is exposed to the infectious disease at a future date. The rapid response that ensues will prevent or very much reduce the likelihood of disease and is also likely to reduce the chance of infection spreading to other animals and, in the case of certain zoonotic diseases such as rabies and leptospirosis, to ourselves.

What side effects might be expected following vaccination?

Vaccines work by stimulating the immune system, which may result in generally mild and short term effects.

These include:

  • Lethargy and malaise
  • A small rise in body temperature
  • Stiffness
  • Local swelling or tenderness at the site of the injection.

Whilst no obvious effects are often noted, most expected adverse reactions are of this type described above.

Very rarely, more significant abnormal responses are reported where an animal may have developed an underlying sensitivity to a component of the vaccine. These typically relate to allergic-type responses (hypersensitivity). These may be self-limiting but can prove serious requiring medical treatment.

Should your animal experience this post vaccination please seek immediate veterinary care.

Vaccination schedules for dogs paper thumbnail

How commonly are adverse events reported?

The reporting incidence for adverse events for all dog vaccines in the UK are rare with 21.3 reports for every 100,000 doses sold with the majority of such reports relating to non-serious reports1. For a more comprehensive discussion of dog vaccination schedules and vaccine safety please see below.


Further information


A dog and cat laying inside together

Why do we vaccinate pets?

Vaccination plays a pivotal role in keeping your pets healthy and safe.

A vet and assistant examining a dog

Titre testing


Serological titre testing involves taking a blood sample to determine the levels of antibodies...

Woman holding her cat and a dog sat in front

Vaccination safety


Pet vaccines are tested thoroughly for both safety and efficacy before they are authorised...


1. VMD position paper on Authorised Vaccination Schedules for Dogs - (3/11/2020)

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