How do vaccines work?

Most vaccines are given by injection under the skin although some may be given as a spray up your dog or cat's nose. They all work by training the white blood cells in your pet's body how to recognise and attack the viruses or bacteria contained in the vaccine. This should prevent infection with that particular bug if your pet is in contact with it again. Current vaccines fall into two main categories: 'live vaccines' which contain a strain of the bug which has been altered so that it cannot cause disease but does stimulate immunity, and 'dead vaccines' in which the bug has been killed by heat or chemicals. Each type has their pros and cons - live vaccines generally give better and longer-lasting protection but they can sometimes cause more side effects. Live vaccines are not recommended for certain groups of animals, such as pregnant females.

Do vaccines always work?

The quality of vaccines available today is very high but occasionally an individual animal may not get the full protection from the vaccine. This may be because the animal was already ill or was stressed when it was vaccinated and its immune system wasn't working properly. Your vet will examine your pet before vaccination and if any signs of illness are detected, will delay vaccination until he/she is well again.

Can vaccinations be dangerous?

Often your pet will seem 'off colour' for a day or two after its vaccination and the injection site may also become tender and swollen. If these effects do not wear off it is worth taking your pet back to see your vet. If you are concerned about any symptoms in your animal do not hesitate to contact your vet for reassurance or advice.

Injection site fibrosarcomas have been shown to be significantly more common in the USA than in the UK, perhaps due to the type of vaccines that are used. This type of tumour develops at the site of vaccination on the neck, and is thought to be due to a long-term reaction to a substance called an adjuvant that is added to a vaccine to make it more effective. Some vets vary the site of injection each year to try and reduce the chances of this occurring. The risk of this tumour occurring in the UK is very small and the benefits of vaccination far outweigh this risk.

Horses occasionally have a local injection site reaction or mild muscle stiffness. The risk to your horse of contracting and suffering serious or fatal consequences of tetanus and influenza is many, many times greater than the risk of your horse having an adverse reaction to a vaccination.

Infectious disease may not seem very common because most pets are protected by vaccination. Vaccine manufacturers issue data sheets to which the veterinary profession must adhere and these state that to be fully vaccinated against potentially fatal diseases, pets must receive regular annual vaccinations.