How do Vaccines work? 

When horses are infected naturally, their immune system produces antibodies along with a type of white blood cell called a killer cell. Together this helps the horse fight disease. Both antibodies and killer cells are specific for a particular infectious organism and can continue to circulate within the immune system for varying periods of time (often years) after the horse has recovered from the disease. This is known as “immunological memory” and helps to protect the horse from getting the disease again. However, the limitation with this natural immunity is that this response can take days to generate the appropriate antibodies and white blood cells, by which time the disease can cause significant harm and spread to other horses. Over time, the immune response weakens further, unless the horse is re-exposed to the same infection, meaning it may be able to contract that particular disease again.

Vaccines stimulate an immune response in horses by fooling their body’s immune system into believing that a disease is attacking them and thus creating antibodies and killer cells to fight infection. This means that when natural infection does occur, the immune system is able to produce a much faster and stronger response. It is this strong response, as a result of vaccination, that prevents the disease becoming debilitating and spreading to others.

Vaccines contain antigens (disease causing organisms) that are altered slightly but are still recognisable to the horse’s immune system.

Therefore when a horse is vaccinated the immune system is primed and able to fight the disease it is immunised against. In turn this means that a vaccinated horse should not suffer the ill effects of the disease itself.

Why should I vaccinate my horse?


  • To prevent my horse from contracting the disease.
  • To prevent unnecessary suffering associated with having a particular disease.
  • To prevent additional unplanned expenses – the cost of treatment is often much more than the cost of a vaccination.
  • To prevent loss of use – a sick horse should not be ridden or exercised until it has completely recovered. For example it is recommended that if your horse has a high temperature associated with a respiratory disease, then it should have one week off for every day that it had a high temperature.
  • To prevent yard closures and sporting cancellations – horse movements on a yard may have to be restricted or cancelled if a horse is diagnosed with an infectious disease.
  • To prevent the spread of disease via shedding of the infectious organism by infected horses.

As well as considering the above points, as a horse owner you should ask yourself:

  1. Is my horse at risk of disease?
  2. Is my horse putting other horses at risk of disease?

If the answer is “yes”, then you should consult your vet about whether a vaccine exists to help protect your horse and others within the herd from infectious disease(s).

General vaccine FAQs

What should my horse be vaccinated against?

The most common diseases to vaccinate against are equine influenza and tetanus, although in some circumstances it may also be advisable to protect your horse against strangles and herpes infection. Your vet will be able to advise you on a vaccination or prevention programme and the frequency of booster vaccinations. If you are concerned about any diseases during breeding, for example herpes virus or rotavirus, you should also talk to your vet for information on a suitable prevention programme.

How often should I vaccinate my horse?

How often you vaccinate a horse very much depends on the disease, which is being vaccinated against, the vaccine manufacturers’ guidelines and competition governing bodies e.g. FEI regulations and The British Horse Racing Authority guide lines. A priming vaccination course may be required depending on the disease being vaccinated against, and booster times can vary too. Your vet will guide you as to the course of vaccinations required, but it is your responsibility to remember when your horse’s vaccination is due!

I don’t compete my horse so do I need to vaccinate?

Yes, especially if your horse has any contact with other horses. It is easy to overlook the importance of influenza as a disease as outbreaks are relatively rare. However the disease is highly contagious, debilitating for your horse and can be distressing for you to witness. A horse doesn’t need to leave home to be at risk of tetanus, as the bacteria are found in the soil, and most cases of tetanus are fatal.

How effective is vaccination?

As in humans, vaccination is never a 100% guarantee since it relies on the ability of each horse to individually mount a satisfactory immune response. As a result, you should never vaccinate a horse that is unhealthy or stressed. A combination of having a complete vaccination programme and enough of the population vaccinated (> 85% of the herd - herd immunity) is how viruses and bacteria are kept at bay. The gold standard is to have a whole yard vaccinated and have a good biosecurity plan in action. This will help reduce the risk of an infectious organism causing problems within the herd.

What should I do if my horse appears sick following vaccination?

Horses, like humans, can sometimes feel unwell following immunisation, horses also can appear off-colour or have sore muscles after being vaccinated. This is not usually a cause for concern, but if you are worried you should talk to your vet.

Can I vaccinate my pregnant mare and what can I vaccinate her against?

  • Flu and Tetanus: It is advisable to keep your pregnant mare up to date with her flu and tetanus vaccinations so that this immunity can be passed to her foal via the colostrum. Most ‘flu/tet’ vaccines are licensed to use in pregnancy. Speak to your Veterinary Surgeon about the most suitable time to give your mare her booster vaccination.
  • Herpes: To reduce abortion caused by EHV-1 infection pregnant mares should be vaccinated during the 5th, 7th and 9th month of pregnancy.
  • Strangles: It is not recommended to vaccinate against strangles during pregnancy.

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