Sweating causes heat which makes the fluid in the injection move through the body faster and therfore not giving enough time for the immune system to form proper antibodies and could make the horse get a mild case of influenza or tetanus.

With regards to the comment below:

The modern injections are no different to the old ones with regard to travelling through the body so sweating & over heating are still out of the question.

Injections work by putting small amounts of infection into the body so that the body can produce antibodies against it so that if they ever get the full virus it won't floor them. But if they are allowed to sweat etc and don't build the antibodies, you are in effect giving them a virus not stopping it in the future.

Hope this helps x

Katie we will have to disagree i also have great expertise dealing with horse health, although i wouldn't call anothers comment garbage. There is plenty of evidence on the internet to back me up if you google horse vaccinations.

Not sweating after a jab is an old fashioned thing, based on the old vaccines. I don't really understand it, but it was to do with a slight change in the horse's immunity shortly after it was vaccinated. Nowadays it's not really relevant, because the vaccines have changed. A day's hunting or eventing isn't really recommended, but charging about in the field or going for a short ride won't do any harm.

I'm with pink on this one and MY vet still doesnt want the horses sweating even with modern vaccines.

Pinkpotions, that is such a load of garbage!!!

Horses musn't sweat BEFORE having a vaccine as you mustn't put the injection through wet skin. The same applies when it is raining - horses should be kept in before they are jabbed when the weather is bad. Water/sweat allows pooling of organisms, mud, dead skin etc, and when the needle is passed through the skin it drives these particles in with it. This then increases the chance of developing an abscess at the site. Swabbing the skin beforehand with spirit or scrub actually does the same, so it is actually worse to try and clean the skin.

Sweating AFTER a flu/tet jab is absolutely fine - I always tell my clients to go out and ride immediately following the jab if they want to. The only reason people used to be told not to ride after a jab was the older vaccines used to cause a stiff neck and make the horses a little off colour. Owners were therefore told to give the horse a few days off to allow it to recover.