How can you prevent cold sores and is there an injection to prevent them?

There has been talk of a vaccine against cold sores over the years, but at the moment, I am not aware that such a treatment is generally available in the UK. In any case, the vaccine will have to be given during childhood and it is unlikely that this will be effective once the virus has invaded your body.
 
The best way to prevent cold sores will be to avoid the specific trigger factors. In particular, stress seems to be the most significant as it depresses immune function.
 
Echinacea has been used to help build up the immune system and Propolis will help heal an outbreak.