In general terms, a vaccine contains an organism or organisms that cause a specific disease or diseases, however, the organisms are changed in such a way that they won't cause illness yet are still recognized as "foreign". Consequently, they are able to stimulate the horse's immune system to mount a protective response.
The two common ways of "changing" an organism are to inactivate it (or ‘kill' it) or modify it during the vaccine's manufacturing process. This latter approach yields a vaccine that contains a "modified live" organism. Attenuated vaccines and some modified live vaccines might also contain another substance called an "adjuvant" which is included to increase the vaccine's ability to stimulate an immune response. It is frequently thought to be the adjuvant that is associated with local swelling, pain, redness that may develop at the site of the vaccine administration in some individuals.
Many of the vaccines used in the horse contain inactivated viruses and are administered by the intra-muscular route, meaning they are injected into a muscle mass. Common locations for giving vaccines are the muscles of the neck, hind leg, rump and pectoral (chest) region. A few equine vaccines contain modified live organisms. In Canada, these vaccines protect against certain respiratory diseases and are administered into the nasal cavity. The modified live organisms are able to produce a local immune response that is fairly quick in onset, as well as a systemic one similar to inactivated vaccines given into muscle.
In horses that have never been vaccinated, two doses given at a few weeks apart are needed to produce optimal levels of immunity. The first dose will stimulate humoral immunity. In some cases, cell-associated immunity is also stimulated but this takes time. By approximately two weeks (more or less), the antibody level will have reached its highest level before it begins its decline. After the second dose, the antibody level not only increases faster but also reaches a higher level and lasts longer before declining.
Foal vaccination, as recommended in the current vaccination guidelines produced by the American Association of Equine Practitioners (AAEP), consists of a series of three doses of vaccine with a longer interval of between the second and third doses than between the first and second. As to the age at which a foal should receive its first vaccine, this will depend on several factors including the vaccine status of the mare and whether or not she had been "boostered" during pregnancy and in some cases, the type and kind of vaccine. The specific vaccine needs of pregnant mares and foals should be discussed with your veterinarian.