Just as in humans, most vaccinations cause at most very mild outward symptoms after the vaccine is given. Some horses may be a little more lethargic than usual, have some mild tenderness at the injection site, or stiffness in the neck if that is where the vaccine was administered. Occasionally, the horse will develop a mild and transient fever, and this is usually noticed if the horse goes off his feed. These usually resolved uneventfully, although sometimes anti-inflammatories are given.
Some horses develop a small lump at the injection site, and some develop a larger region of swelling and tenderness. A smaller number can develop at abscess at the site of injection. In very rare cases, more serious systemic effects can be seen—anaphylaxis, hives, or purpura hemorrhagica (a systemic inflammation of blood vessels, especially in the limbs). Contact your veterinarian immediately if any of these symptoms are observed.
These adverse reactions cannot always be predictable. It is therefore wise to avoid scheduling vaccinations in the 2 weeks prior to a competition, performance event or clinic, auction, or long distance transport. Also, for horses in FEI competition, the required influenza vaccination cannot be administered in the 7 days prior to the start of the competition, so careful planning is advised.