If a human arrives for an influenza vaccination from a vaccine clinic or pharmacy, one of the first questions asked is, “Are you feeling well today?” If the immune response is actively engaged fighting another virus or bacterial process, the response to a vaccination might not be optimal. Therefore, if your horse demonstrates a nasal discharge, has a mild fever, or is recovering from another medical issue, we usually delay vaccination until their immune system is better able to respond to the vaccine reliably.
Vaccination FAQ - Can I have my horse vaccinated if (s)he has a mild “cold?”