Vaccination site bacterial infections are usually due to infection with staphylococci or streptococci. Occasionally, enteric or anaerobic organisms are the cause of bacterial superinfection of vaccinations. Occlusive dressings may also lead to more frequent occurrence of infection, the possibility of anaerobic infection, and more serious disease.

The actual frequency of bacterial infections is unknown. Infants and young children experience this adverse event more commonly, in part because of unfettered scratching of the vaccination site and, in some instances, unsanitary behavior. In some of the more serious viral complications, bacterial superinfection may also occur as a consequence of both the necrotic tissue and the immune deficiency of the vaccinee.

In the immunodeficient, particularly those with antibody or polymorphonuclear WBC deficiency as a component, bacterial infection is common and may be due to unusual organisms.