breakthrough infection

infection occurring during a vaccine trial in which a patient becomes ill with the disease that the vaccine is designed to prevent (Medicine)

A breakthrough infection is an infection caused by the live virus in a vaccine or, simply, a vaccinated individual becoming sick with the same illness that the vaccine is meant to prevent. Often, the infection in the vaccinated individual is milder than if contracted naturally. A breakthrough infection can be especially dangerous in locations where herd immunity exists.
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