According to Dawn Bowdish, a professor of pathology and molecular medicine at McMaster University, this so-called sterilizing immunity was a key factor in eliminating smallpox. “The [smallpox] vaccination caused sterilizing immunity, meaning that you don’t carry any of the virus. Sterilizing immunity may have been a lofty goal for COVID-19 vaccine manufacturers, though not necessary to curb disease. The oral polio vaccine (OPV) generates localized intestinal immunity, preventing infection and protecting against disease and transmission. But ample precedent points to vaccines driving successful containment of infectious diseases even when they do not provide perfectly sterilizing immunity.