In addition to protecting against polio by inducing antibodies that kill the virus, OPV activates other protective mechanisms, including an innate immune system, thus making people resistant to infections caused by other viruses and bacteria. For example, in large scale multicenter clinical trials conducted in the 1970s during outbreaks of seasonal influenza, OPV protected more people from influenza than most flu vaccines do. Furthermore, observational studies in many countries suggested that the hospitalization rate and the overall mortality among children immunized with OPV were consistently lower compared with unimmunized children, even in the absence of poliovirus in communities.
Related studies revealed that similar nonspecific protection can be induced by immunizing people with measles vaccine, tuberculosis vaccine (BCG) and some other live attenuated vaccines. These observations suggest that the nonspecific protective effects are a result of boosting innate immunity that is our body’s front-line defense against infectious agents. This protection would last for a period of several weeks or months preventing or reducing the severity of disease in immunized individuals and slowing down the spread of COVID-19.