According to an article in Science Magazine today, the Oral Polio Vaccine (OPV) may provide significant protection against COVID-19.

The article points to a long understood phenomenon of attenuated virus vaccines in their ability to proactively protect against other pathogens.

...an increasing body of evidence suggests that live attenuated vaccines can also induce broader protection against unrelated pathogens likely by inducing interferon and other innate immunity mechanisms that are yet to be identified.

Studies in the 60s and 70s showed the OPV demonstrated this effect particularly well. Over a study of more than 60,000 individuals, OPV immunized individuals showed 3.8-fold reduction in influenza morbidity, even reducing symptoms and increasing healing in individuals with genetal herpes simplex virus.

OPV not only demonstrated positive effects against viral infections but also had oncolytic properties, both by directly destroying tumor cells and by activating cellular immunity toward tumors
They were also used in more than 90,000 individuals for nonspecific protection and found to reduce incidence of seasonal influenza and acute respiratory diseases (2, 3). Subsequently, in Bulgaria, mass immunization with OPV helped to control a 1975 outbreak of unrelated acute poliomyelitis-like disease caused by Enterovirus

The article goes on describing OPV as a kind of health tonic, where infant mortality in Guinea-Bissau was reduced by 32%.

Early evidence shows the novel coronavirus may be no exception to what is often referred to as "trained immunity."

Both poliovirus and coronavirus are positive-strand RNA viruses; therefore, it is likely that they may induce and be affected by common innate immunity mechanisms. There are multiple important advantages to using OPV: a strong safety record, the existence of more than one serotype that could be used sequentially to prolong protection, low cost, ease of administration, and availability.

The article proposes OPV be administered to a population at once, lending to the vaccine's high efficacy in herd immunity.