https://www.frontiersin.org/articles/10.3389/fimmu.2020.611337/full https://www.frontiersin.org/articles/10.3389/fimmu.2020.6113... This explains why we need to

https://www.frontiersin.org/articles/10.3389/fimmu.2020.611337/full https://www.frontiersin.org/articles/10.3389/fimmu.2020.6113... This explains why we need to develop a vaccine that provides mucosal immunity to defeat COVID.