>>12164912>but is correlation correlated with causation?In tests with rodents, it's causation.
https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa644/5848814>Noncontact transmission was found in 66.7% (10/15) of exposed naive hamsters. Surgical mask partition for challenged index or naive hamsters significantly reduced transmission to 25% (6/24, P = .018). Surgical mask partition for challenged index hamsters significantly reduced transmission to only 16.7% (2/12, P = .019) of exposed naive hamsters. Unlike the severe manifestations of challenged hamsters, infected naive hamsters had lower clinical scores, milder histopathological changes, and lower viral nucleocapsid antigen expression in respiratory tract tissues.So not only did a surgical mask reduce risk of infection, but it also reduced severity of infection when it occurred. I think far too much importance has been placed on masks and their ability to prevent infection, and their ability to reduce severity has been overlooked.
Another hypothesis is that masks can serve as a form of variolation, essentially serving the same purpose as a vaccine without the vaccine. If masks prevent noticeable infection in most people, then they can actually allow for a (mostly) safe spread and development of herd immunity.
https://www.nejm.org/doi/full/10.1056/NEJMp2026913>While we await the results of vaccine trials, however, any public health measure that could increase the proportion of asymptomatic SARS-CoV-2 infections may both make the infection less deadly and increase population-wide immunity without severe illnesses and deaths. 
