Huge meta-study (n=172) concludes chance of transmission falls from 17.4% to 3.1% when wearing face masks:
Physical distancing was also very important. Eye protection, less so.
Chu, Derek K., Elie A. Akl, Stephanie Duda, Karla Solo, Sally Yaacoub, Holger J. Schünemann, and COVID-19 Systematic Urgent Review Group Effort (SURGE) study authors. 2020. “Physical Distancing, Face Masks, and Eye Protection to Prevent Person-to-Person Transmission of SARS-CoV-2 and COVID-19: A Systematic Review and Meta-Analysis.” The Lancet, June.
Some other interesting results from a study on the Plague Marines aboard the aircraft carrier USS Theodore Roosevelt:
Looks like washing hands more than normal made no difference.
As Lyman Stone notes, what is especially striking is that this refers to individual protection, not community based protection (mask compliance was ~75% but entirely after the infection had become widespread).
Payne, Daniel C. 2020. “SARS-CoV-2 Infections and Serologic Responses from a Sample of U.S. Navy Service Members — USS Theodore Roosevelt, April 2020.” MMWR. Morbidity and Mortality Weekly Report 69. https://doi.org/10.15585/mmwr.mm6923e4.
I know I am beating a dead horse, having already written a couple of effortposts on this topic. Nonetheless, always nice to have priors confirmed, even if most people have already made up their minds.