From a research team from the University of Hong Kong:
"In our systematic review, we identified 10 RCTs that reported estimates of the effectiveness of face masks in reducing laboratory-confirmed influenza virus infections in the community from literature published during 1946–July 27, 2018. In pooled analysis, we found no significant reduction in influenza transmission with the use of face masks....There is limited evidence for their effectiveness in preventing influenza virus transmission either when worn by the infected person for source control or when worn by uninfected persons to reduce exposure. Our systematic review found no significant effect of face masks on transmission of laboratory-confirmed influenza."
https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article?fbclid=IwAR1paMwS5sVoyd3pXunigSCHln7NpWG8DSLA628XCpJNqVVY3DHHX0Sv2oo
From a mask study with our own Marines at Parris Island specifically regarding Covid:
For those who agreed to mask up, by day 14, 2.8% of the 1,848 tested positive. Of those who didn't, 2.3% of the 3,402 who didn't mask up tested positive by day 14.
https://www.nejm.org/doi/full/10.1056/nejmoa2029717
From a study in Denmark specifically regarding Covid:
"Although the difference observed was not statistically significant, the 95% CIs are compatible with a 46% reduction to a 23% increase in infection...The recommendation to wear surgical masks to supplement other public health measures did not reduce the SARS-CoV-2 infection rate among wearers by more than 50% in a community with modest infection rates, some degree of social distancing, and uncommon general mask use."
https://www.acpjournals.org/doi/10.7326/m20-6817