The CDC published a brief report this week that pulls together the evidence that mask usage is highly effective when most, if not all, people are using face coverings consistently and correctly (covering mouth and nose). There are plenty of references for those who want to read the original scientific reports or journal articles. Some of the conclusions of the report are based on "observational and epidemiological studies" done since January 2020 in multiple countries.
I remember reading the NY Times article about the two hair stylists in Missouri who worked while they had COVID-19 before they were tested, but didn't infect any customers. Everyone was required to wear a face mask back in May while in the salon, both employees and customers.
A new study looks at what might have been a dangerous outbreak — had it not been for vigilant mask wearing.
www.nytimes.com
Headlines this week point to the idea that this is the first time that the CDC has mentioned that a person wearing a decent face mask gets some level of protection. However, when it comes to cloth face masks reducing community spread is based on universal usage so that people who are infected are much less likely to infect someone else. In particular people who have no idea that they are shedding SARS-CoV-2 because they don't have any symptoms--yet--or mild symptoms that they think are due to another cause.
Nov. 10, CDC
Scientific Brief: Community Use of Cloth Masks to Control the Spread of SARS-CoV-2
https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html
" . . .
Conclusions
Experimental and epidemiological data support community masking to reduce the spread of SARS-CoV-2. The prevention benefit of masking is derived from the combination of source control and personal protection for the mask wearer. The relationship between source control and personal protection is likely complementary and possibly synergistic14, so that individual benefit increases with increasing community mask use. Further research is needed to expand the evidence base for the protective effect of cloth masks and in particular to identify the combinations of materials that maximize both their blocking and filtering effectiveness, as well as fit, comfort, durability, and consumer appeal. Adopting universal masking policies can help avert future lockdowns, especially if combined with other non-pharmaceutical interventions such as social distancing, hand hygiene, and adequate ventilation."