A large randomized community-based trial in which 4862 healthy participants were divided into a group wearing medical/surgical masks and a control group found no difference in infection with SARS-CoV-2 (76)
Here is the study: https://apps.who.int/iris/rest/bitstreams/1319378/retrieve
From page 8 of the report:
Guidance on mask use in community settings
Evidence on the protective effect of mask use in community settings
At present there is only limited and inconsistent scientific evidence to support the effectiveness of masking of healthy people in the community to prevent infection with respiratory viruses, including SARS-CoV-2 (75). A large randomized community-based trial in which 4862 healthy participants were divided into a group wearing medical/surgical masks and a control group found no difference in infection with SARS-CoV-2 (76)...