In October 2019, the World Health Organization published guidelines for the use of non-pharmaceutical interventions during a potential influenza pandemic.
In the section on face masks, they reviewed 10 randomized, controlled trials on face masks which included over 6,000 participants.
“Ten RCTs were included in the meta-analysis, and there was no evidence that face masks are effective in reducing transmission of laboratory-confirmed influenza.”
They also state:
“Reusable cloth face masks are not recommended. Medical face masks are generally not reusable, and an adequate supply would be essential if the use of face masks was recommended. If worn by a symptomatic case, that person might require multiple masks per day for multiple days of illness.”