Efficacy studies for COVID-19

A WHO-funded systematic review by Chu et al. (27 June 2020) published in The Lancet found that the usage of face mask could result in a large risk reduction of infection with epidemic-causative betacoronaviruses, in which N95 or similar respirators accounted for a larger risk reduction than disposable surgical or other similar masks.[41] Masks were found to be protective for both healthcare workers and people in communities exposed to infection; evidence supported masking in both healthcare and non-healthcare settings, with no striking differences detected in the effectiveness of masks between the settings.

A study by Chan et al. (30 May 2020), using SARS-CoV-2-challenged and non-infected Syrian hamsters placed in closed units with unidirectional airflow to test the effect of surgical mask partitions, concluded that SARS-CoV-2 can be transmitted by respiratory droplets or airborne droplet nuclei and that such transmission could be reduced by usage of surgical masks, especially when worn by infected individuals.[124] The results showed that transmission was reduced from 66.7% to 25% and that the exposed hamsters who became infected showed less severe manifestations of the infection than their virus-challenged counterparts as measured by clinical scoring, histopathology, and viral nucleocapsid antigen expression.[124] Yuen Kwok-yung, one of the researchers, said that "The findings implied to the world and the public is that the effectiveness of mask-wearing against the coronavirus pandemic is huge," but cautioned that a risk of infection still remains.

A report by Hendrix et al. (17 July 2020) detailed that 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19—with both the clients and stylists wearing face coverings—resulted in no symptomatic cases reported among all clients and no positive tests among the 67 people who volunteered to be tested.[128] This case was highlighted when the CDC reiterated that Americans should wear masks in their 14 July 2020 press release.