We included 44 new RCTs and cluster‐RCTs in this update, bringing the total number of randomised trials to 67. There were no included studies conducted during the COVID‐19 pandemic. Six ongoing studies were identified, of which three evaluating masks are being conducted concurrent with the COVID pandemic, and one is completed.
Many studies were conducted during non‐epidemic influenza periods, but several studies were conducted during the global H1N1 influenza pandemic in 2009, and others in epidemic influenza seasons up to 2016. Thus, studies were conducted in the context of lower respiratory viral circulation and transmission compared to COVID‐19. The included studies were conducted in heterogeneous settings, ranging from suburban schools to hospital wards in high‐income countries; crowded inner city settings in low‐income countries; and an immigrant neighbourhood in a high‐income country. Compliance with interventions was low in many studies.
The risk of bias for the RCTs and cluster‐RCTs was mostly high or unclear.