Protocol: a systematic review+ (SR+) to combine associative and mechanistic evidence on the efficacy of face masks in reducing transmission of respiratory diseases

Mechanistic evidence is evidence about how an intervention works. A 2023 Cochrane review, which was restricted to randomised controlled trials (RCTs), concluded that evidence on the efficacy of face masks was weak, conflicting and non-definitive. A 2024 narrative review, which included RCTs plus mechanistic evidence on masks and mask mandates, concluded that evidence of efficacy was strong, consistent and definitive. These strikingly contrasting conclusions reflect differences in how evidence is valued. Orthodox synthesis methods (e.g. those used for Cochrane reviews, informed by GRADE criteria) classify mechanistic evidence as lower quality than RCT evidence, but this position has been challenged by (among others) philosophers, non-RCT researchers and advocacy groups. We seek to include mechanistic evidence in a systematic review of mask efficacy.