COMMENTARY: What can masks do? Part 1: The science behind COVID-19 protection

Confusion continues to abound over the effectiveness of masks to protect people from COVID-19, and recent news stories touting imperfect studies are only compounding the situation. First, there was confusion and then intransigence about the modes of transmission, with the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) insisting for many months after SARS-CoV-2 emerged that the virus was transmitted only by large droplets or hand contact. Only fairly recently did those agencies finally recognize that inhalation of small infectious particles in the air ('aerosols') is a more important mode of transmission. That was followed by substantial misinformation and misunderstanding about the role of cloth face coverings, surgical masks, and respirators in preventing SARS-CoV-2 spread. Then we had misunderstandings and poor messaging about the efficacy of different interventions, the effectiveness of one intervention versus another, and why interventions should be considered in a particular order or hierarchy.