Can Face Masks Reduce Transmission of SARS

Overall, the core intervention induced 29 out of every 100 people to wear masks, increased physical distancing and reduced the number of people with symptomatic SARS-CoV-2 infections by 9 percent. Surgical masks were especially effective in reducing symptomatic SARS-CoV-2 infections and reduced symptomatic infections in people 60+ by 35 percent. 

Use of masks: In villages that received the intervention, mask use increased by 29 percentage points (from 13 percent in the comparison villages to 42 percent in treatment villages). Mask use increased most in mosques (37 percentage points). A pilot and our scale-up activities suggest that in-person public reinforcement is a critical part of the intervention.

Physical distancing: The intervention also increased physical distancing in villages that received the intervention, contrary to concerns that mask-wearing would provide a false sense of security and promote risky behaviors like failing to maintain appropriate physical distance from others. On average, physical distancing increased by 5 percentage points across all locations in intervention villages, but the change was larger in some locations than others. In markets, people were substantially more likely to physically distance (7 percentage points increase). In mosques, researchers observed no change. 

Symptomatic Covid-19: The intervention reduced Covid-19 (symptomatic SARS-CoV-2 infections) and Covid-like symptoms. Free mask distribution and promotion reduced the proportion of people who reported Covid-like symptoms on average by 11 percent, which was driven mainly by the effects of surgical masks. Villages where cloth masks were distributed experienced a 5 percent reduction in symptoms, while villages where surgical masks were distributed saw a reduction of 12 percent. About a third (40 percent) of those who reported Covid-19 symptoms agreed to have their blood tested for SARS-CoV-2. On average, researchers found a decrease of symptomatic SARS-CoV-2 infections of 9 percent. This reduction was higher in villages that wore a surgical mask (12 percent) and in these, among individuals aged 60+ (35 percent). 

Behavioral Nudges or Incentives: None of the behavioral nudges or incentives at the community- or household-level led to additional increases in mask-wearing or physical distancing. Neither text message reminders, public signage, monetary and non-monetary incentives, altruistic messaging, nor verbal commitments had any effect on mask-wearing above and beyond the core intervention. Mask color had a statistically significant but minor impact on mask adoption. 

This study has several implications for policymakers. First, there is clear evidence that community mask-wearing can reduce Covid-19. The effects were substantially larger in communities where surgical masks were distributed, consistent with greater filtration efficiency measured in laboratories, and surgical masks reduced 1 in 3 symptomatic infections among individuals aged 60+. 

Second, community mask-wearing can be increased through a combination of four core intervention elements, now called the “NORM” model, which stands for “No-cost mask distribution, Offering information, Reinforcement to wear masks, and Modeling by local leaders." Many other factors did not increase mask-wearing: researchers found no evidence that public commitments, village level incentives, text messages, altruistic messaging, or verbal commitments changed mask-wearing behavior in this context, which underscores the importance of rigorous, real-world testing before rolling out behavioral interventions at scale. 

Whether people with respiratory symptoms should generally wear masks to prevent respiratory virus transmission––including for viruses other than SARS Co-V-2––is an important area for future research. The findings from this study suggest that such a policy may benefit public health.