In a First, Randomized Study Shows That Masks Reduce COVID-19 Infections

 In the first part of the study, completed this spring, the researchers found that a “cocktail” of four interventions, including endorsements from community leaders and reminders from roving monitors, increased mask wearing to 42% in the targeted villages, up from 13% in control villages, as recorded by an observation team. That study’s recommendations have now reached more than 100 million people, having been adopted by governments across South Asia and drawn the support of major companies and NGOs.

In the second part of the study, the researchers surveyed people in the targeted and control villages about COVID-19 symptoms. Those who reported symptoms were asked to provide blood samples, which were tested for COVID-19 antibodies. The results showed that there were 9.3% fewer symptomatic infections in villages that were targeted in the mask promotion program.

https://insights.som.yale.edu/insights/in-first-randomized-study-shows-that-masks-reduce-covid-19-infections

Results: There were 178,288 individuals in the intervention group and 163,838 individuals in the control group. The intervention increased proper mask-wearing from 13.3% in control villages (N=806,547 observations) to 42.3% in treatment villages (N=797,715 observations) (adjusted percentage point difference = 0.29 [0.27, 0.31]). This tripling of mask usage was sustained during the intervention period and two weeks after. Physical distancing increased from 24.1% in control villages to 29.2% in treatment villages (adjusted percentage point difference = 0.05 [0.04, 0.06]). After 5 months, the impact of the intervention faded, but mask-wearing remained 10 percentage points higher in the intervention group

The proportion of individuals with COVID-like symptoms was 7.62% (N=13,273) in the intervention arm and 8.62% (N=13,893) in the control arm. Blood samples were collected from N=10,952 consenting, symptomatic individuals. Adjusting for baseline covariates, the 2 intervention reduced symptomatic seroprevalence by 9.3% (adjusted prevalence ratio (aPR) = 0.91 [0.82, 1.00]; control prevalence 0.76%; treatment prevalence 0.68%). In villages randomized to surgical masks (n = 200), the relative reduction was 11.2% overall (aPR = 0.89 [0.78, 1.00]) and 34.7% among individuals 60+ (aPR = 0.65 [0.46, 0.85]). No adverse events were reported.

[The percentage of individuals in a population who have antibodies to an infectious agent is called seroprevalence.]

https://www.poverty-action.org/sites/default/files/publications/Mask_RCT____Symptomatic_Seropositivity_083121.pdf