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Normalizing Community Mask-Wearing: A Cluster Randomized Trial in Bangladesh

Our take —

This was a very large, well-designed, and well-conducted cluster-randomized controlled trial of a multi-pronged intervention program to encourage mask-wearing in Bangladesh from November 2020 to January 2021. The trial found that the intervention package (which included mask distribution, public role-modeling, and encouragement to non-mask-wearers in public settings) more than tripled public mask usage behaviors (from 13% to 42%) without diminishing observed physical distancing. The study is the largest and best-designed trial to date of a non-pharmaceutical intervention to combat SARS-CoV-2. The second half of the study, which will assess whether the intervention affected SARS-CoV-2 transmission and COVID-19 outcomes, is in progress as of June 2021.

Study design

Randomized Controlled Trial

Study population and setting

This study, designed to estimate the impact of public health interventions on mask-wearing behavior, took place in 600 rural and peri-urban villages containing 341,830 adults throughout Bangladesh. From an initial sampling frame of 1,000 rural and peri-urban unions (each union consists of ~80,000 people), the authors used pairwise randomization to select 300 intervention and 300 control unions, and selected one village (containing the largest market in the region) in each union. The intervention was rolled out in waves starting from November 2020 to January 2021, and lasted for 8 weeks with 10 weeks of surveillance. The intervention group villages received a multi-pronged set of interventions, designed in conjunction with local leaders. The basic intervention had five elements: 1) one-time mask distribution and promotion to households, 2) mask distribution in markets 3-6 days per week, 3) mask distribution at mosques on three Fridays, 4) mask promotion in public spaces and markets, during which non-mask-wearers were encouraged to wear masks, and 5) role-modeling and active promotion of masks by religious and other community leaders. In addition, intervention villages were cross-randomized to four additional interventions: 1) cloth vs. surgical masks, 2) village-level incentives if a given level of mask-wearing was achieved vs. no incentives, 3) public commitments via provision of signs to households vs. no public commitments, and 4) additional text message reminders vs. no reminders. There were also three additional household-level cross randomizations: 1) messages focused on community protection vs. self-protection, 2) twice-weekly text message reminders, and 3) asking households to make a verbal commitment to mask-wearing. The main outcomes for this study were measured by an enumerator who recorded behaviors in a public space in each village. The enumerator noted whether people were wearing masks, the colors of those masks (indicating randomization group), whether both mouth and nose were covered, and whether people were keeping physical distance from each other. Finally, the authors conducted an economic evaluation to estimate the degree to which this intervention was cost-effective in preventing deaths. This was the first stage of a larger study designed to estimate the effectiveness of mask-wearing in preventing SARS-CoV-2 transmission.

Summary of Main Findings

The estimated prevalence of mask-wearing behavior was 13% in the control group and 42% in the intervention villages, implying that the intervention increased mask-wearing behavior by 29%. Models controlling for baseline characteristics of villages (baseline mask-wearing, baseline SARS-CoV-2 symptom prevalence) produced the same result. Physical distancing behavior also increased from 24% in the control arms to 29% in the intervention arms, suggesting that increased mask wearing did not cause net risk-compensating behaviors. Mask wearing behavior effects persisted for the full 10-week period of the study. None of the cross-randomized interventions (e.g. changes in messaging, monetary incentives, etc.) had any substantial effects on mask-wearing behavior. Evidence from pilot studies indicated that the primary mechanism of the intervention’s effectiveness was promotion of mask-wearing in public spaces. A preliminary cost-effectiveness analysis suggested that this intervention was relatively cost-effective.

Study Strengths

This study benefited from being a very large, well-designed, and well-conducted cluster randomized controlled trial of a realistic intervention. The large-scale cluster randomization allowed the study to robustly examine the impact of this non-pharmaceutical intervention on directly observed behavior, rather than on self-reported behavior.


One key limitation is that the persons recording the behavior data could not be blinded to whether the village was a control or intervention arm, leaving open the possibility that data recorders could have been influenced by knowledge of the study arm. People from control villages may have acquired masks or encountered mask promotion in nearby intervention villages, which would dilute the apparent effectiveness of the intervention. Additionally, the cost-effectiveness analysis employed large and unrealistic assumptions about the translation of mask-wearing behavior to health outcomes. This study, the first stage of a larger trial, only assessed behavioral results and did not assess the impact of the intervention on SARS-CoV-2 transmission or COVID-19 outcomes. Finally, it is unclear whether or how the intervention would need to be modified to obtain similar results outside the setting of Bangladesh.

Value added

This study is the largest and best-designed randomized controlled trial to date of a realistic non-pharmaceutical intervention on SARS-CoV-2 transmission. The design and scale of this study allows for extremely high confidence in the utility and robustness of its results; it is likely that these results may apply to interventions outside this setting.

This review was posted on: 20 June 2021