Associations between wearing masks, washing hands, and social distancing practices, and risk of COVID-19 infection in public: a cohort-based case-control study in Thailand

Objective. To investigate whether wearing masks, washing hands and social distancing practices are associated with lower risk of COVID-19 infection. Design. A retrospective cohort-based case-control study. All participants were retrospectively interviewed by phone about their preventive measures against COVID-19 infection. Setting. Thailand, using the data from contact tracing of COVID-19 patients associated with nightclub, boxing stadium and state enterprise office clusters from the Surveillance Rapid Response Team, Department of Disease Control, Ministry of Public Health. Contacts were tested for COVID-19 using PCR assays per national contact tracing guidelines. Participants. A cohort of 1,050 asymptomatic contacts of COVID-19 patients between 1 and 31 March 2020. Main outcome measures. Diagnosis of COVID-19 by 21 April 2020. Odds ratios for COVID-19 infection and population attributable fraction were calculated. Exposure. The study team retrospectively asked about wearing masks, washing hands, and social distancing practices during the contact period through telephone interviews. Results. Overall, 211 (20%) were diagnosed with COVID-19 by 21 Apr 2020 (case group) while 839 (80%) were not (control group). Fourteen percent of cases (29/210) and 24% of controls (198/823) reported wearing either non-medical or medical masks all the time during the contact period. Wearing masks all the time (adjusted odds ratio [aOR] 0.23; 95%CI 0.09-0.60) was associated with lower risk of COVID-19 infections compared to not wearing masks, while wearing masks sometimes (aOR 0.87; 95%CI 0.41-1.84) was not. Shortest distance of contact >1 meter (aOR 0.15; 95%CI 0.04-0.63), duration of close contact ≤15 minutes (aOR 0.24; 95%CI 0.07-0.90) and washing hands often (aOR 0.33; 95%CI 0.13-0.87) were significantly associated with lower risk of infection. Sharing a cigarette (aOR 3.47; 95%CI 1.09-11.02) was associated with higher risk of infection. Type of mask was not independently associated with risk of infection. Those who wore masks all the time were more likely to wash hands and practice social distancing. We estimated that if everyone wore a mask all the time, washed hands often, did not share a dish, cup or cigarette, had shortest distance of contact >1 meter and had duration of close contact ≤15 minutes, cases would have been reduced by 84%. Conclusions. Our findings support consistently wearing non-medical masks, washing hands, and social distancing in public to prevent COVID-19 infections.