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

Background: Effectiveness of personal protective measure against COVID-19 infection is largely unknown. Methods: We conducted a retrospective case-control study, using a cohort of contact tracing records in Thailand. A total of 1,050 asymptomatic contacts of COVID-19 patients between 1 and 31 March 2020 were retrospectively interviewed by phone about their protective measures against COVID-19 infection. Cases were defined as asymptomatic contacts who were diagnosed with COVID-19 by 21 April 2020. Multilevel mixed-effect logistic regression models were used Findings: 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 independently associated with lower risk of COVID-19 infection 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. 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, maintained distances >1 meter and spent ≤15 minutes with close contacts, cases would have been reduced by 84%. Interpretation: Our findings support consistently wearing masks, washing hands, and social distancing in public to protect against COVID-19 infections. Combining measures could substantially reduce infections in Thailand. Funding Statement: The study was supported by the DDC, MoPH, Thailand. DL is supported by the Wellcome Trust (106698/Z/14/Z). Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: As this study was part of the routine situation analysis and outbreak investigation of the DDC MoPH Thailand, it was not required to obtain ethics approval and no written informed consent was collected. However, the study team strictly followed ethical standards in research; that is, all individual information was strictly kept confidential and not reported in the paper. The DDC MoPH Thailand approved the analysis and reporting of data in aggregate.