A case-control study of behavioural and built environment determinants of COVID-19 transmission in sheltered markets

Abstract

BackgroundIn July 2021, amid occasional COVID-19 community transmission, an outbreak in a seafood wholesale centre propagated secondary clusters in sheltered markets in Singapore. This period offers a unique chance to study COVID-19 transmission risk factors related to environmental features and personal behaviours in markets. Only 40 % of the population was vaccinated during this period. MethodsA two-tier case-control study was conducted. The first tier entailed site assessments of markets where primary cases, presumed to have acquired COVID-19 at the seafood wholesale centre, worked. Markets with secondary cases (case markets) were compared against those without (control markets). The second tier comprised telephone questionnaires administered to individuals who worked at or visited the three markets with the highest number of secondary cases, with respondents either testing positive or negative for SARS-CoV-2. ResultsAssessment of 38 markets showed that markets “seeded” with more primary cases were more likely to experience a secondary outbreak (adjusted odds ratio, aOR: 2.0; 95 % confidence interval, CI: 1.3–4.7). From 333 visitor responses, maintaining social distancing (aOR: 0.34; 95 % CI: 0.11–1.06) and completing vaccination (aOR: 0.25; 95 % CI: 0.07–0.95) decreased infection risk, while using public washrooms increased the risk (aOR: 3.12; 95 % CI: 0.87–11.00). Among 86 staff responses, statistically significant risk factors were gender and mask type: male staff (aOR: 0.14; 95 % CI: 0.02–0.62) and those wearing surgical/N95 masks (aOR: 0.10; 95 % CI: 0.01–0.71) had lower infection risk. Wider aisles of the markets correlated with a higher chance of secondary transmission (aOR: 2.5; 95 % CI: 1.3–6.7). ConclusionThese findings underscore the importance of considering both built environment features and individual behaviours in devising effective measures to mitigate COVID-19 transmission in markets and community hubs in general. The observed association of wider market aisles with increased risk could be attributed to these facilitating the gathering and intermingling of more individuals, underlining the importance of maintaining social distancing. The identified individual risk factors among different populations within the market environment highlight the need for population-specific measures for infection prevention.