Researchers and transnational public health organizations have recognized aerosol transmission as an essential route of COVID-19 transmission. Therefore, improving ventilation systems is now adopted as a core preventive measure. As young children aged 2-6 in kindergartens generally lack vaccine protection and multiple infection clusters have been identified during the pandemic, we aimed to quantify the risk of aerosol transmission in kindergartens in Taipei, Taiwan. From August to November 2021, we conducted on-site visits and continuously monitored indoor air quality indicators including carbon dioxide (CO2) in a kindergarten located in northern Taiwan. We utilized the Wells-Riley model to estimate the basic reproduction number (R0) of each classroom and staff office, with input parameters including the number of occupants, duration of their stay, and indoor/outdoor CO2 concentration. Contagious settings were defined as those where the R0 estimate exceeded 1. We conducted a scenario/sensitivity analysis to assess the effect of simulated improvement measures. During school hours, the average concentration of CO2 in each classroom and the staff office was often more than 400 ppm higher than the outdoor levels. The R0 estimates gradually increased from Monday to Friday and throughout school hours, corresponding to the hourly and daily distribution of the CO2 concentration, which could not dissipate completely during off-duty time. The R0 estimates during school hours ranged from 3.01 to 3.12 in classrooms with a maximum of 30 occupants. To lower the R0 estimate, it is imperative to substantially reduce the number of occupants, the duration of their stay, and indoor CO2 concentration. The risk of outbreaks of cluster infections in kindergartens should not be underestimated. Feasible strategies to mitigate this risk should include improving ventilation systems through engineering control and limiting the number of indoor occupants and their time staying indoor through administrative control.
