Quantitative risk assessment of COVID-19 and serious illness among spectators at mass gathering events with vaccine-testing package implementation

While mass gathering events have resumed in conjunction with vaccine-testing (VT) packages, their effects on reducing COVID-19 risk remain unclear. Here, we used an environmental exposure model to analyze the effects of vaccinations and proof of negative test results on reducing infection risk and serious illness among spectators at mass gathering events. We then analyzed the difference in risk with and without VT and regular seat zoning. Risk of infection and serious illness were quantified using a model incorporating parameters such as vaccination coverage, vaccine prevention effectiveness, and sensitivity of polymerase chain reaction (PCR) or qualitative antigen tests. When vaccine prevention effectiveness was 50% (corresponding to 4 months for the delta variant and 1-2 months for the omicron variant after the second vaccine dose), the risk of infection and serious illness among vaccinated spectators were 0.32-0.40 and 0.13-0.16 times of those who tested negative, respectively. In contrast, the risks of infection and serious illness among vaccinated spectators without measures such as mask wearing were 4.0 and 1.6 times higher than those among unvaccinated spectators with such measures, respectively. The risk of infection with an 80% vaccination coverage and a vaccine prevention effectiveness of 20% (corresponding to 5-6 months for the delta variant or 3-4 months for the omicron variant after the second vaccine dose) was comparable to that of a 20% vaccine coverage and a vaccine prevention effectiveness of 80% (corresponding to 1-3 months for delta variant after the second vaccine dose). Regarding zoning, there was little difference in risk with a vaccination coverage of [≥]80%. Adherence to individual measures after vaccination and maintenance of high vaccine effectiveness among spectators at stadiums are important for reducing risk of infection and serious illness. Furthermore, seat zoning did not affect overall infection risk reduction.