COVID-19 infections following outdoor mass gatherings in low incidence areas: retrospective cohort study

ObjectiveIndoor mass gatherings in counties with high COVID-19 incidence have been linked to infections. We examined if outdoor mass gatherings in counties with low COVID-19 incidence are also followed by infections. MethodsWe retrospectively examined COVID-19 incidence in 20 counties that held mass gathering rallies (19 outdoor and 1 indoor) in the United States in August-September 2020. They were compared to the rest of the United States counties. We utilized a 7-day moving average and compared the change on the gathering date and 15 days later, based on the 95% confidence interval. For control counties we used the median of the gathering dates. SettingThe United States Population8.4 million in the counties holding mass gatherings, and 324 Million in the rest of the counties in the United States. Main Outcome MeasureChange in COVID-19 incidence rate per 100,000 capita during the two weeks following mass gatherings. ResultsIn the two weeks following the gatherings, the COVID-19 incidence increased significantly in 14 of 20 counties. The county with the highest incidence increase (3.8-fold) had the 2nd lowest incidence before the gathering. The county with the highest decrease (0.4-fold) had the 3rd highest incidence before the gathering. At the gathering date, the average incidence of counties with gatherings was lower than the rest of the United States, and after the gathering, it increased 1.5-fold, while the rest of the United States increased 1.02-fold. ConclusionThese results suggest that even outdoor gatherings in areas with low COVID-19 incidence are followed by increased infections, and that further precautions should be taken at such gatherings. What is already known on the topicMass gatherings have been linked to COVID-19 infections, but it is less clear how much it happens outdoors, and in areas with low incidence. What this study addsCOVID-19 infections increased significantly in 14 of 20 counties that held mass gathering rallies in the United States, 19 of which were outdoors. The county with the highest incidence increase (3.8-fold) was outdoors and had a low incidence before the gathering. The average incidence of all 20 counties with gatherings was lower at the gathering day compared with the rest of the United State, and it increased 1.5-fold following the gatherings. Our findings suggest a need for precautions in mass gatherings, even when outdoors and in areas with a low incidence of COVID-19.