An interesting preprint published by Miron and others in Medrxiv retrospectively examined COVID-19 incidence in 20 countries that held mass gathering rallies in the USA in August to September 2020. 

They were compared to the rest of the United States’ counties. 

They utilised the seven day moving average and compare the change on the gathering data 15 days later, based on a 95% competent interval. 

In 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 second lowest incidence before the gathering. 

The county with the highest increase 0.4 fold, had the third highest incidence before the gathering. At the gathering date, the average incidence of counties at 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. 

These results would 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 mass gatherings. 

Dr Paul Ettlinger 
London General Practice 

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