  Large-scale events like the UEFA Euro~2020 football (soccer) championship
offer a unique opportunity to quantify the impact of gatherings on the spread
of COVID-19, as the number and dates of matches played by participating
countries resembles a randomized study. Using Bayesian modeling and the gender
imbalance in COVID-19 data, we attribute 840,000 (95% CI: [0.39M, 1.26M])
COVID-19 cases across 12 countries to the championship. The impact depends
non-linearly on the initial incidence, the reproduction number $R$, and the
number of matches played. The strongest effects are seen in Scotland and
England, where as much as 10,000 primary cases per million inhabitants occur
from championship-related gatherings. The average match-induced increase in $R$
was 0.46 [0.18, 0.75] on match days, but important matches caused an increase
as large as +3. Altogether, our results provide quantitative insights that help
judge and mitigate the impact of large-scale events on pandemic spread.
