## 4 Discussion and Conclusions  
With this study, we found that, in 17 out of 22 (77%) countries involved in the 2020 European football championship, there has been a changepoint in the number of daily new SARS-COV-2 cases during the tournament, falling on average 14.97 days (95% CI 12.29-17.47) after the first match they played. Not only that, the case rate of the new daily infections was inverted for all these 17 countries, changing from a decreasing trend to an increasing one. We have quantified this inversion by measuring, for each national infection curve, the halving time before the change and the doubling time after it; they are respectively, on average: 18.07 days (95% CI 11.81-29.42) days and 29.10 days (95% CI 14.12-49.78). Figure 5: Latvia, Lithuania, Moldova, Norway, Romania, and Serbia not participating countries. Yellow space: duration of the tournament. Red vertical line: first match. Purple vertical line: last match. Green vertical line: last hosted match. Blue vertical line: changepoint. Blue space: CI amplitude for the changepoint. Green segment: case rate trend before the changepoint. Red segment: case rate trend after the changepoint. Blue bell-shaped peaks, grey segments, black dots, and rightmost and leftmost \(y\) axis: same as in previous figures. There are five countries that break the pattern, and the presence of which could be seen as a first limitation of this study. Nonetheless, a careful consideration of the situation of these countries could provide a plausible explanation to this behavior. For example, it is evident that, for many of them (the UK, Russia, and Portugal), it is not possible to detect a changepoint in the infection rate which is coincidental with their participation in the tournament. This is because the inflation of the new COVID-19 cases was already in effect, in all these countries, when the tournament started, with the effect of the football championship probably absorbed into that inflation. The causes for this premature upturn of SARS-COV-2 cases are quite clear for the UK, which was the first European country to face the Delta variant. Portugal, instead, could have been the first European country to face the tourism impact, with many early tourists coming just from the UK. The situation in Russia, because of its enormous geographical extension, is, instead, too complex to look for a single explanation. Sweden, which reports COVID-19 numbers four days a week, entails a difficult interpretation, with our model not able to spot plausible changepoints. In regards of Sweden, it should not be forgotten that this was a country where very different strategies for managing the pandemic were adopted, without resorting, for example, to national lockdowns. Obviously, at the current stage of our research, no inference can be drawn regarding the existence of a relation between this fact and the results we achieved concerning this country. The situation for Turkey is different. It seems to follow the pattern, with an easily identifiable changepoint, coincidental with a reversion in the decrease/increase trend of the new COVID-19 cases. Nonetheless, this changepoint comes a bit too late (29 days after its first match). Hence, our decision was not to consider it as a further evidence in favor of the investigated link. A second limitation of this study is that it ignored the possible effects of other confounding factors that could have played a role. Unfortunately, there are too many, and they are also too country specific, in many cases, to be considered as a whole. Nonetheless, the following two facts should also be considered. A general trend toward the decrease in the new daily SARS-COV-2 cases had already begun during the beginning of the 2021 spring, in almost all the considered countries, as an effect of the vaccination. In response to the benefits of the vaccines, almost all these European countries had consequently begun to lift the restrictions that were imposed to combat the third wave of the contagion.