I read with great interest the study by Suñer et al. on the association between mass gathering events and risk of the coronavirus disease 2019 (COVID-19). This study assessed the infection risk associated with the events based on a sophisticated control-matched design and analyzed risk factors for infection through a questionnaire survey of event participants. The latter analysis showed that 74.7% of respondents wore masks at all or most of the time during the event, and that mask-wearing was a protective factor for infection (odds ratio = 0.64 [95% confidence interval: 0.54–0.77]). Masks are not only effective in reducing the risk of acquiring infections among exposed individuals, but also in preventing the spread of infection by viruses emitted from those already infected. The finding in this study related to the former effect; however, it is also important to evaluate the effectiveness of masks with respect to the latter. For example, what is the difference in the mask-wearing ratio among the uninfected and already infected participants at the time of the event? Also, is there any difference in the risk of post-event infection between participants who accompanied infectors who wore masks and those who accompanied infectors who did not wear masks? With the worldwide relaxation of mask-wearing in public, this study is of great value in that it prompts a re-examination of the significance of the effect of mask-wearing in public, especially at mass gathering events. I would be grateful for the authors’ opinions on this point.
Declaration of interests
MM has attended the new coronavirus countermeasures liaison council jointly established by the Nippon Professional Baseball Organization and Japan Professional Football League as experts without any rewards.