The hockey-related cases have been especially striking, epidemiologists have said, because clubs followed Centers for Disease Control and Prevention
limits on gathering size and had numerous social distancing measures in place. In retrospect, one mistake by some clubs was that until recently masks had been required on ice for only the two players doing the initial faceoff for the puck — although many players wore clear face shields, which theoretically should have a similar effect.
“We’re watching hockey very carefully because it’s the first major sport that’s been played indoors predominantly and also during the winter months,” said Ryan Demmer, an epidemiologist at the University of Minnesota’s School of Public Health.
Demmer said the cases provide some of the first real-world evidence to support early theories about the importance of how people breathe, ventilation, and the social
dimensions of transmission.