The group found that when a new NHL team moved into a city, it resulted in a 24.6 per cent increase in the local flu mortality rate, or about 20 more flu deaths per city per year. A new NFL team resulted in a 17 per cent increase, or 13 additional flu deaths.
Less affected, but still statistically significant, are cities that become home to new NBA or MLB franchises, which experienced increases in influenza mortality rates of 4.7 per cent and 5.3 per cent respectively. These increases each represent about three additional deaths annually.
Soebbing said some of the results could be chalked up to the time of year the sport is played. Baseball, for instance, runs from spring into the fall, which is typically the shoulder season for influenza. The bulk of the NFL games border peak flu season, while hockey and basketball run throughout the flu season.
As to why the NBA and NHL have marked differences, Soebbing said the difference in the number of teams the respective leagues grew by and when they expanded might be factors, adding that additional research is needed to explore these differences.