Flu cases in the U.S. start to take off after Thanksgiving, when more people fly to be with family.
The CBC reports that John Brownstein of Children’s Hospital Boston and his colleagues looked at how flu spreads between cities and regions and calculated the rate of spread each year.
They also found that the airline shutdown of 9/11 delayed the flu season.
In the years before 9/11, flu deaths in the U.S. tended to peak around Feb. 17, the researchers reported in the journal PLoS Medicine. In the flu season after the attacks, the peak date was delayed by 13 days, to March 2. In France, where there were no flight restrictions, there was no delay in the 2001-2002 season.