9/11 and the flu

Apparently the traveling freeze post-9/11 delayed the flu season:

During the first five flu seasons, flu deaths peaked on or around February 17. But in 2002 the peak was delayed by two weeks, until March 2. In the years that followed, the peak moved back to February, they wrote in the online journal Public Library of Science-Medicine.

And during the 2001-2002 flu season, it took 53 days for flu to spread across the United States, 60 percent longer than the average time of 33 days.