Sept. 11, 2006 -- The drop in U.S. air travel following the terrorist attacks on Sept. 11, 2001 delayed America's flu season by two weeks, according to a new study.
The finding suggests lessening air travel could affect the spread of a flu virus.
The research appears in the October edition of Public Library of Science Medicine.
Researchers included John Brownstein, PhD, of the Massachusetts Institute of Technology, Children's Hospital Boston, and Harvard Medical School.
Brownstein's team tracked air travel, flu's spread, and the flu season's peak in the U.S. from 1996-2005. For five of those nine years, U.S. flu deaths peaked on Feb .17, give or take two days.
But after 9/11, the U.S. flu season had a "markedly delayed peak, on March 2, 2002, 13 days later than average," write the researchers.
The following flu season's peak was also later than normal, coming at the end of February 2003. Air travel hadn't fully recovered by then.
Over the next two years, flying picked up, and flu season returned to its normal peak time, the researchers note.
For comparison, France, which had no similar downturn in flying after 9/11, saw no delay in flu season for any year during the study, the researchers say.
September may be the critical month for new flu strains to enter the U.S. from other countries, the researchers say, adding that U.S. flu seasons start in October or November.
Brownstein's team isn't blaming flu's spread totally on air travel. Flu has been around a lot longer than airplanes, and you don't have to be a jetsetter to catch the flu.
It's still unclear exactly how flu seasons work.
But the findings "do suggest that fluctuations in airline travel have an impact on large-scale spread of," the researchers write.
Would banning flights halt a major, global flu outbreak? The researchers aren't sure.
Even with a "significant travel ban" flu might spread faster than vaccines could be made and distributed, write Brownstein and colleagues.