Flu Season Delayed After 9/11

Sept. 12, 2006 — The sharp dip in air travel after Sept. 11, 2001, slowed the spread of flu and delayed the onset of the 2001-2002 flu season in the United States, according to a new study.

The study, published in PLoS Medicine, suggests air travel facilitates the spread of infectious disease. The World Health Organization (WHO) and the U.S. government are already considering enacting flight restrictions in response to possible pandemics.

"Air travel is an important mode of the worldwide movement of flu viruses," lead author John Brownstein told Discovery News. "Although there are other mechanisms, air travel can quickly bring infected individuals to areas where they can mix with susceptible populations."

Brownstein, a faculty member of the Harvard/MIT Children’s Hospital Informatics Program, added, "Planes themselves have been shown to be good incubators of viruses. Poor air circulation can increase the probability of person to person transmission."

Brownstein and his colleagues gathered influenza mortality data from the Centers for Disease Control and Prevention (CDC) for the nine flu seasons from 1996-2005. They also compiled weather and international airline volume data.

During the first five seasons, flu mortality consistently peaked around February 17. This peak was delayed until March 2, 2002, during the 2001-2002 season.

Laboratory surveillance information from the WHO and the CDC further revealed that it took 53 days for the flu to spread across the United States following the 9/11 attacks. This was 60 percent longer than the average spread time of 33 days. In France, where flight restrictions were not imposed, there was no delay.

The study also found that, for international flights, September is a key month for the introduction of foreign flu strains. International flights fell 27 percent- from 4.9 to 3.5 million passengers- post 9/11, which contributed to the two-week delay in peak flu mortality.

Less traffic to and from Asian countries especially seems to affect flu spread.

"New flu viruses can originate from anywhere, but they most often come from Asia," Brownstein explained. "There are a number of reasons that might explain this, including population density, tropical environment, large avian reservoir for flu viruses, and close quarters, including close contact with poultry."

For domestic flights, November Thanksgiving travel appears to jumpstart flu’s spread. The drop in travel during November 2001 was one of the biggest predictors of the 2001-2002 season’s delay in peak flu mortality.

While researchers previously suspected that air travel contributed to the spread of infectious disease, they were not able to establish such a direct link since travel volume during flu seasons usually occurs at steady rates.

"The post-September 11th flight ban was a natural experiment on the effect of flight restrictions on disease spread," Brownstein said.

In addition to proposed airline travel restrictions to curb possible future pandemics, experts are also considering installing better air filtration systems on planes.

Joe Lundquist, a filtration technologist at New York’s Pall Corporation, said, "The flying public needs to be armed with the facts and informed about the risks of cabin air transmission of viruses, bacteria and fungi that can cause illness."

Former PanAm flight attendant Diana Fairechild, author of the book Jet Smarter, and Lundquist advise air travelers to wash their hands frequently and to avoid touching their eyes, nose and mouth. They also recommend opening the overhead gasper nozzles, available on most airplanes, to provide a steady flow of filtered air.