The arrival of flu season was delayed when air traffic stopped after the Sept. 11 attacks, a finding that suggests a ban on air travel could buy valuable time in the event of a pandemic, a study's authors say.

Researchers in the U.S. studied influenza seasons for nine regions in the U.S. between 1996 and 2005.

John Brownstein says restricting air travel could delay the arrival of a pandemic flu, giving healthcare workers more time to prepare.
(CBC)

Study author 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.

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 2001-02 season.

Dr. John Oxford doesn't think the delay from restricting air travel would make a difference in slowing the spread of a pandemic.
(CBC)

As air travel in the U.S. picked up again in 2003, flu deaths returned to their traditional mid-February peak.

The study also showed seasonal flu cases in the U.S. start to take off after American Thanksgiving, when more people fly to be with family.

The researchers tried to look for other reasons for the lag after Sept. 11, such as weather and vaccination rates, but couldn't find any link.

"Really, it was just the travel, the number of people on flights, domestic and international, that influenced the influenza season," said Brownstein.

The findings prompted the team to ask: if restricting air travel could delay flu season, could it also delay the arrival of a flu pandemic?

Every day might count

The delay would not be enough to make a difference, said Prof. John Oxford, an influenza expert at University College London.

"If you stop 99.99 per cent of the flights coming into your country, you do two things," said Oxford. "You blow the feet off the economy, which is not good, and you don't stop the virus coming in. You delay it a little bit."

Others say that in a pandemic, every day will count.

"This could provide significant public health lead time for things like stockpiling antiviral therapy, manufacturing a vaccine," Brownstein countered. The researchers acknowledged their findings cannot prove travel restrictions would change the course of a flu pandemic, but they say it shows air travel plays a major role in the annual spread of flu. 

Canada's pandemic flu plan doesn't call for restrictions on air travel, and it might be needed, said Dr. Ron St. John of the Public Health Agency of Canada.

"There will be a dramatic decrease in air travel, without any need for restriction," said St. John. "People will just stop travelling, and that was evident during SARS."