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G.eneral H.ealth : Other N.ews Last Updated: Nov 19th, 2006 - 12:21:58

Air Travel Restrictions Delayed Spread of Winter Flu in 2001
By Sara Andrews
Sep 12, 2006, 16:30

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12 Sep, ( - A sharp drop in air travel following the 9/11 terrorist attacks may have played a role in delaying the spread of winter flu across the US in 2001, new research finds. The study says that restricting air travel may allow time for countries to stockpile vaccinations in the event of bird flu pandemic in future.

Previous research has already documented a scenario where restricting airline travel may halt the spread of influenza, but these estimates were based on computer-simulated data. The current study published online yesterday in PLoS Medicine is the first concrete report that offers statistical evidence that a future pandemic flu may be controlled by placing restrictions on airline travel.

John Brownstein of the Children's Hospital Boston and his colleagues mapped the link between airline travel and the spread of flu between 1996 and 2005.

The data on flu was provided by the Centers for Disease Control and Prevention (CDC), while the airline monthly manifest provided data on the number of passengers traveling during a month.

The researchers found that changes in the rate of spread of flu and the timing of peak mortality each year seemed to correspond with yearly fluctuations in monthly airline passenger volume. According to their estimates, November was the best predictor of the speed of influenza spread.

"Travel restrictions to infected areas may provide public health coordinators lead time to stockpile vaccine, for example, said lead researcher John Brownstein of the Children's Hospital in Boston, Massachusetts. The study also suggests that travel during the Thanksgiving holiday could be a predictor of the spread of influenza.

This effect was most pronounced in the travel restrictions placed after the 9/11 terrorist attacks in 2001. Airline manifests show a 27 percent drop in passenger traffic amounting to 1.7 million passengers. Generally the peak date for influenza mortality in the US is February 17, but in 2002 this scenario was delayed for a fortnight. The peak mortality rate was attained on March 2.

In France where no such travel restrictions were placed in 2001, there was no change in the peak mortality date.

Computer-based models say that airline traffic volumes must drop at least 99 percent if such an effect is to be seen with pandemic bird flu. "Our study suggests that you could get more benefit than the simulations show," Brownstein said, adding that a 50 percent reduction in air travel could translate into a delay of about one month.

"When we first looked at our data, we noticed that the 2001-2002 flu season was highly aberrant," said Kenneth Mandl, M.D., a co-author of the study at Harvard and Children's. "At first we thought it was a problem with the data, but then we realized we were seeing the shadow of September 11th cast upon the influenza season."

Researchers estimate that a delay of just one or two weeks might allow public-health officials time to distribute antiviral drugs or vaccines and isolate those already infected.

"It will only make a difference if you were racing to put a control measure in place," said Ira Longini, of Fred Hutchinson Cancer Research Center in Seattle, Washington. "It's a natural experiment. I've never seen this done before for flu or other infectious diseases."

Dr. Brownstein and colleagues acknowledge that the mechanism behind airline travel and delay in spread of influenza is not properly understood, but "our findings do suggest that fluctuations in airline travel have an impact on large-scale spread of influenza."

Influenza is caused by a variety of viruses, which sometimes change the nature of infection usually by mutating to other types or by simply increasing their antigenicity. The World Health Organization estimates that the annual death rate from influenza is between 250,000 and 500,000 globally.

A pandemic was last seen in 1918-19 and was known as Spanish flu pandemic, which killed 40 million to 50 million people worldwide. Almost 20 percent of the world's population suffered and 5 percent of them were killed. The strain was unusual in killing many young and healthy victims, unlike common influenzas that kill mostly newborns and the old and infirm.

Experts fear that the circulating H5N1 bird flu virus may mutate into an easily transmissible form between humans. If that happens it may trigger a worldwide pandemic. Although the bird flu virus has infected 244 people and killed 143 the WHO has warned that it could turn out to be worse if the key mutation occurs.

The current study has highlighted the fact that restricting air travel may allow for a theoretical pandemic to be controlled by distributing vaccines and drugs quickly. "What this study shows to a policymaker who wants to use evidence in making a decision ... stopping air travel is very likely to give a real delay and slow the spread," Brownstein said.

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