The onset of the flu season in the US has been shown to be influenced by air travel. After flights were restricted following the 9/11 terrorist attacks in 2001, the flu season started about two weeks later than usual.
In both the northern and southern hemispheres, influenza epidemics occur annually during the winter flu season. Given the concerns about global epidemics of serious strains of flu, such as occurred in 1918, as well as the more recent, localized outbreaks of bird flu, it is important to know more about how flu spreads round the world. Might airline travel play a role in the spread of the infection across large distances? If so, reducing or restricting air travel might be useful in the early stages of a serious global epidemic. Research at the Children’s Hospital Boston, now published in PLoS Medicine, has investigated the effects of air travel on the annual spread of flu in the US.
The researchers analyzed government records on deaths from flu and pneumonia in cities from nine regions of the US between 1996 and 2005. For each year, they determined the time it took for the epidemic to spread across the country and the date of the national peak in flu deaths. They used government estimates of passenger air travel to explore any connection with the timing of the annual flu epidemics.
They found the usual time taken for a flu epidemic to reach peak levels across the US was about two weeks, and that the national peak date was within two days of the average date—February 17. Flu spread more slowly in years when the number of domestic air travelers was lower. However, the most striking finding was that the peak date of the US flu season following September 11, 2001 was delayed, by 13 days, to March 2. Following the terrorist attack, there were restrictions on air travel and many people themselves chose to use other forms of transport.
The peak date then returned to February 17 over the subsequent two flu seasons, when international airline travel went back to its previous level. In contrast, the researchers found no delay in the 2001/2002 flu season in France, where there were no flight restrictions. Although other factors may possibly have been involved, the findings suggest that air travel affects both the peak date and the rate of spread of flu. This does not prove that travel restriction would be effective in altering the course of a flu pandemic, but it does provide evidence that air travel plays a significant role in the annual spread of flu in the United States. The findings will help plan for the next flu pandemic.