Post-9/11 travel restrictions delayed 'bird flu'

The onset of the flu season in the USA 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.

Background.

In both the northern and southern hemispheres, influenza epidemics occur annually during the winter “flu season.” Although the disease maps out a remarkably similar pattern in most years, little is known about the specific mechanisms by which geographic spread occurs. Given the perennial possibility of influenza global epidemics (pandemics) such as occurred in 1918, 1957, and 1969, as well as the more recent, localized outbreaks of avian influenza (“bird flu”) in which a high proportion of affected people have died, we need to understand how influenza spreads in order to limit the destructive impact of future pandemics.

Why Was This Study Done?

In theory, airline travel might be expected to play a role in the spread of influenza across large distances. If so, reducing or restricting air travel might be an appropriate public health intervention in the early stages of an influenza pandemic. This study was performed to identify specific effects of air travel on the annual spread of influenza in the United States.

What Did the Researchers Do and Find?

The researchers analyzed weekly government records on deaths from influenza and pneumonia in cities from nine regions of the US during the nine influenza seasons between 1996 and 2005. For each year, they determined the time it took for the epidemic to spread across the US and the date of the national peak in influenza deaths. They then used government estimates of passenger air travel to explore any connection with the timing of the annual flu epidemics.

The analysis found that the usual time for an influenza epidemic to reach peak levels across the US was approximately two weeks, and that the national peak date fell within two days of the average date, February 17, in five of the nine seasons. In general, influenza was found to spread more slowly during years when the number of domestic air travelers, particularly during November, was lower. Also, the peak of the influenza season was found to come later during years when the number of international air travelers, particularly in September, was lower. These results, based on reported deaths from pneumonia or influenza, were corroborated using data from an influenza virus surveillance program, and could not be explained by variations in winter temperatures or by different types of influenza virus circulating in different years.

Of note, the peak date of the US influenza season following September 11, 2001, was delayed by 13 days to March 2, consistent with marked reductions in airline travel following the terrorist attack, and then returned to February 17 over the subsequent two influenza seasons as international airline travel returned to its previous levels. In contrast, the investigators found no delay in the 2001–2002 influenza season in France, where flight restrictions were not imposed.

What Do These Findings Mean?

While this study does not demonstrate that travel restriction would be effective in altering the course of a flu pandemic, it does provides evidence that air travel plays a significant role in the annual spread of influenza in the United States. Although other factors, related or unrelated to the decrease in air travel after September 11, may have affected the course of the 2001–2002 influenza season, the general findings across several years suggest that air travel affects both the peak date and the rate of spread of influenza. These findings merit consideration in the process of preparing for the next influenza pandemic.