Monday, April 27, 2009

Airline travel and the spread of flu: Implications for the swine flu outbreak

by B. N. Sullivan

airlinerOver the past several days there has been an explosion of news about an outbreak of a new kind of flu virus, Swine Influenza A/H1N1 (AKA swine flu), that began infecting large numbers of people in Mexico, and then spread to several other countries, including the United States. We in the aviation community may not like to admit it, but air travel almost certainly has played a central role in the rapid spread of the disease outside the area where it first emerged.

Even as I write this, countries around the world are initiating screening of passengers arriving at gateway airports in an attempt to detect those who may be carriers of swine flu. Discussions are underway about travel restrictions to and from infected areas. Undoubtedly this outbreak -- and some of the measures being implemented (or at least being considered) -- will have a negative effect on the civilian air transport industry, which is already suffering from the global economic crisis that preceded the emergence of swine flu.

Should restrictions on air travel be imposed? Should public health concerns take precedence over the economic impact on the aviation industry? That's a tough call, and I'm glad I don't have to make it.

What is undeniable is that there is indeed a link between air travel and the spread of flu. That link was established several years ago by some scientists who studied the role of air travel in the spread of seasonal flu. It's an interesting story with great relevance to the present situation.

A team of researchers reviewed public records of deaths across the United States due to flu (influenza) and pneumonia for the nine year period from 1996 through 2005. They were looking for patterns in the timing of deaths attributed to seasonal flu outbreaks, and the geographic spread of flu mortality across the United States.

It is well known that 'flu season' occurs annually during the cooler months of the year, but the researchers wanted to find out more exact information about the timing of this yearly health problem, and about how the flu is spread.

They found that the timing of the peak of the flu season seemed to be related to seasonal patterns of passenger airline travel volume.

Specifically, they found that the seasonal increase in domestic air travel volume that begins around Thanksgiving in the U.S. was quite consistently followed by an increase in the rate of influenza spread. In years when holiday travel volume was lower, influenza spread more slowly. Conversely, during years with higher traffic volume, especially around Thanksgiving, influenza spread across the country more rapidly.

They also found that the volume of international air travel was significantly correlated with when the peak number of flu-related deaths occurred in a given year. They noted that in years when the number of international travelers was lower, especially during early Autumn, the flu season peaked later, and vice versa.

But when they dug a little deeper into the data, they discovered something even more interesting.

The patterns in the first five years they studied were remarkably stable. The timing of the peak number of flu-related deaths occurred in mid-February in each of those years. In fact the actual date when the peak occurred was within two days of February 17th for each of those first five years.

Then 9/11 happened, and we all know what followed in regard to air travel. First there was the ground stop, and when airline schedules resumed, passenger volume was quite low for many months. Then, over the next few years, annual passenger volume gradually increased.

Guess what: rates of spread for flu deaths around the U.S., and the timing of the peak of the flu season during those same years reflected the patterns in passenger volume. During the 2001-2002 season, flu spread across the country more slowly than in any other year of the nine studied, and the peak was weeks later.

Over the next three years, as passenger volume gradually increased, the peak of the flu season gradually moved to an earlier date -- until the final year they studied (2005) when it was right back to February 17th!

In the words of the researchers themselves:
The flight ban in the US after the terrorist attack of September 11, 2001, and the subsequent depression of the air travel market provided a natural experiment for the evaluation of the effect of flight restrictions on disease spread. The importance of airline activity was highlighted by the delayed peak of influenza in 2001-2002 following the period of reduced flying activity. This finding is further validated by the absence of a similar delay in influenza activity in France, where flight restrictions were not imposed.
Of course there are many other factors besides airline travel that can influence the spread of seasonal flu. Nevertheless, the findings of this study do confirm to a certain extent what many have always suspected: that air travel during flu season does indeed figure into how and when the flu is spread.

One final issued raised by this study's results is the extent to which restrictions on air travel might assist in inhibiting the spread of other infectious disease epidemics. Of particular concern these days is the prospect of the spread of swine flu.

In light of the findings from this study about how air travel figures into the spread of seasonal flu, we have to wonder if a similar effect would be found for swine flu, since it can readily be passed from human to human in a manner similar to seasonal flu.

The researchers said, "Our results suggest that limiting domestic airline volume would have a measurable impact on the rate of spread of an influenza pandemic, and particularly on spread across regions."

Given the many other factors that influence the spread of disease in epidemics, they believe that the primary benefit of air travel restrictions during an epidemic or pandemic would be to alter the rate and timing of the spread of the disease, not to prevent it outright. But, in the event of a deadly pandemic, slowing the spread even for a few weeks might give public health officials a bit more time to put in place other measures to fight the disease.

Source: Brownstein JS, Wolfe CJ, & Mandl KD (2006) Empirical Evidence for the Effect of Airline Travel on Inter-Regional Influenza Spread in the United States, PLoS Med 3(10): e401

NOTE: This is the first in a series of articles about swine flu and air travel that I will post here on As a researcher, I have been involved for years with topics relating to the health and well-being of people who work in the civilian air transport industry, and I run a website ( devoted to that subject matter. I am preparing a special page for that website with information about swine flu, tailored to the information needs of people who fly for a living.

More to follow...