The slump in air travel that followed the 9/11 attacks delayed the influenza season of that year, according to researchers from Harvard University and the Massachusetts Institute of Technology in Boston.

The study provides the first evidence on the effect of air travel on the spread of flu and could support enforcement of air travel restrictions in cases of outbreak of pandemic diseases.

Using nine years of weekly influenza and pneumonia mortality data (1996 to 2005), the researchers measured the rate of influenza spread across nine regions and related them to estimates of air travel volume.

They found the national peak date for flu mortality in 2001-02 came in early March -  13 days later than the previous average. The peak then gradually returned to February.

The researchers also found out that seasonal flu spread more slowly when the number of domestic air travelers was low and it peaks later when the number of international air travelers was lower.

What does this all mean? Researchers say airline passenger volume explains about 60% of the year-to-year variation in the spread of flu across the country. The study also suggests, in case of bird flu outbreak, limiting domestic airline volume may have help reduce its spread across regions.

Severe Acute Respiratory Syndrome, better known as SARS, is one such epidemic that spread through air travel. First reported in Asia in February 2003. The virus quickly spread to more than two dozen countries in Asia, North America, South America, and Europe. When the Global Outbreak of 2003 was finally contained, 8,098 people worldwide became sick with SARS, leaving 774 people dead.