Scientists have found the first real evidence that restricting air travel can delay flu's spread - a finding that could influence US government plans for battling the next influenza pandemic. Air travel has long been suspected of playing a role in flu's gradual spread around the globe each year. But Monday, Boston researchers said they have finally documented it: The drop in air travel after the 9/11 terrorists attacks seemed to delay that winter's flu season by about two weeks. "This is the first time that a study's been able to show a direct link between the numbers of people traveling and the rate of spread of a virus," said John Brownstein, an epidemiologist at Children's Hospital of Boston, who led the new research. "These data show such a striking effect," added Dr. Kenneth Mandl, his co-author and a pediatric emergency physician at Children's. Other scientists stress that the study does not prove restricting air travel really helps in the long run - there was no drop in the number of deaths, just a delay. So if a pandemic were to strike, the question is whether a mere two-week delay would outweigh the economic chaos of severe travel restrictions. People easily spread the flu through coughs, sneezes and germy hands. But scientists don't understand how a community outbreak ripples outward until each winter's flu strain spreads across countries. Plus, every few decades a new and virulent flu strain causes a worldwide epidemic; better understanding of those geographic patterns might help stem the next such pandemic. Previous studies suggest that young children who bring the flu home to older relatives spark community outbreaks, which spread between US cities and states when the sick go to work instead of recuperating at home. Brownstein and Mandl were studying flu's spread across larger distances when they stumbled onto what they first thought must be an error: In the winter of 2001-2002, influenza took longer than usual to spread from coast to coast. Then it struck them: International and domestic travel plunged right after the September 11 attacks - fall months when flu would have been starting to percolate. Could there be a connection? The researchers analyzed government records of deaths attributed to flu and pneumonia, from nine US regions, between 1996 and 2005. During the first five flu seasons, flu-attributed deaths consistently peaked on or around February 17. But in the 2001-02 flu season, mortality didn't peak until March 2, almost two weeks later, they reported Monday in the online science journal PLoS Medicine. The next two flu seasons gradually crept back to that February peak. Next Brownstein and Mandl compared their flu data to Transportation Department monthly air-travel estimates. The winter's flu strains begin arriving from abroad in September, they concluded: For each flu season, the September volume of international passengers most strongly predicted when US flu mortality would peak. In September 2001, that volume dropped 27 percent. Thanksgiving is when flu's cross-country spread takes off, the study found. Domestic air travel was down in November 2001, too. NIH's Fauci cautioned that flu patterns are notoriously fickle. If the researchers had looked back to 1992, they would have spotted a similar delay that makes the September 11 winter so striking, he said. It's not that lots of people catch the flu from sneezy fellow passengers, although that's possible. Instead, Brownstein says travelers who may start a trip before flu's symptoms kick in infect the people they're visiting - or they catch the virus in one city and carry it back home. Winter temperatures or milder flu strains did not explain the 2001-02 variation.