In a startling measure of just how widely a new disease can spread, researchers found that more than 2.3 million people flew from Mexico to more than 1,000 cities worldwide in March and April as the swine flu epidemic was unfolding.
Passengers traveled to 164 countries, but four out of five of those went to the United States. That fits with the pattern of the epidemic, say researchers reporting their findings Monday in the New England Journal of Medicine.
The research shows promise in forecasting how a new contagion might unfold, indicated one government health official who praised the work.
"We share a common interest in this issue: If we map the global airline distribution network, can we anticipate, once a virus emerges, where it is likely to show up next?" asked Dr. Martin Cetron of the U.S. Centers for Disease Control and Prevention. He leads CDC's division of global migration and quarantine.
The new swine flu virus was first reported in the United States in mid-April, but the first large outbreak was in Mexico at about the same time. Health officials believe cases of the new virus were circulating in Mexico in March.
Scientists have long assumed a relationship between air travel and spread of the virus. But the new research for the first time confirmed the relationship, said Dr. Kamran Khan, who led the study. He is a researcher at St. Michael's Hospital in Toronto.
For years, Khan and his colleagues have been working on a system to use air travel information quickly to determine how a new contagion is likely to spread around the world.
Their data sources include the International Air Transport Association, an international trade association representing 230 airlines and the vast majority of scheduled international air traffic.
The study showed the majority of passengers flew to the United States, with Canada a distant second and France a more distant third.
More than 90 percent of the time, Khan and his colleagues accurately matched air traffic volumes to which countries did and did not suffer swine flu outbreaks as a result of air traffic.
The top 11 destination cities from Mexico were all in the United States. Los Angeles was the leader, receiving about 9 percent of all passengers from Mexico, and New York City was second, with about 5 percent.
In contrast, the only South American entry in the top 40 destination cities was Buenos Aires, at No. 22. Passengers were even fewer when it came to cities in neighboring Guatemala and other Central American countries.
The data show not only how disease spreads out of Mexico, but also that air travel is mainly among more industrialized countries, experts said.