Work-related commuting appears to be the single biggest factor influencing the spread of influenza from region to region, a government-led analysis of flu outbreaks over three decades shows.

While children may drive the spread of flu within households and even within communities, adults are largely responsible for spreading the disease over more distant areas, according to the analysis, conducted by researchers from the National Institutes of Health.

The finding has important implications for health officials developing strategies to limit the impact of a possible bird flu outbreak, infectious disease experts tell WebMD.

The analysis is published in the March 31 issue of the journal Science.

“If this is correct it means that limiting commuting to and from work could be an important strategy for slowing the spread of flu during a pandemic,” says Andrew Pavia, MD, who leads the public policy committee of the Infectious Disease Society of America.

Pandemic Would Spread Quickly

The analysis shows that flu epidemics spread more quickly and efficiently in more densely populated areas than in sparsely populated ones.

For this reason, a flu outbreak starting in California spreads to other states more quickly than one that originates in a less populous state such as Wyoming, study co-author Mark Miller, MD, tells WebMD.

In fact, the analysis showed that flu seasons are more likely to start in California than in any other state.

Miller and colleagues at the NIH’s Fogarty International Center conducted their analysis by comparing state-by state progression of influenza outbreaks in the U.S. between 1972 and 2002 with statistics on movement patterns among the population during the same period.

The researchers reported that the spread of seasonal flu across the country typically takes between five and seven weeks. But they project that future pandemic flu outbreaks would spread much more quickly, within two to four weeks, because the population would likely have little prior immunity to the virus.

Limiting Social Mixing

The regional spread of flu correlated more closely with the movement of adults to and from work than with any other single factor.

The findings suggest that infection control strategies aimed at limiting social interaction -- known in public health circles as social distancing -- could have a major impact during a flu pandemic, says Pavia, who is chief of infectious diseases at the University of Utah.

He points out that we are in a much better position to limit social mixing today than in the past.

“In 1918 [when pandemic flu killed millions] people had to go out and shop for food every day, they had to go to the post office to mail a letter, and they communicated face to face,” he says. “Today, we can fill our freezers with food and our children can do their school work via the Internet. And large numbers of people could work from home.”

As part of the planning for a possible bird flu pandemic, corporate leaders have been asked to develop strategies for limiting employee interactions. Influenza expert William Schaffner, MD, tells WebMD that the response has been mixed.

Schaffner is chief of the department of preventive medicine at Nashville’s Vanderbilt University School of Medicine.

“Depending on the company, that has been given anywhere from very little to quite profound thought,” he says.

By Salynn Boyles, reviewed by Brunilda Nazario, MD

SOURCES: Viboud, C. Science, March 31, 2006; online edition. Cecile Viboud, Fogarty International Center, National Institutes of Health, Bethesda, Md. Mark A. Miller, MD, associate director for research, Fogarty International Center, National Institutes of Health, Bethesda, Md. William Schaffner, MD, chairman, department of preventive medicine, division of infectious disease, Vanderbilt University School of Medicine, Nashville, Tenn. Andrew Pavia, MD, professor of pediatrics and medicine, University of Utah; chairman, public policy committee, Infectious Disease Society of America.