Commuting Plays a Major Role in the Spread of Influenza

Commuters may play a key role in spreading the flu virus across the United States, according to a study published online in Sciencexpress, March 30.

    The authors analyzed data on the spread of flu viruses between 1972 and 2002 in the lower 48 states and the District of Columbia. They then correlated this with information on human travel, including commuting and long-distance trips.

    The results indicated that people traveling to and from work are largely responsible for spreading the flu from city to city and state to state. The speed at which the virus spreads depends in large part on where it originated, explained co-author Mark Miller, the associate director of research at the National Institutes of Health’s Fogarty International Center.

    Connectivity, not size or population, is the most important factor in predicting how long it will take a flu virus to reach neighboring cities or states, Miller said. For example, the paper explains, an outbreak in relatively isolated Wyoming would take longer to spread to other states than an outbreak in California, which is well connected to other states and nations.

    The paper notes that the spread of a virus that begins the flu season often takes place in California. Miller added that an outbreak of avian flu in humans would likely begin in California as well, because of the state’s large population. Some people speculate that high rate of travel between California and Asia makes the Golden State particularly susceptible, because many strains of influenza originate in Asia.

    But Miller noted that there is no sure way to predict where such an outbreak would take place.

    An avian flu epidemic is difficult to predict, Miller said, because it could enter the United States having already mutated to be transmissible between humans. On the other hand, this mutation could occur in the United States, after the virus spread to birds in the country.

    The study found that in a normal flu season, a virus takes five to seven weeks to cross the country, Miller said. In years when a more virulent strain causes a pandemic, the virus spreads more quickly. If the virus is a strain -- such as avian flu -- that the general population has never encountered before, it could spread across the nation in two to four weeks, he added.

    These results indicate that encouraging people to work from home could significantly slow down a potentially pandemic virus’ spread, Miller said.

    He thinks policy makers should study possible methods for controlling a pandemic, such as identifying populations that should be vaccinated to slow the spread of an outbreak.

    Miller and his colleagues plan to expand their research to include data about children, who are thought to be responsible for spreading flu viruses through local communities. They also plan to look at the spread of flu viruses in other countries, he said.