THURSDAY, March 30, 2006 (HealthDay News) -- If a bird-flu pandemic does hit the United States, it may well start in California and spread across the country in just two to four weeks.
And the best way to slow its spread would be to have workers stay at home.
That's the scenario drawn from results of a computer model created by researchers at the U.S. National Institutes of Health's Fogarty International Center. And while the results of that computer model should be interpreted with caution, it is based on data from ordinary flu epidemics for the last three decades, said study author Dr. Mark A. Miller, associate director for research at the center.
"The unique feature of this model is that it challenges conventional wisdom, which says that flu is spread by children bringing it back to the household," Miller said. "That may be true at the household level, but regionally it is spread by adults."
That's why measures to keep people at home could slow the spread of infection, Miller said. Another finding in the study is that states with large populations, such as California, are more likely to reach epidemic levels of the flu at the same time than less-populous states, where transmission tends to be more erratic, he said.
So California, the most populous state, would be the most logical place for a pandemic to start, Miller said. Another factor pointing toward California is that bird -- also called avian -- flu is expected to arrive from Asia, he said.
As for the speed of spread, the estimate is based on ordinary epidemics. "What we see is that epidemics with more pathogenic viruses spread more quickly, two to four weeks versus five to seven weeks for less pathogenic viruses nationwide," Miller said.
The findings appear in the March 31 issue of the journal Science.
The Fogarty researchers used epidemiological data on seasonal flu epidemics that have occurred yearly in the United States since 1972. They connected that information with data from the Census Bureau and the federal Department of Transportation, looking at variations in yearly epidemics from state to state and links with local flows of people to workplaces.
Bird flu is pathogenic, but it does not yet spread easily from person to person; close exposure to an infected bird is needed to cause a human infection. The danger will come when, and if, a mutation makes human-to-human transmission easy.
Since 2003, the H5N1 bird flu virus has been detected in 45 countries in Africa, Asia, and Europe. More than 100 people have died after coming into contact with infected poultry.
The model developed by the Fogarty researchers can go just so far in predicting what might happen if such a mutation occurs, Miller said. This model notably doesn't include previous pandemics, just ordinary epidemics, and a pandemic might have different characteristics, he said.
Still, the model can help plan for ordinary, predictable epidemics by showing how they start and spread, Miller said. It's also not the first of its kind, he said: "We did a similar model to explain the spread of measles."
For more on bird flu, visit the U.S. Centers for Disease Control and Prevention.