Bird Flu Picks a Genetic Lock

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The most frightening aspect of avian flu has always been its astonishing virulence, but the human death rate in hard-hit northern Vietnam has fallen to 34% this year, down from almost 80% for the entire country in 2004. Good news? Not if you're an epidemiologist. Investigators for the World Health Organization (WHO) have raised concerns that even though the H5N1 bird-flu virus appears to be weakening, it may be adapting better to human beings—potentially opening the door to a flu pandemic.

Researchers have found that as the fatality rate dropped in northern Vietnam, there has been an increase in the number of cases clustered close together and in the age of those infected—signs that the virus may be finding more efficient ways to infect people, including human-to-human transmission, the principal barrier to a pandemic. The falling death rate could mean that this process of adaptation is accelerating. "In gaining the ability to go from one person to another, a virus may well lose its virulence," says Dr. Jeremy Farrar, director of the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Ho Chi Minh City. The 1918 Spanish flu, for example, the worst pandemic in history, had a fatality rate of 2.5%. But it was extremely contagious, infecting hundreds of millions.

The data from Vietnam is still far from conclusive, and the reduced fatality rate may be due to more experienced investigators detecting the sort of mild cases they might have missed last year. But that wouldn't explain the difference between situations in Vietnam's north and in the south, where the death rate has remained high and infections have remained comparatively low. Either way, public-health experts are preparing for the worst. Says Dr. Peter Brown, a WHO epidemiologist: "If we wait until we definitely know there is a problem, it may be too late."