U.K. and U.S. teams used computer models to work out the possible scenarios that could occur if the H5N1 avian influenza virus mutated and became capable of spreading from human to human. The result could be deaths on the scale of the 1918 Spanish flu epidemic that claimed between 20 million and 40 million lives. However, a combination of surveillance and the targeted use of antiviral drugs could halt it, the teams told the journals “Nature” and “Science.”

The models used by both teams were nased on incidences and controls used in Thailand, one of the countries at highest risk from bird flu. More than 50 people have died from the virus in southeast Asia since the first human cases were reported in 1997.

Currently, the H5N1 avian influenza poses a limited threat to humans because it is difficult to transmit from person to person. But health experts fear the H5N1 virus could acquire this ability, causing an influenza pandemic that could kill as many as 50,000 people in the United Kingdom alone.

Professor Neil Ferguson of Imperial College in London and his colleagues found two specific conditions that would have to be met to limit an outbreak of human-transmissible avian influenza to fewer than 200 cases. First, the virus would have to be identified while confined to about 30 people. Second, antiviral drugs would have to be distributed rapidly to the 20,000 people nearest the infected individuals.

The Imperial College group estimated that an international stockpile of 3 million doses of antiviral medication would be sufficient to contain an outbreak. But the drug stockpile would have to be deployable anywhere in the world on short notice. “It’s an enormous undertaking and will require cooperation among governments on a large scale,” Ferguson said.

Another team from Emory University in Atlanta, Ga., led by Dr. Ira Longini, simulated an outbreak in a population of 500,000 in rural Thailand in a variety of settings, including households, schools, workplaces and a hospital. Provided targeted use of antiviral drugs was adopted within 21 days, the Emory team predicted it would be possible to contain an outbreak as long as each infected person was not likely to infect more than an average of 1.6 people. If there was more infectivity than this, household quarantines would be necessary.

“Our findings indicate that we have reason to be somewhat hopeful,” said researcher Elizabeth Halloran. “If — or, more likely, when — an outbreak occurs in humans, there is a chance of containing it and preventing a pandemic.”

She added that early intervention could at least slow the pandemic, reducing mortality until a vaccine matched to the virus strain could be produced.

The World Health Organization said the models would help to improve pandemic influenza preparedness planning. A spokesman said: “Several countries have already purchased stockpiles of antiviral drugs, and WHO has taken steps to establish an international stockpile. National and international stockpiles of antiviral drugs may be an essential component of comprehensive international pandemic preparedness that also includes vaccine development and disease surveillance.”

Another medical expert noted that while this analysis is encouraging, Thailand may not have been the best model. Thailand has been relatively successful in controlling avian influenza, and the majority of the human cases and deaths have occurred in Vietnam. There have been no human cases or deaths in Thailand since the end of 2004, compared with 60 cases and 19 deaths during the same period in Vietnam.

The Vietnamese government also has reported that approximately 70 percent of poultry in the Mekong Delta region is thought to be infected with H5N1, and the country’s health department is implementing a program to vaccinate millions of birds, in both northern and southern provinces, beginning in August.

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