If pandemic influenza hits in the next year or so, the few weapons the United States has to keep it from spreading will do little, a new computer model shows.

A pandemic flu is likely to strike one in three people if nothing is done, according to the results of computer simulation published in Thursday's journal Nature. If the government acts fast enough and has enough antiviral medicine to use as preventive dosings — which the United States does not — that could drop to about 28 percent of the population getting sick, the study found.

"Both cases we came up with were very pessimistic," said lead author Neil Ferguson of the Department of Infectious Disease Epidemiology at Imperial College in London. "There is no single magic bullet for stopping pandemic flu."

So far this year, H5N1 bird flu — which is not yet pandemic flu because it doesn't move easily between people — has infected 204 people and killed 113, according to the World Health Organization. Most of the human cases and deaths have been in Asia, but birds with the disease have been found in Europe.

Ferguson's computer simulation is the second released this month and is more pessimistic than one led by Timothy Germann of Los Alamos National Laboratory, who said the flu could be less infectious and that efforts could slow it a bit.

Measures such as closing schools to halt breeding grounds and the use of the antiviral Tamiflu could reduce the disease's toll, Ferguson said. But efforts to stop flu from entering American borders — usually on planes with sick passengers — won't work, he said. At most, they can buy a couple of weeks of delay before the disease sets in, he said.

If the United States were like Britain and had enough antiviral medicine for one quarter of the population to be used before people get sick, computer models show that the number of people getting sick would drop from about 102 million to about 84 million in America, Ferguson said.

Bill Hall, spokesman for Department of Health and Human Services, said his agency has 28 million courses of the antiviral (9.3 percent of the U.S. population), but acknowledged that on hand, there's only enough medicine for 5 million people (1.7 percent). The other 23 million courses are on order and should arrive by the end of the year. The plan is to have 81 million courses (27.1 percent) by 2008, he said.

One course of treatment for people involves ten doses.

"Twenty-five percent doesn't go very far and we don't have anywhere near that," said study co-author Donald Burke, professor of international health and epidemiology at Johns Hopkins University's School of Public Health. "If it does occur before we have enough drug and enough vaccine, then the epidemic will have a substantial impact."

If a country gets enough Tamiflu for half its population, it could then act aggressively in dosing families of flu-struck patients and that could cut the flu attack rate by 75 percent, Ferguson said. So instead of 102 million infected people in the U.S., it would be 33 million.

But even Germann, who conducted the more optimistic study, said no one knows which computer model is closer to reality.

"It would have to be a very weak pandemic strain for us to be able to stop it right now," Germann said this week. "Most likely we wouldn't be completely prepared."