Experts have long said Roche's Tamiflu is not a cure for the avian flu, but the drug is thought to help slow the severity or spread of the disease in the event of an outbreak in humans. Governments have rushed to order stocks, prompting a worldwide run on the drug and criticism of the company for its apparent refusal to share the Tamiflu patent with generic drug makers in this time of potential emergency.

The assessment might be overly optimistic, however, Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis, told reporters Tuesday.

"I believe that anti-viral drugs really represent a tool -- a limited tool," said Osterholm, who also is associate director of the National Center for Food Protection and Defense, an agency within the Department of Homeland Security. "What we don't know is if Tamiflu will work."

Tamiflu and similar drugs can mitigate flu severity and slow disease spread -- if it is taken within 24 hours to 48 hours of infection. The drug has been shown to have some effectiveness against H5N1, the viral strain that has sickened more than 120 persons and killed more than 60 in Southeast Asia.

Studies have suggested Tamiflu has some effectiveness against H5N1, but little real-world data show whether it is effective in infected humans, Osterholm said.

H5N1 replicates more rapidly and infiltrates a wider range of lung cells than do other more common flu forms when they enter the body. The aggressive infection sets off a rapid and massive release of chemicals, which attack the immune system and destroy it.

One key -- but as-yet-unanswered -- question about the drug is whether it needs to be given earlier or in higher-than-usual doses in avian flu cases.

"Frankly we just don't know," Osterholm said.

Limited animal studies have suggested Tamiflu must be administered before infection even occurs in order to be effective, he added.

The federal government has announced plans to increase its Tamiflu stores, from 2 million to more than 20 million doses. The drug is on back order from Roche, because dozens of other governments also have requested the drug.

"It's being sold right now as almost the Cipro of post-9/11," said Osterholm, referring to mass purchases of the antibiotic ciprofloxacin, after mailed anthrax attacks killed 5 people in the United States in the fall of 2001.

"Tamiflu is designed to be active against all clinically relevant influenza viruses and key international research groups have demonstrated, using animal models of influenza, that Tamiflu is effective against the avian H5N1 strain circulating in the Far East," William M. Burns, chief executive officer of Roche's pharma division, said in a statement.

Regarding the criticism that the company has refused to suspend its Tamiflu patent so generic drug makers could help fill stockpile orders, Roche spokesman Terrence Hurley said the company has not refused official requests to do so.

"To date, we have been approached by just one country in the Far East, and are having conversations with them right now," Hurley told UPI.

Sen. Charles Schumer, D-N.Y., told reporters that the company's negotiations with one Asian government "is not good enough."

Schumer said he had contracted Roche Tuesday to request that it grant five licenses allowing U.S. companies to manufacturer generic emergency supplies of the medication. He also said he would formally request that the Bush administration suspend the company's patent if it does not act on the request within 30-days.

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