Roche’s influenza drug Tamiflu suppresses the often deadly Avian flu strain seen in Vietnam, which experts fear will soon cause a human pandemic, U.S. researchers said on Monday.

They said tests in mice showed the drug, licensed for use against influenza in general, could suppress the newest strain of H5N1 virus that is sweeping though flocks of poultry in Vietnam, Cambodia, China and elsewhere in Asia.

Public health experts say the avian flu virus is mutating and fear it could develop the ability to spread easily from person to person and kill millions in a flu pandemic.

The H5N1 strain has killed more than 50 people in Asia since 2003. More than 140 million chickens have been killed in the region in a bid to halt the disease.

“We need to know whether antiviral drugs can prevent and treat avian flu, because in the early stages of a global outbreak, most people would be unvaccinated,” said Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, which funded the study.

“If a pandemic occurs, it will take months to manufacture and distribute a vaccine to all who need it.”

The team at St. Jude Children’s Research Hospital in Memphis, Tennessee, tested 80 mice with the drug, known generically as oseltamivir.

None of the mice that got a placebo and then were infected with the Vietnam strain of H5N1 survived. Five of 10 mice given the highest daily dose of oseltamivir for five days survived.

But writing in the “Journal of Infectious Diseases,” the researchers said eight of 10 mice given the drug for eight days lived.

This will help experts decide how much drug to use and how long to treat people should the virus begin to spread among humans.

The researchers found the new Vietnam strain is much more virulent than a 1997 variant of H5N1 that killed six people in Hong Kong.

“The H5N1 avian flu viruses are in a process of rapid evolution. We were surprised at the tenacity of this new variant,” said St. Jude researcher Elena Govorkova. “Our results provide baseline information that will be needed for further studies on preventing and treating avian flu with antiviral drugs.”

Governments are buying and stockpiling doses of Tamiflu to use in case of an avian flu pandemic, but experts say they will need many more than the few million doses now on hand.