An experimental vaccine appears to be effective against a strain of flu virus that experts fear could spark a devastating pandemic, offering the first evidence that any inoculation could provide a powerful weapon against the deadly microbe, a federal health official said Saturday.

Two doses of the vaccine produced an immune system response potent enough to neutralize the virus in tests on 113 volunteers who were injected as part of a federally sponsored study being conducted at three U.S. universities.

“This is very good news,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. “This is the first vaccine that anybody has that has been tested to show that you can actually produce a robust immune response.”

Public health authorities are alarmed by a strain of flu virus known as H5N1, which has been spreading primarily in birds across Asia and in Russia. It also has infected nearly 100 humans in the past 18 months, killing about half of them. If the virus starts to spread efficiently among humans, experts fear it could trigger a global pandemic that could kill millions.

Much more testing will be needed to determine exactly how the vaccine could be used, and other hurdles remain, including being able to produce and distribute large quantities of vaccine in the event of a pandemic, Fauci said. But he said the results represent a crucial milestone.

The federal government has already purchased 2 million doses of the vaccine and plans to buy more. The results will help officials determine how to package the vaccine for use if necessary.