"This is a sort of interim measure," said Norman Baylor, director of the FDA's Office of Vaccines Research and Review in Rockville.
In clinical trials, the vaccine for the H5N1 strain of bird flu provoked an immune response in 45 percent of people. By contrast, vaccines for a seasonal flu protect 70 percent to 90 percent of those who get a shot, according to the FDA.
The bird flu vaccine, made by Sanofi Pasteur of Swiftwater, Pa., has other drawbacks. The time it takes to administer the two required 90-microgram doses - about a month - was longer than health officials would like, Baylor said. The amount of flu protein needed for a single course of the vaccine was also high, limiting the number of shots available.
Despite the drawbacks, epidemiologists said the vaccine offered a solid start. "Anything you can do to get a margin of safety or survival, people are going to want that," said Dr. Scott P. Layne, a public health expert at the University of California, Los Angeles.
The H5N1 strain has killed millions of birds and 172 humans since 2003, according to the World Health Organization. Most of the deaths have been in Asia and involved direct contact between birds and people, but experts fear the virus could mutate into a form that is easily transmitted from person to person, triggering a pandemic for which doctors have few defenses.
Sanofi's vaccine, based on a virus isolated in Vietnam, will not be available commercially. The federal government has already secured about 6.5 million doses.