The Food and Drug Administration approved the United States' first human vaccine for bird flu Tuesday, saying it could slow a possible pandemic despite its modest effectiveness.
"This is a sort of interim measure," said Norman W. Baylor, director of the FDA's Office of Vaccines Research and Review in Rockville, Md.
In clinical trials, the vaccine for the H5N1 strain of bird flu provoked an immune response in 45% of people. By contrast, vaccines for a seasonal flu protect 70% to 90% of those who get a shot, the FDA said.
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 also was high, limiting the number of shots available.
Despite the drawbacks, epidemiologists said the vaccine offered a solid start in the fight against a potential bird flu pandemic.
"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 UCLA.
The H5N1 strain has killed millions of birds and 172 humans since 2003, the World Health Organization said. Most of the deaths have occurred 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 of the vaccine.
The government did not need FDA approval to begin building its stockpile, but approval does make it easier to dispense the vaccine to the public, said Bill Hall, a spokesman for the Department of Health and Human Services.
For one thing, the FDA will not have to issue emergency authorization to use the vaccine.
The federal stockpile also includes about 6.5 million doses of another Sanofi vaccine, based on an H5N1 strain from Indonesia, that has not yet received FDA approval.
If a pandemic strain emerges, it will probably take months for manufacturers to prepare a vaccine perfectly matched to it.
In the meantime, the Sanofi vaccine approved Tuesday could offer some protection for healthcare workers and military personnel who would probably be among those to get the first injections, Hall said.
"It's like an insurance policy," he said.
Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, applauded the FDA approval but said he doubted Sanofi could make enough of the vaccine.
Even if every vaccine manufacturer in the world turned all its production efforts toward producing the H5N1 shots, he said, they would churn out only enough to vaccinate 12 million to 20 million people a year -- a fraction of what is needed in the U.S., let alone the world.
"The reality is, this vaccine will have only a very limited impact on any potential pandemic caused by H5N1," he said.