(Recasts; adds details, background) By Lisa Richwine WASHINGTON, April 17 (Reuters) - The first bird flu vaccine for people won U.S. approval on Tuesday as an interim measure in case an influenza pandemic strikes before a better immunization comes along. The vaccine made by French company Sanofi-Aventis <SASY.PA> <SNY.N> will not be sold commercially. It is being stockpiled by the government for use if the H5N1 bird flu virus mutates to a form that can spread easily from person to person. The Food and Drug Administration said two injections given 28 days apart may provide "limited" protection if a pandemic occurs. About 45 percent of people who got the vaccine in a study developed an immune response to the virus. The vaccine is "sort of an interim measure" until better ones are developed, said Norman Baylor, director of the FDA's Office of Vaccines Research and Review. Several companies are working on other versions. "Ideally, yes, you would like a vaccine that would have a higher efficacy," Baylor told reporters. A single shot and a lower dose also would be preferred, Baylor said. The dose needed for the new Sanofi vaccine is higher than used in the seasonal flu vaccine. Still, "we feel as part of pandemic preparedness it would be best to have a licensed vaccine. Our review suggests this vaccine is safe and effective," Baylor said. David Williams, president of Sanofi vaccine unit Sanofi Pasteur, said in a statement the approval was "a significant milestone in pandemic preparedness." Sanofi manufactures the vaccine at a plant in Swiftwater, Pennsylvania. The U.S. Department of Health and Human Services said it had already purchased 13 million doses of the Sanofi vaccine, enough to inoculate 6.5 million people. The vaccine was approved for people age 18 to 64. Studies in other age groups are ongoing. The most common side effects reported were pain at the injection site, headache, "general ill feeling" and muscle pain, the FDA said. The H5N1 avian influenza virus has killed 172 people out of 291 known to have been infected, according to the World Health Organization. If it acquires the ability to pass person-to-person easily, it would spark a pandemic and scientists fear it could kill tens of millions of people. If H5N1 does mutate, it is unclear if vaccines developed now would still work against a pandemic strain. Manufacturers could tailor a new vaccine to that strain, but current production methods take months. Research on the Sanofi vaccine was conducted by the National Institutes of Health as part of the government's efforts to prepare for a flu pandemic. Several companies, including Novartis AG <NOVN.VX> <NVS.N> and GlaxoSmithKline Plc <GSK.L> <GSK.N>, are developing bird flu vaccines with adjuvants, substances that can boost immune response. Sanofi's stock closed down slightly on the New York Stock Exchange on Tuesday, off 5 cents at $46.14.
