A bird flu vaccine won federal approval for the first time Tuesday as a stopgap measure against a potential pandemic until more effective vaccines can be developed.

The vaccine is the first to win Food and Drug Administration approval for use in protecting humans against the H5N1 influenza virus. It would be used if the strain mutated into a form that spread easily from person to person, sparking a pandemic.

The Sanofi Aventis SA vaccine already is being stockpiled for use in an outbreak of bird flu. It will not be commercially available. Approval came on the recommendation of FDA advisers, who in February said the vaccine would be better than nothing.

"The threat of an influenza pandemic is, at present, one of the most significant public health issues our nation and world faces," FDA commissioner Dr. Andrew von Eschenbach said. "The approval of this vaccine is an important step forward in our protection against a pandemic."

The government plans to buy and stockpile enough bird flu vaccine for 20 million people, including emergency and health care workers. It also could be given to military personnel before they are deployed abroad.

In a clinical trial, the two-shot series appears to provide protection to just 45 percent of adults who received the highest dose. By comparison, seasonal flu shots, given singly, typically protect more than 90 percent of young, healthy adults.

Since it began ravaging Asian poultry farms in late 2003, the H5N1 strain of bird flu has infected nearly 300 people worldwide, killing more than half of them.

The Sanofi vaccine is given in two 90-microgram doses given about a month apart. The two shots contain 12 times the 15-microgram dose contained in regular winter flu shots.

"You'd like to respond to a pandemic quicker than that and ideally you'd like a vaccine where you had one dose and would need less antigen. Those types of vaccines are under study and development," Norman Baylor, director of the FDA's vaccine office, told reporters, adding later: "At this point, this is where we are."

Those other studies are looking at the use of immune boosters, called adjuvants, to try to improve the effectiveness of the H5N1 vaccine. Eventually, adjuvants also might shrink the needed dose for a pandemic vaccine closer to what's now used in seasonal shots.

Scientists also are studying the development of cell-based vaccines, which would be faster to produce in an outbreak of deadly flu than are current egg-based vaccines.

Sanofi called FDA approval "a first key step" in protecting people at increased risk of exposure to bird flu during the early stages of a pandemic. The shots are for adults ages 18 through 64.

Paris-based Sanofi makes the shots at a Swiftwater, Pa. plant run by its vaccines business, Sanofi Pasteur.

Pandemic flu info: http://www.pandemicflu.gov/