Initial testing showed the first U.S. vaccine against H5N1 bird flu virus is safe, but only partly protective, researchers reported Wednesday in The New England Medicine Journal.

The vaccine sparked a protective immune response in 54 percent of the 451 volunteers who got two shots of the highest dose, which is 90 micrograms of antigen, 12 times the use in the regular winter flu shot.

The experimental bird flu vaccine sparked a slightly lower immune response in 70 percent of the recipients, but scientists could not determine whether such a response is protective.

Humans have never been exposed to the H5N1 bird flu virus, and it takes time for the immune system to ramp up to fight unique types of influenza, said lead researcher Dr. John Treanor of the University of Rochester in New York.

The good news is that the vaccine seems to be safe, even at very high doses.

Researchers are giving the receivers a third dose to see if the vaccine would work better, while other studies are seeking to strengthen the effect of the vaccine at regular dose through adding immune-enhancing chemicals, alum or MF59.

The vaccine is made by a unit of Sanofi-Aventis and based on a version of H5N1 virus culled in Vietnam in 2004. It is unknown if the vaccine could partly protect from a slightly different version that emerged in Indonesia last year.

"We have a long way to go," Dr. Anthony Fauci, chief of the National Institute of Allergy and Infectious Diseases, told a telephone briefing Wednesday.

The test findings mean that the United States has only stockpiled enough H5N1 vaccine to protect 4 million people in a pandemic, he said. That falls far short of the government goal of protecting 20 million Americans.

The latest World Health Organization figures show that H5N1 bird flu has since 2003 caused more than 180 human infections and 105 deaths. Experts fear the virus could mutate quickly to transmit easily from human to human, triggering a global pandemic that could kill tens of millions.

At the current high doses of H5N1 vaccine, vaccine factories worldwide could only produce enough vaccine to fully immunize 75 million people, or 1.25 percent of the global population, Mayo Clinic flu specialist Dr. Gregory Poland said in an accompanying editorial in the Journal.