U.S. health officials said on Wednesday they have started human tests of a vaccine against avian flu, which experts believe could kill tens of millions of people if it becomes easily passed from person to person.

The vaccine, made by Sanofi Pasteur, will be tested in 450 healthy adults in Rochester, New York; Baltimore and Los Angeles, the National Institute of Allergy and Infectious Diseases said.

“While there have been relatively few cases worldwide of H5N1 avian influenza infection in humans, the public health community is concerned that the virus will develop the capability of efficiently spreading from human to human and thus create a risk for a worldwide pandemic,” NIAID Director Dr. Anthony Fauci said in a statement.

“The initiation of this vaccine trial marks a key advance in our efforts to prepare to respond to an avian flu pandemic.”

The vaccine is made from an inactivated H5N1 avian flu virus isolated in 2004. The Phase I study is meant to test the vaccine’s safety - not whether it protects against the infection that has wiped out millions of birds in Asia and killed dozens of people in Vietnam, Thailand and Cambodia since the end of 2003.

“If the vaccine is shown to be safe in adults, there are plans to test it in other populations, such as the elderly and children,” the NIAID said.

“Between January 2004 and March 11, 2005, there were 69 confirmed cases of and 46 deaths from H5N1 infection in humans reported to the World Health Organization,” it added.

Normal influenza kills between 250,000 and 500,000 people a year globally but avian flu could be much deadlier.

It is unclear how high the fatality rate is, as people have been found to be infected without serious symptoms, and people who were believed to have died of other causes were later found to have been infected with bird flu.

Usually, people catch the virus directly from birds such as chickens or ducks. Geese and wild birds and mammals such as cats can also carry the virus.

“To date, there has been a small number of cases where human-to-human transmission of the virus may have occurred. However, public health experts fear that the virus may evolve into one that is more easily transmitted between people,” the NIAID said.

“If this were to happen, a worldwide pandemic could follow.”

Chiron Corp. of Emeryville, California, also has a U.S. government contract to make an H5N1 bird flu vaccine.