The government ordered enough vaccine Monday to protect an additional 2.7 million people against bird flu, adding to a stockpile for use in an outbreak of the deadly virus.

The $199 million in contracts went to three companies — Novartis AG, Sanofi-Pasteur and GlaxoSmithKline PLC — for 5.3 million doses of vaccine. Two of the 90-microgram doses are required to vaccinate a single person against the deadly Asian bird flu known as H5N1.

When delivered, the newly ordered bird flu vaccines will roughly double the existing stockpile, which now has enough vaccine for about 3 million people. The government plans to eventually buy enough vaccine for 20 million people, including emergency and health care workers.

"Having a stockpile of influenza vaccine that may offer protection against the H5N1 virus is an important part of our pandemic influenza preparedness plan," said Mike Leavitt, secretary of the Department of Health and Human Services.

Researchers are studying techniques that would reduce the amount of antigen — the substance that prompts the body's immune system to respond to the virus — in each vaccine. Using less of the active ingredient could stretch supplies and enable the vaccination of millions more people.

The bulk of Monday's order went to Sanofi-Pasteur for 3.7 million doses, under a $118 million contract.

The government said last week that some of the bird flu vaccine it stockpiled earlier is growing weaker with age. It's not known how long vaccines in the existing stockpile would remain viable.