Washington ? The government said Thursday it was purchasing 75 million doses of a new generation anthrax vaccine under an $877.5 million contract — the first awarded through a federal program to develop and stockpile antidotes to biological and chemical weapons.

The five-year contract with VaxGen Inc. will provide enough vaccine to treat roughly 25 million people. The company expects to begin delivery by 2006. According to the contract, the first 25 million vaccine doses would be delivered within two years and the balance within three years, said Lance K. Gordon, the company’s president.

The doses will be added to U.S. reserves to protect against a terrorist attack using anthrax spores.

VaxGen receives no payment until the vaccine is delivered, Health and Human Services Secretary Tommy Thompson told reporters.

“The company is putting a lot at risk to be involved in this. But we also are doing a giant service to companies like VaxGen” by providing incentives to make the vaccines, Thompson said. “It is going to make America a lot safer.”

President Bush signed the new program, called Project BioShield, into law on July 21 with the promise of spending $5.6 billion to develop remedies against possible bioweapons.

VaxGen of Brisbane, Calif., was one of two biotechnology company competing for the HHS contract to develop the new anthrax vaccine. The company still needs to obtain licensing from the Food and Drug Administration. The FDA will review the company’s vaccine production and individual lot testing to ensure the anthrax vaccine is safe and effective.

The existing vaccine is manufactured by Lansing, Mich.-based BioPort Corp. for the Pentagon under a $245 million contract. In response to a federal judge’s order in late October, the Pentagon halted the mandatory vaccinations for the military — six shots spaced over 18 months.

The military would decide for itself whether to switch to the VaxGen product, but could not use it before the product is licensed, an HHS spokesman said.