WASHINGTON?The Bush administration signed a contract Wednesday to buy 155 million doses of smallpox vaccine from a British firm in case terrorists try to spread the deadly virus.

The contract with Acambis Inc. will bring the United State’s stockpile to 286 million doses of the vaccine by the end of next year, promising protection for every American should bioterrorists ever attack with the all-but-extinct virus.

The vaccine can be administered four days after exposure to smallpox and still offer protection. For that reason?and because the vaccine can cause some rare but deadly side effects?officials have no plans to resume the routine vaccinations of Americans that ended in 1972.

The government has 15.4 million doses of smallpox vaccine on hand, and each of them will be diluted to create five doses, bringing the on hand total to 77 million.