Extra stocks of vaccine have been found
The long-forgotten stockpile was discovered by drug company Aventis Pasteur in France.
In theory, it is enough to counter virtually any bioterrorism threat involving the virus.
The US had launched a desperate hunt for supplies of the vaccine after the September 11 attacks after it was revealed that its own stocks would prove inadequate in the face of a biological attack.
Other pharmaceutical companies had been racing to produce more supplies as part of a "biodefense initiative".
Still working
The Aventis vaccines are said to have been produced some decades ago - but are thought likely to still be potent against the virus.
It was not revealed how so many doses came to be lost for so long, or who found them again.
It is possible that the vaccines could even be diluted to produce more doses.
Price negotiations
The US is now in negotiations with the firm about the price of the vaccines, and any liability faced by them should their age cause problems in patients.
A spokesman for the federal office of health preparedness described the find as "a great insurance policy".
Currently, the US has an estimated 15m doses of smallpox vaccine, and tests are underway to see if these can be diluted to make them stretch further.
President Bush expressed a wish to have a stockpile which would include a vaccine for every citizen.
One in three dies
The UK government refuses to reveal how many doses of vaccine it has in readiness for biological attack.
Smallpox is one of the most feared potential biological weapons.
The variola virus, which causes it, spreads quickly and kills 30% of the people it infects.
Two governments, the US and Russia, keep supplies of the virus itself following the eradication of the disease in humans more than two decades ago.