An experimental smallpox vaccine appears to work nearly as well as an older vaccine in protecting against the deadly virus, according to federal documents released Tuesday.

The newer vaccine, ACAM2000, showed by separate measures that it would be nearly as effective as the older vaccine, Dryvax. ACAM2000 is derived from Dryvax, which is no longer made. Both vaccines pose similar risks of serious side effects, including itch, rash and pain as well as rare cases of inflammation of the heart and surrounding sac.

The federal Food and Drug Adminsitration released its review of studies of the vaccine ahead of an advisory committee meeting Thursday.

Even though ACAM2000 remains experimental, the United States already has stockpiled 192.5 million doses of the vaccine, according to its manufacturer, Acambis Plc. Acambis anticipates its vaccine, if approved by the FDA, will be used on deployed military personnel and others at risk of smallpox if there is an outbreak.

Acambis also awaits a contract, pending approval, to maintain the ongoing capacity to make more of the vaccine. Dryvax is currently the only U.S.-approved smallpox vaccine. However, Wyeth Pharmaceuticals Inc. no longer makes it.

A panel of FDA advisers is scheduled to review Acambis' application on Thursday and make a nonbinding recommendation on whether ACAM2000 merits approval. Acambis hopes to win such approval by Aug. 31.

The U.S. ended routine childhood vaccination against smallpox in 1971. After the Sept. 11, 2001, terrorist attacks, however, the Bush administration ordered some military personnel vaccinated and recommended shots for front-line health care workers. The government since has stockpiled enough smallpox vaccine for everyone in the country.

Acambis Plc is based in Cambridge, England, and Cambridge, Massachusetts. The company ran into trouble in November when it failed to win a further U.S. government smallpox vaccine contract.

Dryvax and ACAM2000 both are made using the vaccinia or cowpox virus, which is closely related to the smallpox virus.

In 1980, world health authorities declared smallpox eradicated. The virus had been fatal in about 30 percent of cases.