Geneva ? Acting on fears of bioterrorism, the 191 World Health Organization members on Saturday formally reversed a long-standing order for the destruction of all smallpox virus stocks and recommended they be retained for research into new vaccines or treatment.

The World Health Assembly, the U.N. health agency’s top decision-making body, decided to back an earlier recommendation by WHO Director-General Gro Harlem Brundtland to drop a 2002 deadline for destroying the virus, held at top security laboratories in the United States and Russia.

In January, the WHO’s 32-member Executive Board approved Brundtland’s policy. Late Friday, a World Health Assembly committee representing all member nations also backed the move.

Under the ruling, no new target has been set for destroying the stocks, which are held at two secure laboratories at the U.S. Centers for Disease Control and Prevention in Atlanta and a similar Russian facility in Siberia. The virus stocks will instead be retained for research into new vaccines or treatment for smallpox. The World Health Assembly will receive updates on the research at its annual meetings.

Kenneth Bernard, U.S. assistant surgeon general, told the assembly smallpox research was necessary because the “events of Sept. 11 have underscored the extent that terrorists are willing to go to.”

“In recent years, experts have come to see smallpox as a No. 1 deadly threat,” and the danger of deliberate use was “small but growing,” he said. “We regard the potential release of smallpox as a critical national security issue, not only for us but for the entire world.”

Research also would help people suffering from HIV/AIDS, whose weakened immune systems could not stand existing smallpox vaccines, he added.

The assembly said that full research results should be made available to all WHO member nations. WHO officials said that decision came after developing countries on the assembly said they feared rich nations would keep the results to themselves.

Smallpox used to kill 3 million to 4 million people per year and left millions more hideously scarred and blind. It was declared eradicated in 1979 after a massive WHO-spearheaded campaign, and the virus samples were placed in the U.S. and Russian laboratories.

In 1996, WHO set a target of mid-1999 for destruction of the virus. But after U.S. and Russian resistance, it extended the deadline to the end of this year.

China previously had called for the destruction of the stocks, claiming their very existence presented an enormous risk to the world. But the Chinese delegation agreed to drop the deadline, provided the research was completed as soon as possible and a new date was set for destruction at a later meeting of the assembly.

Although international teams carry out regular checks of the virus storage facilities to ensure security standards, there have been long-standing fears that samples may have found their way into the hands of so-called rogue nations.