By diluting long-stored doses, the United States now has more than enough smallpox vaccine to protect everyone in case of a bioterrorist attack, a top health official has said.
Testing some of the 86 million doses of vaccine that came to light last March shows that they can be watered down and still offer potent protection against smallpox.
Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said Sunday that the diluted vaccine has been tried on more than 100 volunteers to see if it still works. The results show this cache alone contains enough to vaccinate everyone in an emergency.
"This is very reassuring," Fauci said. He spoke at an infectious disease conference in San Diego sponsored by the American Society for Microbiology.
The last smallpox case in the United States was in 1949, and routine vaccination stopped in 1972. However, the Soviet Union stockpiled tons of the virus during the Cold War, and experts worry some of that might be used as a weapon by terrorists.
Many experts assumed that vaccination would begin again once new smallpox cases were discovered. But last week, government officials said they are planning to offer smallpox vaccine before such an attack to all 280 million Americans.
However, that would involve distributing vaccine that is still being made for the government. The first of that supply should arrive early next year from Acambis Inc., which has been contracted to make 209 million doses.
In the meantime, health officials would have to rely on batches of vaccine left over from decades ago if smallpox is somehow released before the new vaccine is ready. Until recently, the only known supply was 15 million doses of a vaccine called Dryvax, made 20 years ago. Government studies showed it could be diluted 5 to 1, making 75 million doses.
Still, that was not enough. In March, the pharmaceutical firm Aventis Pasteur announced that it was giving the government 86 million doses that had sat nearly unnoticed in a walk-in freezer at a remote mountainside lab in Pennsylvania since the 1970s.
No one knew, though, whether that vaccine, like Dryvax, could be diluted.
Fauci said Sunday that tests show the Aventis Pasteur vaccine can be watered down at least as well as the other vaccine. That means that, when cut 5 to 1, there will be more than 300 million doses.
"We really are fully protected. We have enough doses to cover everyone right now," said Dr. Michael Osterholm of the University of Minnesota, a bioterrorism adviser to Health and Human Services Secretary Tommy Thompson.
Whether everyone would need to be vaccinated, however, is unclear. Many adults were vaccinated in childhood, and even though defenses against the virus fade over time, experts assume many retain some protection against smallpox.
"It is not true that no one in the population is immune to smallpox," said Dr. Julie Gerberding, head of the national Centers for Disease Control and Prevention. "But we can't tell who they are. In event of an attack, we will have to assume no one is immune and respond accordingly."
By DANIEL Q. HANEY, AP Medical Editor