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Washington --- Americans vaccinated for smallpox 50 or more years ago still may be immune to the disease, scientists said Sunday.
Scientists had feared immunity to the disease had "run out" for people vaccinated prior to the mid-1970s, when vaccinations ended in America.
New research shows that smallpox-specific antibodies remain in the blood of persons who received the vaccine as long as 75 years ago, said Mark Slifka, a viral immunologist at Oregon Health and Science University.
Less plentiful but also present are white blood cells --- search-and-destroy T-cells --- that are specific to smallpox, he said. Together, the antibodies and the T-cells mean that people who have been vaccinated probably are either immune or resistant enough to have only mild cases, he said.
"That's good news for the people who were vaccinated, and for people who have never been vaccinated also," Slifka said in a telephone interview, "because it means that with from 145 [million] to 150 million Americans walking around with resistance to the smallpox virus, our 'herd immunity' is high."
That means that if the virus were deliberately introduced into the United States, it would spread much more sluggishly than had been feared.
Scientists had assumed that half of the population has never been vaccinated and the other half received its vaccine so long ago that it would not be effective. On this basis, they believed the overall population would be highly vulnerable and the disease would spread rapidly.
Natural smallpox was eliminated in the 1970s by global eradication campaigns organized by the World Health Organization and the Centers for Disease Control and Prevention in Atlanta. Two small samples of the virus were preserved, one at the CDC and one in the Soviet Union.
With the end of the Cold War, U.S. intelligence and public health officials have concluded that the Soviets violated agreements and experimented with the smallpox sample, possibly developing large reserves of the virus for use as biological weapons. Since the whereabouts of this virus is now unknown, the U.S. Departments of Health and Human Services and Defense have contracted for development of new vaccines.
Smallpox immunity results from having a related disease, vaccinia. Since Slifka and his associates do not have access to smallpox virus, they exposed serum samples of hundreds of people who have been vaccinated to new vaccinia virus, using the vaccinia as a stand-in for smallpox. The tests revealed vigorous antibody action and smaller quantities of T-cells.
Since the antibodies and T-cells that react against vaccinia are the same ones that protect against smallpox, the scientists concluded that smallpox immunity also remains high. Slifka and other scientists at the Oregon university published their conclusions Sunday in the journal Nature Medicine.
Copyright 2003 The Atlanta Journal-Constitution
