Antibodies to vaccinia, the live virus used in the smallpox vaccine, were present in more than 90% of the 306 people tested, and remained fairly constant whether the participants were vaccinated a year ago or as far back as 1928, says researcher Mark K. Slifka of the Oregon Health and Science University. "From one to 75 years out, the levels were in the same range," he says.

But tests also showed that the level of infection-fighting white blood cells, called T-cells, declined with age over a long period of time, he says. Both types of immunity — antibodies and white cells — are needed to provide full protection, he says.

Between eight and 15 years after the shot, T-cell levels drop by half, Slifka says, but "if you begin with very high T-cell levels that could still be a large number."

Slifka, who presented the findings Tuesday in Washington, D.C., at a meeting of the American Society for Microbiology, says 90%-94% of people ages 36 to 96 were vaccinated at least once, an estimated 140-150 million people in the USA.

He says his studies show greater immunity in people vaccinated twice, the second vaccine acting as a booster shot, but people who received smallpox vaccine even as many as 10 times had no higher immune response than that.

The new study is considered preliminary. But the findings bolster observations from decades ago, when smallpox was still circulating around the world, says infectious-disease specialist William Schaffner of Vanderbilt University. From the '40s to the '60s, he says, doctors saw that people who were exposed to smallpox even 30 years after vaccination "had measures of protection against disease, and that if disease occurred, it was less severe."

A small study at the University of North Carolina found a similar long-term immune response in 13 lab workers vaccinated 35 years ago or longer.