WASHINGTON (AP) _ Millions of Americans already vaccinated against smallpox may retain at least some protection many years later, a study indicates.
Medical experts have assumed that protection from the vaccine lasted three years to five years and declined after that.
A report scheduled for the September edition of the journal Nature Medicine indicates that lab tests can detect immune response in 90 percent of vaccinated people for many years, some for as long as 75 years.
The study, led by Mark Slifka of Oregon Health and Science University, was being published in Monday's online issue of the journal.
While Slifka and colleagues were able to measure immune response in people's blood, the question remains whether that response is strong enough to actually protect from disease.
How much response is necessary for protection is unclear.
``The stuff that they did really does look comforting,'' said Dr. John Treanor of the University of Rochester School of Medicine and Dentistry in Rochester, N.Y.
``But it wouldn't be safe to assume that you were going to be immune to the disease based on the fact that you can still detect some immunity,'' said Treanor, who was not involved in the study.
Dr. Anthony Fauci, director of the National Institute of Allergy and infectious Disease, agreed that the relationship between immune response and protection from disease is not well understood.
Fauci noted that the finding does not address the fact that half of Americans have never been vaccinated for smallpox, nor does it change the government's need to pursue vaccine for emergency responders.
People who will be activated to respond in the event of an outbreak need optimal protection and to get that they need a recent vaccination, he said.
There has not been a natural case of smallpox in the world since 1977. Still, stocks remain in labs and there is concern the virus could fall into the hands of terrorists.
Preparing for that possibility, the government has required vaccination for about 500,000 members of the armed forces and sought to get several million health and emergency workers to volunteer for the vaccine.
The vaccine is not currently being offered to the general public. Last week, the Institute of Medicine, a health policy advisory center to the government, urged that individuals get the vaccine only as part of closely monitored clinical trials because of concerns about serious side effects.
In the same issue of Nature Medicine, Richard Weltzin of Acambis, Inc., in Cambridge, Mass., reported that he and colleagues are developing a vaccine adapted from the existing one. In animal models and a human clinical study, the researchers report, the vaccine appears to be equally effective at inducing immunity and may be safer.
Slifka's team looked at the response of antivirus antibodies and by T-cells. They are a part of the immune system that remembers previous encounters with bacteria and viruses and mount a defense when the germs attack again.
Researchers found that while T-cell memory of vaccinia gradually declined over the years, most people had a consistent antibody response for many years.
Based on this finding, the researchers said the risks from any smallpox outbreak would be less than had been thought for people who had previously been vaccinated. But people born since the mid-1970s, when routine vaccination was stopped, would still be in danger.
``This research shows that significant immunity levels last for many decades, perhaps throughout a person's entire life,'' Slifka said.
They studied more than 300 people who had been vaccinated; some of them several times.
The finding ``also shows that repeated vaccinations provide a short-term boost in immunity but, over time, do not create a sustained higher level of protection compared to those persons vaccinated only once,'' he added.
Treanor said, however, that there have been cases where people who were vaccinated later became infected with smallpox, but had a milder case than people who were never vaccinated.
``The risk increases the further you get from vaccination, but it's never as bad as not having been vaccinated at all,'' he said. ``It's not like you're protected against smallpox one day and you wake up the next day and you're not.''