Those smallpox shots millions of Americans got as children may still offer some protection, a new study suggests.
But half of all Americans have never received the vaccine, and many scientists believe protection wanes over time for those who did.
The vaccine has been assumed to offer its best protection for from three to five years.
However, according to a paper scheduled for the September edition of the journal Nature Medicine, lab tests can detect immune response in 90 percent of vaccinated people for many years, some for up to 75 years.
The study, led by Mark Slifka of the Oregon Health and Science University, was prepared for Monday's online issue of the journal.
But the jury is out on a big question: Does the presence of immune antibodies in the blood really indicate protection from the disease, and how much how strong does that antibody response have to be.
"The stuff that they did really does look comforting," Dr. John Treanor of the University of Rochester (N.Y.) School of Medicine and Dentistry said in a telephone interview. "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."
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.
Neither Fauci nor Treanor was part of the research team.
While the last case of natural smallpox occurred in 1977, stocks of the virus remain in labs and concern has been raised that some of the material could fall into the hands of terrorists.
Seeking to be prepared for that possibility, the government has launched a campaign mandating vaccination for about 500,000 members of the armed forces and seeking to get several million health and emergency workers to volunteer for the vaccine.
The vaccine is not being offered to the general public and the Institute of Medicine, a health policy advisory center, last week urged that individuals only get the vaccine as part of closely monitored clinical trials because of concerns about serious side effects.
Meanwhile, Richard Weltzin of Acambis, Inc., in Cambridge, Mass., reports in the same issue of Nature Medicine that he and colleagues are developing a new vaccine adapted from the existing one. In animal models and a human clinical study, he says, the new vaccine appears to be equally effective at inducing immunity and may be safer.
Slifka's team in Oregon looked at the response of antivirus antibodies and by T-cells, a part of the immune system that remember previous encounters with bacteria and viruses and mount a defense when the germs attack again.
They found that while vaccine's power declined gradually, 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. However, 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 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 in Rochester noted, however, there have been cases where people who had been vaccinated became infected later with smallpox, but with a milder case than people 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."