A new study shows that millions of people who received smallpox shots decades ago still retain some immunity to the virus. This could help protect them if bioterrorists launch a smallpox attack. But many experts say this leftover immunity is unlikely to be strong enough to be useful.
The immune system has its own memory. Once a microbe enters the body, white blood cells mobilize to attack it and then remain on guard for years to come, by remembering how it looks and acts.
Because of the potential threat of bioterrorism, immunologists at the Oregon Health and Sciences University wanted to know how well people's immune systems remembered smallpox. How immune are they 25 years after the disease was eradicated and inoculations ended?
Oregon researcher Mark Slifka and colleagues took blood samples from 332 people and measured how many disease-fighting cells responded when confronted with a close relative of smallpox, the vaccinia virus, which is the basis for smallpox vaccine. "What we found is that a substantial number of these people did have measurable immunity indicating in some cases lifelong immunity following a single smallpox vaccination," he says.
The lab tests detected at least partial immunity in 90 percent of the participants, some vaccinated as long ago as 75 years.
The scientists measured the two components of the immune system - antibodies and T-cells. Antibodies try to neutralize a microbe after it first enters the body. T-cells are a second line of defense that responds if the microbe enters the cells.
As Mr. Slifka's team reports in the September issue of the journal Nature Medicine, they found that antibody responses remain strong over time, but the T-cell responses to smallpox weakened.
No one knows what minimum immune response is protective against smallpox. But Mr. Slifka says any level is helpful, especially the high levels among 38,000 U.S. health and emergency workers who have recently been vaccinated under a voluntary government program.
"The expectation would be that they would be fully protected. Others have lower degrees of immunity that are easily detected and it would probably be considered partial immunity," says Mr. Slifka. "What good is partial immunity? In terms of smallpox, it could be the difference between life and death."
However, U.S. government public health officials say the study does not change their view that new vaccinations are needed to offer maximum protection against a bioterrorist attack.
Another smallpox vaccine expert agrees. Thomas Monath of the Cambridge, Massachusetts biotechnology firm Acambis, Incorporated says Mr. Slifka's laboratory tests might not be applicable in the real world. "The measurement of immunity by the sophisticated assays that are currently available have to be put in the context of the long experience, over two centuries, with smallpox vaccination that clearly shows that the longer that interval between vaccination and exposure is, the more susceptible you are to dying of smallpox if exposed or getting severely ill," he says.
Mr. Monath says the Oregon researchers have overestimated the protection that old smallpox vaccinations offer.
The issue of residual immunity is important because medical experts recently recommended against widespread smallpox inoculation, despite Bush administration efforts to make the vaccine available to all who want it. The problem is that it contains bits of live virus that can cause illness in some people and be transmitted to those around them. Experts, led by University of Pennsylvania medical professor Brian Strom, say the potential health risks of the vaccine outweigh the threat of a bioterrorist attack.
"We think, in general, the ethical problems here are major and that in the general sense, it is not warranted for the normal public to get it," says Mr. Strom.
In Cambridge, Massachusetts, Thomas Monath says there is a debate about whether to vaccinate when no clear bioterrorist threat appears imminent. "The risk-benefit equation is one of weighing the threat and the need to be prepared against the likelihood there will be some serious adverse events," he says. "This is not an easy thing and it is controversial and groups have different opinions."
In addition to the 38,000 voluntary inoculations of U.S. health and emergency workers so far, the Bush administration is requiring half a million military personnel to get smallpox shots.