21 May 2003 20:00 GMT

by Mari N. Jensen

Washington, D.C. - People vaccinated against smallpox as long as 75 years ago still have measurable levels of antibodies against the disease, report US microbiologists. The discovery could take pressure of health care services preparing for possible bioterrorist attacks in a country where routine smallpox vaccination ceased over thirty years ago.

Half the US population is older than 35, and of those, 90% have been vaccinated against smallpox, says Erika Hammarlund who has been studying the vaccine at the Oregon Health and Science University in Beaverton. "Therefore we do have some kind of herd immunity [against smallpox]," she told delegates here at the annual meeting of the American Society for Microbiology (ASM). "If there were to be an outbreak, there wouldn’t be the scary scenarios they predicted.”

The Centers for Disease Control and Prevention (CDC) website says that smallpox vaccination “provides high level immunity for three to five years and decreasing immunity thereafter.”

To compare the levels of immunity of those who were recently vaccinated with those vaccinated 25 to 75 years ago, the researchers recruited 306 vaccinated volunteers. In addition, 26 unvaccinated volunteers participated as controls. The vaccinated volunteers, who had been inoculated from one to fourteen times and in 34 different countries, came from 40 different states and the District of Columbia. The researchers took blood from the volunteers, extracted and purified the peripheral blood mononuclear cells (PBMCs), and exposed the cells to smallpox virus. The researchers then assayed the mixtures for CD4 and CD8 cells and also assayed the cells for interferon-gamma (IFN-gamma), an indicator of memory T-cells, and for tumor necrosis factor-alpha (TNF-alpha).

The team found that multiple vaccinations had little effect on long-term CD4 cells and that the half-life of CD4 and CD8 cells was between eight and 15 years. For one measure of immune response, the number of CD4 cells that expressed both IFN-gamma and TNF-alpha, controls had less than ten such cells per million CD4 cells, compared with 586 per million one year after vaccination. Although that response declined with the amount of time since vaccination, a volunteer that had been vaccinated 61 years before still had six times as many double-positive CD4 cells as did the controls. The results are similar for CD8 cells that express IFN-gamma and TNF-alpha: while controls have fewer than ten cells per million CD8 cells, a vaccinated individual had 2,180 per million one year after vaccination and a vaccinated individual had 82 per million 61 years post-vaccination.

Virus-specific antibodies, as detected by enzyme-linked immunosorbence assay (ELISA), were maintained for life after vaccination, reports the team.

Michael Hoffman, a microbiologist at the University of Wisconsin-La Crosse who works with Vaccinia, says the Oregon team’s work suggests that, once vaccinated, “You probably have life-long immunity.”

Childhood infection with other live viruses, such as measles and mumps, confers life-long immunity, says Hammarlund. “That’s where we started – with live virus," she concluded, "why shouldn’t you have immunity for life?”