Smallpox vaccine provides more protection than previously thought

Researchers at Oregon Health & Science University this week announced preliminary study results showing smallpox vaccine protection lasts longer than previously thought. Until now, it was widely accepted that smallpox vaccine protection lasted approximately three to five years. However, early study data shows that significant, partial protection may last many decades after inoculation.

“More than 90 percent of Americans older than 35 have already been vaccinated against smallpox. This translates into about 150 million people who are likely to have at least some level of detectable immunity against this disease,” said Mark Slifka, Ph.D., a researcher at the OHSU Vaccine and Gene Therapy Institute. Slifka and his colleague Erika Hammarlund presented the initial findings at this week’s meeting of the American Society for Microbiology in Washington, D.C.

To conduct their research, Slifka and his colleagues studied blood samples taken from 306 people who had received smallpox vaccinations. These immunizations occurred as recently as one month prior to testing and as long ago as 75 years. Scientists concentrated on two types of immunity in these patients. The first form of immunity is linked to levels of antibody produced in the body in response to the vaccine. These antibodies protect patients by forming the first line of defense against smallpox if an exposure occurs. The second form of immunity is T-cells programmed by the vaccine to attack the smallpox virus.

“What we found was that while T-cell immunity declines slowly over time, antibody immunity can last throughout a person’s life,” said Slifka. “Remarkably, we were able to identify some types of virus-specific immunity in volunteers who were vaccinated up to 75 years ago, indicating that their immune systems remembered the virus for three-fourths of a century after smallpox vaccination. These results are likely to have a profound impact on current models and theories on how rapidly smallpox would spread in contemporary populations because immunity following smallpox vaccination is lasting much longer than people expected.”

However, Slifka believes the pre-event vaccination of first-responders, the military, or members of smallpox task force teams remains necessary. “People with a potential occupational risk for exposure to smallpox should still be vaccinated as a safety precaution” he said. “Likewise, in the event of a smallpox outbreak, those who have come into contact with smallpox victims should be vaccinated or revaccinated as a fundamental safeguard against contracting the disease, and this serves as an important measure against spreading smallpox to others.”

Another important finding of the study was that people who had received vaccinations several times in the past had essentially the same immunity level as those vaccinated once or twice. Researchers believe immunizations create a short-term burst of higher immunity that, over time, appears to return to a set point no matter how many times a person has been vaccinated.

The smallpox vaccine’s active ingredient is a virus called vaccinia, that is closely related to smallpox. However, unlike smallpox, vaccinia does not cause serious health problems in most patients. The virus promotes smallpox protection by causing the body to produce protective antibodies and white blood cells that can search for and destroy smallpox-infected cells.

Future studies in the Slifka lab will focus on determining the level of immunity that could be considered protective in the case of an outbreak. This information would help health officials determine which members of the previously vaccinated population are at highest risk. It would also guide emergency health providers in making decisions about vaccination priorities.

“This research helps illustrate how world events have refocused scientific research and resources to better respond to major health threats that could have devastating effects,” added Jay Nelson, Ph.D., director of the Vaccine and Gene Therapy Institute. “OHSU has proposed a number of initiatives to aid in this national effort.”

Slifka and many of his colleagues are part of the newly formed Pacific Rim Biodefense Center (PRBC). The PRBC is a collaborative effort headed by OHSU. Member institutions will investigate new ways to fight infectious diseases – spread through natural causes or bioterrorism – that threaten human health. The PRBC includes the University of Hawaii at Manoa; the University of Nevada, Reno; Oregon State University; the University of Idaho; Princeton University; and Pacific Northwest National Laboratory (PNNL).

In addition to the PRBC, OHSU has proposed expansion of research and facilities aimed at developing vaccines to combat infectious disease. More information about this effort, called the Pacific Rim Vaccine Initiative, can be found at www.ohsu.edu/prvi.