Researchers also made key discoveries about long-term immune system responses following inoculation. They found that one component of the immune system retained memory of how to fight smallpox for a much longer period than another immune system component. One form of immunity is linked to levels of antibody produced in the body in response to the vaccine. In study participants, these antibody levels remained relatively stable up to 75 years post-vaccination. The second form of immunity is antiviral T-cells programmed by the vaccine to attack the smallpox virus. In study participants, antiviral T-cell levels declined slowly over time with a half-life of approximately 8 to 15 years.

"We found that while antibody immunity can last throughout a person's lifetime, T-cell immunity declines slowly over time," explained Slifka. "This may help explain curious findings in previously gathered data about vaccinated patients who became infected with smallpox at a later time."

Historical data suggests that immunity against lethal smallpox infection can be maintained for many years after vaccination. However, this same data also shows that the level of disease severity increases with the length of time between vaccination and infection in other words more severe cases occur in infected patients vaccinated many years ago compared to infected patients vaccinated more recently. While Slifka says it's difficult at this point to state whether antibody or T-cell levels correspond to death and severity rates, the possibility of a connection exists.

The next step for Slifka and his colleagues is to track immune system responses for those recently vaccinated, the designated "first responders" in the case of a new outbreak. By studying these individuals, scientists will obtain a better understand
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Contact: Jim Newman
newmanj@ohsu.edu
503-494-8231
Oregon Health & Science University
17-Aug-2003