Smallpox Vaccine

In the study, which recruited volunteers from all over the state of Oregon, participants filled out medical questionnaires revealing their vaccination history. Then researchers took blood samples from the participants, and tested for both white blood cells that could kill the virus, and for antibodies against it. In order to compare their results with historical records of immunity following smallpox vaccination, they also used a test that was developed over 50 years ago called the neutralizing assay. "[We] take the serum from a volume and mix it with the virus [and] see if the virus has been neutralized by something in the blood, which is the antibody," says Slifka, who published his research in Nature Medicine.

To make sure the volunteers from Oregon represented a good sample of the entire U.S., the scientists checked where they were originally vaccinated. They found out they represented 42 different states and 34 countries.

"Our study indicates that the vast majority of people that were vaccinated, even 75 years ago, demonstrated some type of antiviral immunity. This indicates that some people have high levels of immunity, others have partial immunity, [and] there is probably about five to ten percent that have lower levels of immunity. But when you compare this at a population level, that still means that there's at least a hundred million people that have strong immunity following the smallpox vaccination. That would at least give partial protection if there were an outbreak of smallpox."

Slifka says that in light of this finding, the federal program to vaccinate healthcare and emergency medical professionals will likely be effective even though it has recruited only 38,000 volunteers instead of the projected 500,000.

"The combination of these first-responders and this preexisting immunity we're finding in the general population would indicate that this has been a successful program," says Slifka. "Even though the numbers haven't reached the original goals of the CDC, each state does now have smallpox task force team members that are able to go into a hot zone if there was in fact a smallpox outbreak."

Slifka says the next step in his research is to be able to determine individual peoples' level of immunity. "We have the technology right now to determine who has high levels of immunity or who has low levels of immunity," he says. "It wouldn't take long at all, within even a year to two years. We can develop these tests and validate them, based on historical data, to determine who would be fully protected against smallpox and who should be revaccinated."
Smallpox is a serious, contagious, and sometimes fatal infectious disease. There is no specific treatment, and the only prevention is the vaccination, which has virtually eradicated the disease. In May 1980, the World Health Assembly certified that the world was free of naturally occurring smallpox.