Immunity against smallpox is retained and can be still be detected for at least 20 years after vaccination. Even people vaccinated in the 1950s may still have a robust immune response after exposure to this serious virus, a research team reports in the Journal of the American Academy of Dermatology.

Routine inoculation against smallpox was discontinued after the disease was considered to be eradicated in 1972, but it was re-initiated at the end of 2002 in reaction to the possible threat of biological warfare.

It was not known if people who were vaccinated earlier were still protected against infection, so individuals vaccinated before 2002 were advised to undergo a repeat immunization.

Since smallpox inoculation campaigns have been renewed, health-care providers should be aware of the variety of responses among previously vaccinated individuals and those who have never been vaccinated, Dr. Eric L. Simpson and associates at the Oregon Health and Science University in Portland advise.

The research team therefore evaluated the immune responses of 23 healthy adults after they received a smallpox vaccine using vaccDryvax (Wyeth Pharmaceuticals, Madison, New Jersey). Nine subjects had never been vaccinated and the other 14 subjects were immunized 24 to 50 years ago, but still had a detectable immune response against the virus.

The current vaccinations were successful and all of the subjects produced a skin sore where the vaccine was injected, which is characteristic of an immune response.

This skin response developed sooner and peaked about 3 days earlier in the previously vaccinated subjects compared with the first-time vaccinees. The diameter of the skim eruption was about twice as wide in those immunized for the first time, averaging about 1.6 centimeters in the repeat vaccinees and 3.7 centimeters in the first-timers.

Antibody response, the production of immune cells in response to exposure to a pathogen, was four-times greater among those who received a repeat inoculation.

Simpson's group believes these findings may help in developing an appropriate smallpox vaccination strategy for large populations based on vaccine history, antibody response, and skin reactions after viral exposure.