Skin test predicts remaining immunity to smallpox

NEW YORK (Reuters Health) - Skin testing with killed vaccinia virus, which is related to smallpox, is a simple and reliable way of predicting residual immunity to smallpox, a study shows.

"Residual immune response decades after smallpox vaccination is important for public health and vaccine development," the study team notes. The current findings suggest that skin testing with inactivated vaccinia virus is useful in assessing this response, they add.

Dr. Myoung-don Oh, from the Seoul National University Hospital in South Korea, and colleagues administered the skin test to 83 subjects, 63 of whom had a history of smallpox vaccination. Two days later, all of the subjects underwent smallpox vaccination.

The appearance of skin lesions after vaccination was used to determine immunity status.

Pustules 8 days after vaccination indicated a lack of residual immunity, whereas a rapidly evolving area of hardening around a central lesion indicated the presence of immunity, the researchers explain in The Journal of Infectious Diseases.

Thirty subjects showed a lack of residual immunity, 34 displayed residual immunity, and 19 had indeterminant results and were excluded from further analysis.

All told, the skin test had an 85 percent sensitivity and a 97 percent specificity in predicting residual immunity to smallpox.

By contrast, other measures of assessing residual immunity, such as the vaccinia-specific T cell response and the presence of neutralizing antibodies, were not as sensitive or specific as skin testing in predicting residual immunity.

SOURCE: Journal of Infectious Diseases August 1, 2006.