BACKGROUND Intradermal skin testing with inactivated vaccinia virus was evaluated for its prediction of residual immunity to smallpox.

METHODS An intradermal skin test was performed with heat-inactivated Lancy-Vaxina. Two days later, the subjects were vaccinated with Lancy-Vaxina. The skin lesions resulting from this vaccination were used as a surrogate marker of residual immunity to smallpox, and this surrogate marker was compared with the available indicators of susceptibility to smallpox.

RESULTS Of the 83 subjects, 30 (36%) showed the typical primary response after vaccination (i.e., absence of residual immunity), whereas 34 (41%) showed the typical revaccinee's response (i.e., presence of residual immunity); the remaining 19 (23%) had an indeterminate response and were excluded from the final analysis. The sensitivity and specificity of the intradermal skin test (induration size, >or=4 mm) for prediction of residual immunity to smallpox were 85% and 97%, respectively, whereas those of a positive vaccinia-specific interferon- gamma -producing T cell response (>or=9 spot forming cells/10(6) peripheral-blood mononuclear cells) were 32% and 63%, respectively, and those of a positive neutralizing antibody (titer, >or=1 : 8) were 79% and 80%, respectively.

CONCLUSION The intradermal skin test appears to be a simple and reliable method for prediction of residual immunity to smallpox.