Prediction of residual immunity to smallpox, by means of an intradermal skin test with inactivated vaccinia virus

Abstract

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, >= 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 (>= 9 spot forming cells/10(6) peripheral-blood mononuclear cells) were 32% and 63%, respectively, and those of a positive neutralizing antibody (titer, >= 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.