Prediction of Residual Immunity to Smallpox, by Means of an Intradermal Skin Test with Inactivated Vaccinia Virus

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BackgroundIntradermal skin testing with inactivated vaccinia virus was evaluated for its prediction of residual immunity to smallpoxMethodsAn 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 smallpoxResultsOf 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-γ-producing T cell response (≥9 spot forming cells/106 peripheral-blood mononuclear cells) were 32% and 63%, respectively, and those of a positive neutralizing antibody (titer, ≥1:8) were 79% and 80%, respectivelyConclusionThe intradermal skin test appears to be a simple and reliable method for prediction of residual immunity to smallpox

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