Clinical and immunological study of percutaneous revaccination in children who originally received smallpox vaccine subcutaneously.

In a large multicenter smallpox vaccination study carried out from 1970 to 1973, it was found that 39% of children who were initially immunized by the subcutaneous route and then challenged percutaneously with a standard vaccination did not have measurable neutralizing antibody upon follow-up. Because of this finding, a percutaneous revaccination study was conducted at the St. Louis study center in 1975 and 1976. There were four study groups, which were composed on the basis of route of primary immunization (subcutaneous or percutaneous) and whether neutralizing antibody was detectable following the original percutaneous challenge immunization. Of 52 children revaccinated, all but four had accelerated reactions. There was no difference in size of lesions or day of maximum erythema among the four study groups. Only 66% of children originally vaccinated subcutaneously who did not have postchallenge neutralizing antibody had measurable neutralizing antibody following revaccination. Transformation studies with vaccinia viral antigen before and after revaccination were performed on lymphocytes from 50 children. There was no appreciable differences in responses either before or after revaccination when the four groups were compared. However, the mean stimulation ratio for the total group increased from 2.4 before revaccination to 4.6 3 weeks later. In primary subcutaneous vaccine recipients without pre-revaccination neutralizing antibody, lymphocyte transformation correlated directly with the neutralizing antibody response.