Durability of vaccinia immunization based on reaction at the rechallenge site

The authors reviewed the immunization records of 621 patients who received vaccinia vaccination in our Occupational Medicine Clinic between July 26, 1994 and October 11, 2001 to evaluate the durability of the vaccine over time, as judged by the patients' local skin reactions following revaccination. Results of the study document a waning immunity to vaccinia immunization in the majority of re-vaccinees. Over 63.4 percent of those previously immunized in the study population were found to lack adequate immunity against vaccinia rechallenge based on their response to it. Of the remaining third, only 6.4 percent elicited an immune response to rechallenge. Overall, 75.5 percent of those receiving vaccinia vaccination in the clinic lacked adequate immunity. Given that this study involved a young, healthy, HIV-negative, pre-screened workforce, the actual projected immunity to vaccinia of the general population should have been significantly lower. Although the study data suggests the persistence of adequate immunogenicity of the vaccinia vaccine stockpile, over the past year an increased frequency (3.7 percent) of vaccine-associated cellulitis was noted, coinciding with the Centers for Disease Control (CDC) decision to allow vaccinia vaccine stockpile to be used after its expiration date of April 2000.