Clinical and immunological responses to undiluted and diluted smallpox vaccine with vaccinia virus of Lister strain

Abstract The potential to increase the supply of vaccine by diluting the vaccinia virus of Lister strain to face possible bioterrorism with smallpox was evaluated. Vaccinia-naive subjects ( n  = 97) were randomized to receive either undiluted or diluted (1:5, 1:10) vaccine, and previously vaccinated subjects ( n  = 122) were randomized to receive either undiluted or diluted (1:10, 1:30) vaccine. Except two subjects who received 1:30 diluted vaccine, the vaccination of all subjects was successful clinically. All subjects had significant vaccinia-specific T cell and antibody responses. The diluted vaccine was not associated with decreased local or systemic reactions, lower T cell responses, or higher antibody titers when compared with undiluted vaccine. Here we show the diluted vaccine of Lister strain can be used in vaccinia-naive subjects and previously vaccinated subjects if viral titer ≥10 8 and 10 7.5  pfu/mL after dilution, respectively. The reactogenicity of vaccinia virus may not be a dose-dependent response.