Neutralizing antibody production in vaccinated humans hasbeen shown to correlate with expression of various genes asso-ciated with innate and cell-mediated immunity, indicating theintegral nature of the adaptive immune system in humans [86].Mice that are incapable of producing poxvirus specific IgG and IgMare not protected by post-exposure MVA vaccination (intravenous)at day 3 post-exposure whereas wild type mice are which indicatesthe importance of neutralizing antibodies [52]. The time it takes avirus to spread to the point where post-exposure vaccination is notbeneficial will depend in part on the incubation period of the virusand the kinetics of the immune response. For example, primaryexposure requires 7–13 days to produce neutralizing antibodiesagainst vaccinia in human studies [87–89], and smallpox in humanshas a combined incubation and prodromal period that lasts 7–17days with a mean of 12.5 days [90]. Based on these data, smallpoxcontacts who have been exposed and have a shorter incubationperiod (