Live rVVs expressing either p24 delta or p27 delta were produced and used to immunize woodchuck hepadnavirus carriers. Upon challenge with infectious HDV, circulating HDV RNA levels appeared to be similar in both controls and vaccinees. Although extended follow-up studies of these animals is necessary before making firm conclusions, including an analysis of circulating HDAg levels, these preliminary results provide no evidence for a protective immunity conferred by the rVVs. In contrast, we have shown in other studies that repeated immunization of woodchucks with purified, recombinant p24 delta subunit does confer significant protection against HDV challenge in some of the vaccinees (A. Ponzetto, et al., this volume). The underlying immunological mechanisms responsible for the different outcome of these varied vaccination regimens remain to be elucidated.