HDV superinfection can only occur in patients with HBV infection because HDV requires the HBsAg protein for its assembly. Therefore, it is expected that the removal of serum HBsAg (from HBV SVPs) and unmasking of the anticipated, pre-existing anti-HBsAg response by REP 2139 will result in the clearance of HBV and HDV from the blood. Furthermore, the enhanced effect of immunotherapy in the absence of serum HBsAg has the potential to provide a durable control of both HBV and HDV infection that will persist after treatment.