Vaccines based on recombinant viruses have gained a great interest because of their ability to stimulate robust humoral and cellular immune responses. Viral vectors were investigated as prophylactic and therapeutic vaccines against many human pathogens such as measles virus, herpes simplex virus (HSV), human papillomavirus (HPV), HIV, and rabies [126] [127] [128] [129] [130] . However, the utility of those recombinant vaccines in the treatment of chronic hepatitis B was not yet evaluated. Preliminary results obtained from the study in chronically HBV-infected chimpanzees immunized with retroviral vector, based on Moloney murine leukemia virus, encoding HBcAg suggest that further investigation of viral-vector based vaccines should be taken into consideration [131] . In the experiment, one of the three therapeutically immunized chronic carrier chimpanzees cleared the virus and showed HBeAg seroconversion. Significant ALT elevations observed in this animal implicate restoration of HBV-specific cytotoxic and humoral responses without causing fulminant hepatitis. Moreover, the other two chimpanzees demonstrated high anti-HBe titers after the therapy and one of them HBcAgspecific CTLs [131] . This study demonstrates not only the benefit of using the recombinant viral-vectors for treatment of chronic HBV infection in primate model, but also the possible advantage of using core antigen-based therapeutic vaccines. Even though the retroviral vector vaccination was well tolerated in the chimpanzees, several clinical trials suggest that gene therapy with traditional retroviral vectors can lead to oncogenesis [132, 133] . Therefore, the usage of another recombinant virus as a carrier of the proteins could be beneficial.