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BACKGROUND: Hepatitis B virus (HBV) infections are still a major health issue, with approximately 350 million people chronically infected with HBV worldwide. Information about the minimum copy number of HBV genomes required for infection would be useful as a reference for drug and vaccine development; for monitoring HBV patients during treatment; for screening of blood, organ, and tissue donors; and for regulating nucleic acid amplification assays for HBV.

STUDY DESIGN AND METHODS: Serum samples from chronic carriers (hepatitis B surface antigen–positive and antibody to HBV core antigen–positive) of the three most common subtypes of HBV were studied; their infectivity titers had been evaluated previously in chimpanzees. The genotypes of the HBV samples were determined by DNA sequences and type-specific amino acids of the S gene of HBV. Copy numbers of HBV DNA were quantified by real-time TaqMan polymerase chain reaction (PCR) and by nested PCR applied to limiting dilutions. The copy number determined for each inoculum was compared with previously defined chimpanzee infectivity titers.

RESULTS: The genotypes of the HBV adw, ayw, and adr inocula were A, D, and C, respectively. The concentration of HBV DNA was determined to be 5.4 × 109, 2.5 × 109, and 3.1 × 108 genome equivalents (geq) per mL for serum samples containing the adw, ayw, and adr, respectively. The chimpanzee infectivity titers per milliliter of these initial HBV-containing serum samples were previously determined to be 107.5 for adw, 107.5 for ayw (MS-2 strain), and 108 for adr.

CONCLUSION: The minimal copy number of HBV DNA in chronic carriers of HBV that can infect the chimpanzee model was estimated to be from 3 to 169 geq based upon the three well-characterized inocula.