Hepatitis B virus (HBV) has been classified into 10 major genotypes, and HBV genotypes C and B are found in the majority of Japanese patients. However, the prevalence of genotype A has been increasing in patients with chronic or acute hepatitis. Here we report a pediatric case of HBV subgenotype A2. A 2-year-old girl was referred to our hospital for liver damage caused by HBV infection. During the pregnancy, her father had developed acute sporadic hepatitis B. The child was born without any complications. She did not receive HBV vaccination at birth because her mother was negative for HBs antigen at the pre-delivery screening; however, her mother developed acute hepatitis B 2 months after delivery. At that time, HBs antigen was detected in the current patient. Phylogenetic full-length sequence analysis revealed HBV subgenotype A2. HBV sequencing was not performed for her parents; therefore, the intrafamilial transmission routes in these cases are unclear, although the authors speculate that, for the current patient, mother-to-child transmission may have occurred. This report illustrates the pitfalls of the selective vaccination strategy in Japan for preventing HBV infection. Universal vaccination to prevent HBV infection might be useful in Japan.
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