This report describes a rare horizontal transmission of hepatitis B virus (HBV) from an unvaccinated 6-year-old boy to his father. The father had been diagnosed with acute hepatitis B 1 month earlier; therefore, when the child visited the clinic with fever, he was screened for HBV markers and diagnosed as an asymptomatic carrier. Neither the child nor his father was vaccinated against HBV, whereas the child\'s mother and sister, having received the HBV vaccination as they were medical staff and a nursing student, respectively, tested negative for the hepatitis B surface antigen (HBsAg) and positive for anti-HBs. We performed a phylogenetic analysis of HBV in the child and his father, and identified 100% homologous strains of identical genotype C. At diagnosis, the father tested positive for IgM anti-hepatitis B core with a high titer, whereas the child tested negative for this marker. These data strongly indicated a child-to-father transmission. In this case, the HBV infection route was speculated as close contact including saliva-based transmission between the child and father, mainly attributed to their daily food habits. When clinicians diagnose patients with acute or chronic HBV infection, the household members should have been examined for HBV markers immediately. If some household members are susceptible to HBV infection, all members should be vaccinated against HBV.

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.