HBVワクチン接種によるHDV排除に向けて：世界戦略、課題、政策のギャップ

Persistent infection with hepatitis D virus (HDV), also known as hepatitis delta, is considered the most severe form of chronic viral hepatitis. HDV is a defective RNA virus that depends on hepatitis B virus (HBV) for propagation. Despite its global distribution, HDV stays a neglected part of the viral hepatitis agenda, often overlooked in surveillance systems and public health policy. This oversight is particularly concerning given HDV's aggressive clinical course, characterized by more rapid progression to cirrhosis, liver failure, and hepatocellular carcinoma (HCC) compared to HBV mono-infection. Mongolia has the highest incidence and mortality rates of HCC worldwide, with approximately 47% of cases estimated to be attributable to chronic HDV infection. Globally, an estimated 12-25 million people are co-infected with HBV and HDV, although the true prevalence is higher due to insufficient screening and incomplete data collection. Because HDV infection is entirely dependent on HBV, prevention of HBV infection through effective vaccination stands for an indirect yet highly effective strategy to curb HDV transmission. The World Health Organization (WHO), together with the global health community, has established ambitious targets to eliminate viral hepatitis as a public health threat by 2030. However, achieving HDV elimination remains particularly challenging due to limited diagnostic capacity, low awareness, and minimal inclusion of HDV in national hepatitis programs. This review explores the intersection of HDV and HBV, focusing on how expanded and optimized HBV vaccination coverage can serve as a cornerstone of global HDV prevention efforts. We examine epidemiological evidence, scientific rationale, policy developments, and key implementation challenges, with particular attention to high-burden settings such as Mongolia. Finally, we propose strategic recommendations to bridge policy and practice gaps in HDV elimination.