Hepatitis B was long called serum hepatitis, a name that reflects the belief that the virus spreads only by percutaneous inoculation of blood. The name is too narrow. The hepatitis B virus (HBV) is highly infectious, and although blood carries by far the highest concentration of virus, transmission also occurs through non-percutaneous routes. Sexual contact is one; close contact between young children is another; and passage from mother to child around the time of birth is the third.
Perinatal transmission is uncommon in the USA and western Europe, but it matters greatly in developing countries, especially in East Asia. It arises most often from chronic hepatitis B in the mother and less often from acute infection acquired during the third trimester of pregnancy. The exact mechanism is not clear, but transmission is thought to happen mainly during delivery. Its efficiency follows the mother’s viral replication: the higher the HBeAg level and the more actively the virus replicates, the greater the newborn’s risk, and the mother is HBeAg-positive in more than 90% of mother-to-child transmission events.
The age at which infection is acquired differs with region, and that difference shapes the disease. In East Asia and Africa, HBV behaves as an infection of the newborn, acquired from the mother. In the USA and western Europe, it behaves as an infection of adolescents and young adults, acquired through new sexual relationships. The two patterns are mirrored in the prevalence of HBsAg seropositivity, which runs at 0.1-0.5% in the general populations of the USA and western Europe but 5-10% in East Asian, sub-Saharan African and tropical countries.
Groups that carry a higher burden of infection include people with Down syndrome, lepromatous leprosy, leukemia, Hodgkin lymphoma, chronic renal failure on hemodialysis, and polyarteritis nodosa, as well as people who inject drugs. Groups that are exposed more often include spouses of acutely infected people, men who have sex with men, prisoners, healthcare workers who handle blood products, and people who need repeated blood transfusions. Transmission through transfusion has become rare with more sensitive screening tests; the residual risk is about 1 in 230,000.
In recent years, the main driver of rising HBV prevalence in Europe has been migration from endemic regions. Public-health measures have pushed in the other direction. Screening programs and vaccination reduced the incidence of HBV in the USA by 80% during the 1990s, and worldwide the prevalence of seropositivity in children aged 6 to 19 years fell by 85%.