Hepatitis B is caused by hepatitis B virus (HBV), a member of the Hepadnaviridae family. HBV is transmitted from person to person through exposure to infected blood or bodily fluids. Exposure may result from sexual contact with an infected person or perinatal transmission from an infected mother to her infant. Infection may also occur as a result of a needlestick injury from an infected person or from blood transfusions.

Perinatal exposure to hepatitis B results in a high rate of transmission of HBV, largely occurring during blood exposures during labor and delivery. The risk that an infant will acquire HBV from an infected mother is dependent on the mother’s serologic status, with a 70% to 90% transmission for infant born to mothers who are hepatitis B surface antigen (HBsAg)-positive and hepatitis E antigen (HBeAg)-positive, and 5% to 20% transmission for infants born to mothers who are mothers who are HBsAg-positive but HBeAg-negative. In older children and adults, approximately 5% of persons who develop an acute hepatitis B infection will develop a chronic carrier state, while neonates acquiring hepatitis B from perinatal transmission have approximately 95% chance of becoming chronic carriers. A chronic carrier state increases the risk for hepatocellular carcinoma and liver cirrhosis.