Risk of hepatitis B in adolescence and young adulthood.

In countries of low hepatitis B virus (HBV) endemicity, infection occurs mainly in adolescents and young adults (15-34 years old). The most important risk factors for infection are heterosexual activity, homosexual activity and intravenous drug use. In industrialized countries, therefore, HBV infection is classified among the major sexually transmitted infections, as more than 50% of infections are spread in this way. Transmission from mother to newborn and during infancy is of less importance, except in some countries of southern Europe and in some southern states of the USA. The highest concentrations of HBV are found in blood of infected individuals, but HBsAg is also present in semen and vaginal and cervical secretions, which forms the biological explanation for sexual transmission of the virus. In epidemiological studies, HBV is, in general, associated with indicators of sexual activity such as number of lifetime or recent sexual partners, years of sexual activity and a history or serological marker of other sexually transmitted infections. Providing immunity from infection before risk-taking behaviour is adopted should be the major control strategy for HBV infection. Just as for other sexually transmitted infections, this can be best achieved by universal vaccination of young adolescents or infants, or both groups.