Worldwide epidemiology of hepatitis A virus infection.

Patterns of hepatitis A virus (HAV) infection and clinical disease differ worldwide, and correlate with socioeconomic conditions (and hygienic and sanitary conditions) of each geographic area. In least developed countries with very poor sanitary and hygienic conditions, HAV spreads readily, and most persons are infected as young children. Because most persons become infected at an age when HAV infection is often asymptomatic, reported disease rates in these areas are low and outbreaks of disease are rare. In developing countries and some regions of developed countries, sanitary conditions are variable, and transmission can predominate in children, adolescents or adults, depending on the geographic region. Paradoxically, since HAV transmission occurs in these areas in older age groups compared with least developed countries where HAV transmission is highly endemic, reported rates of hepatitis A can be higher. In developed countries, sanitation and hygienic conditions are good, and infection rates in children are generally low. Communitywide epidemics can contribute significantly to the burden of disease, as can occasional day care center and common-source outbreaks. In some areas, disease tends to be among specific risk groups, such as travellers to hepatitis A endemic areas, and intravenous drug users among whom hygienic practices may be poor. As countries develop economically with improvement of sanitary conditions, overall endemicity of HAV infection decreases, and disease patterns may change. As the endemicity of HAV transmission decreases, the reported rate of clinical hepatitis A can increase, due to the shift in the average age of infection to an age when clinical illness is more frequent.