[Viral hepatitis A: when a changing epidemiology implies a different prophylaxis]

Major progress has been made in the epidemiology and prophylaxis of hepatitis A virus (HAV) since it was first discovered in stool samples by MacCallum in 1973. Seven types of the virus have been described. Types 1, 2, 3 and 7 occur in humans and types 4, 5 and 6 in monkeys, but genotype is not correlated with antigenic response. Immunization with the HM175 viral strain or natural infection by any of the virus types gives protection against all types of HAV affecting humans. Serological tests detect all types of the virus and make it possible to carry out epidemiological studies anywhere in the world. HAV is common in very young children in developing countries. Most ten-year-old children are immune and have no clinical signs of the illness. HAV infection has become less common in developed countries due to improvements in hygiene. Less than 40% of people under the age of 30 have been immunized. Non-immune individuals have a high risk of developing clinical hepatitis if they become infected and the older the patient at the time of infection, the more severe the disease is likely to be. The introduction of HAV into groups in which the majority of individuals have not been immunized may result in major local outbreaks. Prophylaxis schemes are developed on the basis of epidemiology. Vaccination is not routinely recommended in developing countries, except for visitors to those countries. Vaccination is recommended for individuals in high-risk groups, such as health workers, cooks, sewer maintenance engineers and tourists, in developed countries. In countries with an intermediate level of risk (East Europe, the Middle East and Southeast Asia), there is a high level of HAV infection and a low level of immunization. Vaccination programs are likely to be of value in these countries.