The role of herd immunity in an epidemic cycle of hepatitis A.

An epidemic of hepatitis A took place in Northern Ireland between 1984 and 1990. All ages of persons and geographical areas were involved. Children aged 5-14 years with Belfast addresses were particularly affected. Serologically confirmed cases in this group rose from five cases in 1984 to 93 cases in 1987 and fell to 19 cases in 1990. Of 100 samples of serum of Belfast children aged 5-14 years in late 1985, 23 were hepatitis A IgG positive, 95% confidence limits being 15.2-32.5. Of 100 similar children in late 1989, 19 were hepatitis A IgG positive, 95% confidence limits being 11.8-28.1. Thus a substantial rise in herd immunity may not be necessary for the ending of such an epidemic in a society with good housing and sanitation.