To investigate the epidemiology of infection with delta (delta) agent in a Swedish city, 181 chronic carriers of hepatitis B surface antigen (HBsAg) and 599 patients with acute, self-limited hepatitis B were analyzed for delta antigen and antibody to delta antigen (anti-delta). The study covered the period from 1970 to 1981. The delta agent was found to have been introduced to this population in 1973. Markers of infection with delta agent were almost exclusively found in intravenous drug addicts and their close contacts. The proportion of drug addicts who were chronic HBsAg carriers with anti-delta increased with time and reached 72% in 1979-1981. An episode of acute hepatitis was frequently seen in connection with seroconversion to anti-delta. Among the domestic cases of acute, self-limited hepatitis, no simultaneous infections with hepatitis B virus and delta agent were found before 1975. From 1975 to 1980, between 18% and 44% of the drug addicts with acute hepatitis B were also infected with delta agent.

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To determine the prevalence of antibodies to Hepatitis A virus (anti-HAV) among drug addicts, sera collected in a Swedish city during a ten-year period from 234 drug addicts with acute hepatitis B were tested for anti-HAV. The results were compared with the normal population, where only 3.8% of those born after 1950 were anti-HAV-positive. In individuals born between 1941 and 1965, 8.2% in the normal population and 30.2% of the drug addicts were anti-HAV-positive (p less than 0.001). The level of immunity to hepatitis A among drug addicts ranged from 7.7% to 60% during the ten-year period. Low levels of immunity were seen in the years preceeding outbreaks of hepatitis A among drug addicts. These outbreaks occurred in a cyclic pattern. Higher levels of immunity were seen after each outbreak.

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