Hepatitis A Virus Infections in the United States: Model-Based Estimates and Implications for Childhood Immunization

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Abstract
Objective. The high prevalence of antibody to hepatitis A virus (HAV) in the US population suggests that the incidence of infection is much higher than reported, but the infection rate is difficult to measure directly because of anicteric infection and underreporting. We present a model that reconciles the reported incidence of hepatitis A with the observed prevalence of antibody to HAV and provides an estimate of the true incidence of HAV infection. Methods. In the model, reported incidence of hepatitis A in the United States was adjusted to account first for anicteric infection and then for underreporting and declining incidence over time such that the prevalence predicted by the model approximated that observed in 2 nationwide surveys. Results. The model showed incidence in the susceptible population declining by 4.5% per year. As incidence declined early in the 1900s, the average age at infection increased, leading to a paradoxical increase in the incidence of icteric infection followed by a slow decline. The model estimated approximately 270 000 (range: 190 000–360 000) infections annually from 1980 to 1999, 10.4 times the number of hepatitis A cases actually reported during this period. More than half of these infections occurred in children who were younger than 10 years, most of which would have been clinically unrecognizable as hepatitis. Conclusions. These results suggest a large reservoir of infection in children and that interruption of transmission in children may substantially reduce incidence of hepatitis A overall.