Quantifying the impact of hepatitis A immunization in the United States, 1995-2001.

Hepatitis A rates have declined to historically low rates in the United States. To assess the degree to which this decline was attributable to immunization, we correlated changes in the incidence of hepatitis A with increases in immunization coverage in a Poisson regression model. In a model allowing for herd immunity, an estimated 97,800 hepatitis A cases were averted due to immunization between 1995-2001, including 39% of potential cases in 2001. Assuming no herd immunity; 32,300 cases of hepatitis A would have been prevented. Sensitivity analysis showed that the number of averted cases in this period could range from 45,500 to 172,900. Among children 2-18 years old, vaccination coverage averaged 10% in 2001 and is estimated to have prevented 51% of cases in this age group. These results suggest that much of the recent reduction of hepatitis A rates is attributable to immunization and that immunization has been associated with a strong herd immunity effect.