A mathematical model of hepatitis a transmission in the United States indicates value of universal childhood immunization. 
US recommendations issued in 1999 for hepatitis A (HA) childhood immunization varied according to regional HA incidences prior to vaccination.  Mathematical models of HA transmission, especially those accounting for herd protection, can be useful in formulating new, highly effective recommendations that could lead to disease elimination.  A mathematical model of HA transmission was designed to assess the impact of different vaccination strategies on the evolution of HA infection over time in the United States.  The model represents HA transmission dynamics and is stratified by age and regions defined in the Advisory Committee for Immunization Practices 1999 recommendations.  The model accounts for herd protection and HA importation, using an age-dependent "force of infection" varying over time as a function of the prevalence of subjects with infectious HA.  The model predicts a clear benefit of vaccinating all US children at as young an age as possible.  Nationwide routine immunization at 1 year of age with 70% coverage would prevent 57% of additional cases during the period 1995-2029, compared with the continuation of the regional strategy of vaccinating children at 2 years of age, as recommended by the Advisory Committee for Immunization Practices in 1999.  In contrast, the model also predicts that nationwide routine immunization for children 12 years of age only would result in a 14% increase of HA cases during the period 1995-2029, compared with the number of cases predicted with the regional strategy of the immunization of 2-year-olds.  These findings highlight the importance of accounting for herd protection induced by early childhood HA vaccination.  They also support the very recent Advisory Committee for Immunization Practices recommendations for universal HA immunization of 1-year-olds.