Disease patterns vary in areas of differing endemicity and correlate with socioeconomic status and access to clean water and sanitation. In many parts of the world, economic development and improved sanitation and living standards have resulted in significant shifts in the acquisition of HAV infection from infancy and childhood to older ages.4 In developing countries, where infection is endemic, most persons are infected in early childhood when asymptomatic infection is likely. In developed

countries, where the incidence rate is lower, infection typically occurs at older ages when clinical symptoms become more apparent.5 Community-wide epidemics contribute significantly to the burden of disease in developed countries. In these settings, disease tends to occur CX-4945 cell line in circumscribed groups, such as travelers to hepatitis A endemic areas, or as outbreaks among high-exposure groups such as intravenous drug users or men who have sex with men.5-7 Prior to the introduction of vaccines against HAV in the United States, hepatitis A occurred as large nationwide epidemics approximately every 10 years. The last epidemic occurred during the mid-1990s and affected particularly adolescents and young adults,8 causing substantial morbidity and economic losses estimated at $489 million annually.9 Since the introduction of an HAV vaccine in 1995, the incidence of hepatitis

A has decreased markedly for all age groups; medchemexpress however, an estimated 25,000 new infections still occurred in 2007.10 Approximately 50% of all reported PD98059 hepatitis A cases have no specific risk factors identified.11 In cases where risk factors have been identified, the majority of adults

are international travelers, men who have sex with men, or intravenous drug users.6, 7 Host genetic factors have played an important role in determining the differential susceptibility to infectious diseases such as hepatitis B and C, malaria, HIV/AIDS, tuberculosis, and invasive pneumococcal disease.12 In outbreaks of hepatitis A, some people are infected while others remain uninfected after exposure, suggesting that people vary in their susceptibility to HAV infection.5, 6 However, to date, the number of studies of human genetic variation and HAV infection are few, except for two small candidate gene studies,13, 14 and twin studies showing that genetic factors accounted for ≈36% of the total variability in HAV-specific immune response to vaccination.15, 16 There are no reports investigating host genetic factors for hepatitis A in the United States population, though many public health researchers have focused on behavior, environment, and viral factors. Examining host factors may provide insight into pathogenesis and susceptibility to HAV infection and also help to identify and target high-risk populations for vaccination.