The epidemiology of hepatitis A in Almaty, Kazakhstan: Background for a comparative trial of hepatitis A vaccine versus immune globulin for postexposure prophylaxis.

Abstract

Hepatitis A remains one of the most frequently reported vaccine-preventable diseases in the United States, and much of the world is at high risk of infection with hepatitis A virus. A comparative trial of hepatitis A vaccine versus immune globulin for postexposure prophylaxis is occurring in Almaty, Kazakhstan. Without a placebo control, the trial depends on a historical background of hepatitis A epidemiology in Almaty for generalization of results. Laboratory-based surveillance was implemented to confirm serologically hepatitis A cases and accurately measure incidence. Contacts of identified cases were assessed serologically to measure the proportion still susceptible to hepatitis A, and susceptible contacts were followed prospectively to determine the frequency of secondary transmission in households and schools. Symptoms were recorded and clinical predictors of infection were also analyzed. Results indicated that Almaty is a city with intermediate endemicity of hepatitis A. Incidence was seasonal, and the highest rates occurred in young children, particularly in the Turksibsky and Zhetysusky districts. Seroprevalence data indicated that persons of older age, Kazakh ethnicity, and from the Turksibsky and Zhetysusky districts were likely to have been immune. Household contact imparted higher risk of infection than school contact, especially for children, but also for adults exposed to child cases. Data also indicated nearly half of household contacts had evidence of acute infection by the time of index case identification. Symptoms were identified in 92% of infected household contacts during follow-up, with vomiting and fever most predictive of infection among those without icteric illness. Implications for the comparative trial are discussed.