ABSTRACT
Developing countries with an increasing hepatitis A disease burden may target vaccination to specific groups, such as young children, as an initial control strategy. To better understand transmission of hepatitis A virus in such countries, the authors prospectively studied household and day-care/school contacts of cases in Almaty, Kazakhstan. Overall, by the time of identification of symptomatic index cases, half of transmission had already occurred, having been detected retrospectively. The odds of household contacts' becoming infected were 35.4 times those for day-care/school contacts (95% confidence interval (CI): 17.5, 71.7). Within households, younger age of either index cases or susceptible contacts elevated the odds of secondary infection among susceptible contacts: The presence of a case under 6 years of age raised the odds 4.7 times (95% CI: 1.2, 18.7); and compared with contacts aged 14 years or older, the odds of infection were increased to 7.7 (95% CI: 1.5, 40.3) and 7.0 (95% CI: 1.4, 34.3) among contacts aged 0-6 years and 7-13 years, respectively. Young children are appropriate targets for sustainable hepatitis A vaccination programs in areas undergoing hepatitis A epidemiologic transition. If vaccine is determined to be highly effective postexposure and if it is feasible, vaccinating household contacts could be a useful additional control strategy.