Incidence of hepatitis A virus (HAV) infection in rural Liberia.

To provide background for future hepatitis A vaccine trials, sera were collected from 0- to 4-year-old Liberian infants and their mothers on two occasions an average of 14.75 months apart and tested for antibody to hepatitis A virus (anti-HAV). The prevalence of anti-HAV rose from 2.5% in infants 0-6 months of age to 70% in children 3-4 years of age and did not differ between male and female infants. The annual incidence of new infections was slightly lower in the first year of life (35%) than in the subsequent 3 years, when it averaged 45%. The presence of HBV infection did not affect the incidence of HAV seroconversion. No clinical hepatitis was recognized in the subjects who seroconverted. Dual hepatitis A and B virus infection were observed; these were all clinically inapparent. The extraordinary incidence of HAV infection documented in the present study offers an opportunity for vaccine efficacy trials requiring minimal numbers of subjects.