An outbreak of hepatitis A occurred in a religious community in Indiana, USA. Sixty-nine
cases were ascertained among the 4466 residents over a year, and the highest attack rate was in
children. The management of the outbreak included the widespread use of prophylactic
immune serum globulin (ISG). Despite this, further cases occurred. To guide further ISG
administration, a survey was undertaken to ascertain what proportion remained susceptible to
HAV infection. From a random sample of 600 people in the affected community 440 saliva
specimens (73%) were obtained. Of these, 12·5% were found to be immune (95% confidence
intervals from 9–16%). No changes were made to the ISG administration policy. There was no
evidence to suggest that administration of ISG had any effect on the duration of the outbreak.
There was a low rate of symptomatic infection among young children (less than 10 years); as
ISG does not prevent the spread of the virus its use is not recommended in future outbreak
situations.