Pre-exposure prophylaxis of viral hepatitis with immune serum globulin in an endemic area. A trial in field units of the Israel Defence Forces.

Field units in the Israel Defence Forces represent a high risk group for type A infectious hepatitis. Very large scale post-exposure prophylaxis with immune serum globulin (ISG) has been successfully employed for the past decade with a reduction in incidence and an elimination of overt epidemic cycles. The use of pre-exposure prophylaxis in an indigenous population in whom approximately 70% have antibody to HAV at age 18 was compared in a field trial with the routine post-exposure method. Units were randomized into an intervention group (A) in which all subjects were to receive ISG (79% were in fact immunized), a second group (B) in which 50% of subjects were to be immunized (36% were actually immunized) and a non-immunized control group (C). During the trial incidence rates in the non-immunized study population dropped well below predicted levels. The incidence rates were zero in group A, 1.31/1000 in group B (0.61/1000 among the immunized and 1.72/1000 among the non-immunized) and 2.49/1000 in group C over the 6-month follow-up. Pre-exposure ISG afforded excellent protection and even at the low rate of incidence was cost-effective. The possibility of herd immunity is discussed.