- During the 50's and 60's there were large scale epidemics of hepatitis A every 3-4 years in the Israel Defense Forces. During these epidemics the annual incidence exceeded 10/1000 soldiers at risk. There has been a highly significant decrease in rates during the past 30 years. The average annual incidence of clinically identified viral hepatitis A decreased from an average of 6/1000 during the 60's to 2.5/1000 during the 70's. The decrease coincided with the introduction in the 1970's of wide-scale post-exposure prophylaxis with immune serum globulin (ISG). The incidence was further significantly reduced to 0.5-1.0/1000 with the introduction of pre-exposure prophylaxis with ISG, starting in 1978 (p < 0.001). Other factors probably played a role in accelerating the decline in morbidity, such as improvement in personal hygiene and sanitation facilities, and in waste disposal and other aspects of military environmental health. These improvements were probably more pronounced in the civilian sector, leading to decreased exposure of children to the virus and consequently a higher proportion of seronegatives at induction. Increase in the proportion of recruits without natural immunity to the virus poses greater risk, both in terms of personal morbidity as well as military operational ability. This risk, combined with problems of ISG use and availability, has propelled hepatitis A prevention policy towards the use of the new inactivated vaccines.