Studies of active and passive immunoprophylaxis were carried out in chimpanzees to determine whether a candidate hepatitis A virus (HAV)
vaccine could stimulate antibody to HAV (
anti-HAV) that was qualitatively similar to
anti-HAV stimulated by natural
infection. Normal
immune globulin (Ig) was prepared from plasma obtained from human volunteers before and after vaccination with the HAV
vaccine, and these preparations or commercially prepared Ig were administered to chimpanzees. Protective efficacy was compared to that obtained after vaccination of chimpanzees. As expected, pre-vaccination Ig did not protect chimpanzees against challenge with virulent
hepatitis A. In contrast, chimpanzees were protected against
hepatitis A by Ig prepared from volunteers who had received
hepatitis A vaccine. The protection was qualitatively similar to that afforded by commercial normal Ig containing convalescent
anti-HAV. The minimum protective dose of passively acquired
anti-HAV was approximately the minimum dose detectable by serological means. This information will be useful in calculating minimum acceptable titres of
anti-HAV in normal Ig. Whereas administration of Ig protected chimpanzees against
hepatitis A pathology, it did not protect them from
infection with HAV. Thus, these chimpanzees were protected by classical passive-active immunoprophylaxis. In contrast, chimpanzees actively immunized with HAV
vaccine were apparently protected against both
hepatitis A pathology and HAV
infection. The mechanism of this complete protection is unknown but may simply represent the higher titre of
anti-HAV in the vaccinated chimpanzees, compared to the passively protected animals.