Serum neutralizing antibody response to hepatitis A virus.

Serum neutralizing antibody to hepatitis A virus (HAV) was measured in experimentally infected primates and naturally infected humans by means of an assay based on the autoradiographic detection of viral replication foci in vitro. Infection of primates with either PA-33 or HM-175 strains of HAV elicited antibody capable of neutralizing either strain. Sequential testing of two monkeys showed that neutralizing antibody correlated closely with antibody detected by immunoassay, developed before liver enzyme elevations, and was associated with a substantial reduction in fecal shedding of viral antigen. In tests performed on human subjects involved in an outbreak of hepatitis A, neutralizing antibody was present three to five days before the onset of symptoms and was found in both 19S and 7S immunoglobulin fractions. Immunity to HAV is probably due primarily to neutralizing antibody, and the ability to quantitate this antibody will be helpful in the evaluation of new HAV vaccines.