IMMUNOPRECIPITATION AND VIRUS NEUTRALIZATION ASSAYS DEMONSTRATE QUALITATIVE DIFFERENCES BETWEEN PROTECTIVE ANTIBODY-RESPONSES TO INACTIVATED HEPATITIS-A VACCINE AND PASSIVE-IMMUNIZATION WITH IMMUNE GLOBULIN

Antibodies to hepatitis A virus (anti-HAV) were measured in children f rom two separate vaccine trials (n = 70) 4 weeks after a dose of inact ivated hepatitis A vaccine (VAQTA), The geometric mean titers (GMTs) o f anti-HAV were 49.3 and 45.2 mIU/mL by immunoassay, while reciprocal GMTs of neutralizing anti-HAV were 6.5 and 15.0 by an 80% radioimmunof ocus inhibition test (RIFIT) and 55.6 and 92.0 by antigen reduction as say (HAVARNA), The GMT of antibody detected by radioimmunoprecipitatio n (RIPA) was greater than or equal to 401. These data establish serolo gic correlates of protection against disease and show that RIPA is mos t sensitive for detection of early vaccine-induced antibody, Sera coll ected from adults (n = 20) 7 days after administration of immune globu lin contained similar antibody levels by immunoassay (45.1 mIU/mL) and slightly higher GMTs of neutralizing antibody (27.5 by RIFIT and 146 by HAVARNA) but negligible precipitating antibody (GMT, 5.6), These re sults are best explained by differences in the affinity of antibodies for virus following active versus passive immunization.