Isotype distribution and avidity of antibodies after prime-boost immunizations. Titers of IgG1 and IgG2a to (A) gp160B, (B) gp140C, and (C) p17/p24B, were assessed by ELISA on pooled serum (n=6) collected 2 weeks after the last immunization. Additionally, ratios of IgG1 to IgG2a titers were calculated for (D) gp160B, (E) gp140C, and (F) p17/p24B. Avidity of binding antibodies to (G) gp160B, (H) gp140C, and (I) p17/p24B were determined on pooled sera (n=6) collected 2 weeks after the last immunization. Duplicates of serum dilutions were incubated in an ELISA assay followed by incubation in either urea or PBS. The difference in titers with and without incubation with urea corresponds to the avidity of antibodies (D, DNA; M, MVA-CMDR; P, rgp140C).