Antibody avidity maturation during severe acute respiratory syndrome-associated coronavirus infection

The maturation of virus-specific immunoglobulin G avidity during severe acute respiratory syndrome-associated coronavirus infection was examined. The avidity indices were low ( mean +/- SD, 30.8% +/- 11.6%) among serum samples collected <= 50 days after fever onset, intermediate (mean +/- SD, 52.1% +/- 14.1%) among samples collected between days 51 and 90, and high ( mean +/- SD, 78.1% +/- 8.0%) among samples collected after day 90. Avidity indices of 40% and 55% could be considered as cutoff values for determination of recent (<= 50 days) and past (> 65 days) infection, respectively. Measurement of antibody avidity can be used to differentiate primary infection from reexposure and to assess humoral responses to candidate vaccines.