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