To measure avidity maturation, plates were coated and blocked as described above. After the addition of P2 samples in quadruplicates, two of the wells were rinsed with 6 M urea in PBS-T, whereas the other duplicate wells were rinsed with PBS-T only for 10 min at room temperature. Next, all wells were washed 3 times in PBS-T and subsequent steps were performed as described for the regular indirect ELISA.