ACE2 and infection: In the RBD-ACE2 binding inhibition assay (Figure 3), sera of participants from the ‘prior infection’ group showed higher inhibition capacity against all six strains, including the wild type (81.1 [61.1–91.5] vs. 99.8 [99.7–99.9] %; P < 0.0001), and the Alpha (68.1 [54.4–84.8] vs. 99.8 [99.6–99.8] %; P < 0.0001), Beta (38.4 [6.9–55.1] vs. 99.2 [97.0–99.5] %; P < 0.0001), Gamma (51.1 [38.1–68.7] vs. 99.6 [98.4–99.8] %; P < 0.0001), Delta (78.2 [57.8–83.9] vs. 99.8 [99.7–99.9] %; P < 0.0001), and Omicron variants (0.0 [0.0–18.3] vs. 74.1 [39.4–84.9] %; P < 0.0001).