SP4 and infection: Prior infection with SARS-CoV-2 broadened responses to the S1 and S2 subunits as subpools Sp2 and Sp4 are within the S1 subunit spanning the N-terminal domain and the RBD domain, and Sp7, Sp8 and Sp9 are within the S2 subunit spanning the fusion peptide and the S2’ cleavage site, the HR2 heptad repeat, the central helix and connector domain.