PROS1 and infection: Because virus neutralization can principally be facilitated by various SARS-CoV-2 proteins beyond the S protein [29, 30], differences in titer can stem from the activity against other SARS-CoV-2 proteins, as shown with an in-house ELISA against Nucleocapsid protein (N) with possibly neutralizing [31] or potentially exacerbating consequences [32, 33] and against the Membrane protein (M) [34]: 4 of the 6 participants had elevated responses against N after infection/vaccination (Supplementary Figure  1D).