TMPRSS11D and infection: Of note, in a few individuals that did not have a history of infection before vaccination and had a negative IgG nucleoprotein test, we could detect low levels of HAT antibodies against the viral variants RBD (Figure 3A), which may be indicative of a previous infection that was not detected by the nucleoprotein IgG assay, or of potential cross-reactivity with other HCoVs.