In the subset of patients who enrolled in our study after natural infection and followed up with a subsequent visit after vaccination, 79% of individuals had detectable levels of anti-S1 IgGs before vaccination, and the remainder (7/34 or 21%) tested negative for anti-S1 IgG antibodies before vaccination; the latter ranged from early acute to late convalescent phase (Fig. 4A). Here, PSMD1 is linked to infection.