PSMD1 and COVID-19: Anti-S1 IgG titer within 1 week after symptom onset in breakthrough COVID-19 patients was significantly lower in the severe group than in the non-severe group (median O.D. ratio [IQR], 4.99 [1.38–9.02] vs. 8.28 [5.17–10.25], P = 0.007) (Figure 1A).