Clinical outcomes depending on RBD IgG levels
Serum samples were available for 325/350 residents from 6 NHs in which the blood collected could be organized (92.9%). Causes of missing data are displayed in Figure 1.
By comparison with residents who received two vaccine doses, the RBD-IgG level was lower in non-vaccinated residents and in those with one dose (Table 1).
Residents with higher levels of RBD-IgG had a lower risk of developing SARS-CoV-2 during the outbreak (Table 3). PPV and NPV of a positive RT-PCR by RBD-IgG levels over 1,050 AU/mL (significant response26) were 0.86 and 0.24, respectively, with a sensitivity and specificity of 0.63 and 0.54 (Table S2).
Among the 48 RT-PCR-positive vaccinated residents with blood results, RBD-IgG levels tended to be higher in asymptomatic residents than in those with mild or severe symptoms (median (IQR)[range] 1249 AU/mL (337 ; 3027) [11.00 ; 25,453.00] vs 517 AU/mL (150 ; 1,289) [3 ; 14,631] vs 358 AU/mL (128 ; 1,339) [93 ; 5,824], respectively).
No in vitro serum neutralization activity was found for RBD-IgG levels under 1,050 AU/mL for both SARS-CoV-2 and VOC-α. Above this threshold, the RBD-IgG levels were associated with serum neutralization activity (Figure 2).