Table 3.
Seroprevalence for Each Participant Group, Antigen-Isotype Combination, and Threshold Method
| Isotype-Antigen Combination | Threshold Methoda | Seroprevalence, % (No.) | ||
|---|---|---|---|---|
| HCW (n = 253) | HCW Family (n = 299) | RTW (n = 327) | ||
| RBD IgA | Youden | 51.4 (130) | 34.1 (102) | 29.7 (97) |
| Max Spec | 2.8 (7) | 1.3 (4) | 0.9 (3) | |
| 3 SD | 0.0 (0) | 0.0 (0) | 0.0 (0) | |
| RBD IgG | Youden | 85.4 (216) | 62.2 (186) | 52.9 (173) |
| Max Spec | 2.0 (5) | 1.3 (4) | 0.0 (0) | |
| 3 SD | 0.4 (1) | 1.3 (4) | 0.0 (0) | |
| S trimer IgA | Youden | 19.1 (48) | 12.5 (36) | 38.6 (124) |
| Max Spec | 18.3 (46) | 12.2 (35) | 36.8 (118) | |
| 3 SD | 3.2 (8) | 1.4 (4) | 5.3 (17) | |
| S trimer IgG | Youden | 6.0 (15) | 7.3 (21) | 20.6 (66) |
| Max Spec | 6.0 (15) | 7.3 (21) | 20.2 (65) | |
| 3 SD | 4.8 (12) | 4.5 (13) | 7.2 (23) | |
| N IgA | Youden | 19.9 (50) | 10.8 (31) | 20.2 (65) |
| Max Spec | 9.2 (23) | 3.5 (10) | 8.1 (26) | |
| 3 SD | 1.6 (4) | 0.0 (0) | 0.9 (3) | |
| N IgG | Youden | 15.1 (38) | 12.8 (37) | 15.3 (49) |
| Max Spec | 5.2 (13) | 4.5 (13) | 2.5 (8) | |
| 3 SD | 3.6 (9) | 1.4 (4) | 0.9 (3) | |
| Top Three | Youden | 71.3 (179) | 50.3 (145) | 50.2 (161) |
| Max Spec | 14.7 (37) | 10.1 (29) | 9.3 (30) | |
| 3 SD | 4.0 (10) | 2.1 (6) | 0.9 (3) | |
| All IgA | Youden | 30.7 (77) | 20.1 (58) | 33.6 (108) |
| Max Spec | 10.0 (25) | 4.5 (13) | 12.1 (39) | |
| 3 SD | 5.2 (13) | 1.0 (3) | 3.4 (11) | |
| All IgG | Youden | 15.9 (40) | 11.5 (33) | 25.9 (83) |
| Max Spec | 15.9 (40) | 11.5 (33) | 25.9 (83) | |
| 3 SD | 4.8 (12) | 3.1 (9) | 3.7 (12) | |
Abbreviations: HCW, healthcare workers; HCW Family, family members of participating HCWs; Ig, immunoglobulin; Max Spec, maximum specificity; N, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein; RBD, SARS-CoV-2 receptor-binding domain; RTW, return-to-work employees who had been working remotely from March to August 2020, during the first COVID-19 wave in Worcester, Massachusetts; S trimer; SARS-CoV-2 spike trimer; SD, standard deviation; Top Three, combination of the three antigen/isotype combinations (RBD IgG, N IgG, and RBD IgA) with the largest area under the ROC curve (i.e., with maximum sensitivity and specificity).
The 3SD threshold was determined by calculating the mean + 3 SDs for negative controls; the Max Spec threshold, by setting the cutoff at the highest value of the negative controls for each antigen-isotype combination; and the Youden threshold, by determining the maximum Youden index on the ROC curve.