Table 1:

SARS-CoV-2 seroprevalence survey and participant characteristics

SerosurveyYearDate (epidemiological weeks)ContextTarget sample size
n = 14 000
Included participantsAssays and testing algorithm (antibody to target antigen)*
Sample size
n = 13 765
No. (%) femaleAge, median, yr
12020March 5–13 (10–11)Pre–first wave1000895452 (50.5)44Ortho (S1) and Abbott (NP); any positive specimen also tested with Siemens§ (S1 RBD)
22020May 8–27 (19–22)Post–first wave1000890450 (50.6)45
32020Sept. 17–29 (38–40)Pre–second wave, school start200020001000 (50.0)39.5
42021Jan. 16–27 (2–4)Post–second wave, before broad vaccination200019991000 (50.0)40Ortho(S1) and Abbott (NP); any positive specimen also tested with Siemens§ (S1 RBD) (with supplemental Roche [NP], volume permitting)**
52021May 30–June 11 (22–23)Post–third wave20001991997 (50.1)39
62021Sept. 26–Oct. 8 (39–40)Fourth wave, school start20001990994 (49.9)40Ortho (S1) and Roche** (NP) and Siemens (S1 RBD)
72022Mar. 13–24 (11–12)Post–fifth wave200020001000 (50.0)39.5
82022July 31–Aug. 11 (31–32)Seventh wave, before school start200020001000 (50.0)39.5Abbott (S1 RBD)†† and Roche (NP) and Siemens (S1 RBD)
  • Note: Ig = immunoglobulin, NP = nucleocapsid protein, RBD = receptor-binding domain, S/C = signal to cut-off, S1 = spike 1 protein.

  • * Any seropositivity (vaccine- or infection-induced, or both) defined by meeting assay-specific cut-offs on any 2 of several assays applied per serosurvey. For the first 3 serosurveys in 2020, any dual-assay seropositivity was considered to be from infection. From January 2021 (following S1-based vaccine availability), infection-induced seropositivity required that 1 of the 2 positive assays detect anti-NP.

  • Ortho assay detects total antibody (IgA, IgG and IgM) to recombinant S1 using the Vitros XT 7600 analyzer (Ortho-Clinical Diagnostics). Sample signal was divided by calibrator signal, with resultant S/C ratios of < 1.00 and ≥ 1.00 considered negative or positive, respectively.

  • Abbott assay detects IgG antibody to NP using the ARCHITECT i2000SR analyzer (Abbott Laboratories, Diagnostic Division); S/C ratios < 1.40 and ≥ 1.40 considered negative or positive, respectively.

  • § Siemens assay detects total antibody (IgG, IgM) to S1 RBD using the ADVIA Centaur XP system (Siemens Healthineers); S/C ratios < 1.00 and ≥ 1.00 considered negative or positive, respectively.

  • Roche assay detects total antibody (IgA, IgG and IgM) to NP using the Roche cobas e601 analyzer (Roche Diagnostics Gmbh); S/C ratios < 1.00 and ≥ 1.00 considered negative or positive, respectively.

  • ** With reduced anti-NP sensitivity, especially with the Abbott assay, (17) (21) testing additionally incorporated the Roche (NP total antibody) assay. For the fourth and fifth serosurveys, positivity on either Ortho or Abbott was followed by Siemens, but supplemented also by Roche where specimen volume permitted. In the event of Abbott and Roche discordance, NP positivity on either assay was accepted, with positive Roche replacing negative Abbott finding for anti-S1 plus anti-NP interpretation.

  • †† Abbott assay detects IgG to the S1 RBD using the ARCHITECT i2000SR analyzer (Abbott Laboratories, Diagnostic Division); S/C ratios of < 50.0 and ≥ 50.0 considered negative or positive, respectively.