Elecsys® Anti-SARS-CoV-2 S

Immunoassay for the quantitative determination of antibodies to the SARS-CoV-2 spike protein

Elecsys pack
Immunoassay for the quantitative determination of antibodies to the SARS-CoV-2 spike protein

Elecsys® Anti-SARS-CoV-2 S is an immunoassay for the in vitro quantitative determination of antibodies (including IgG) to the SARS-CoV-2 spike (S) protein receptor binding domain (RBD) in human serum and plasma. The assay uses a recombinant protein representing the RBD of the S antigen in a double-antigen sandwich assay format, which favors detection of high affinity antibodies against SARS‑CoV‑2. The test is intended as an aid to assess the adaptive humoral immune response to the SARS‑CoV‑2 S protein.31

Elecsys® Anti-SARS-CoV-2 S

SARS-CoV-2: An overview of virus structure, transmission and detection

 

SARS-CoV-2, the causative agent of Coronavirus Disease 2019 (COVID-19), is an enveloped, single-stranded RNA Betacoronavirus. Seven coronaviruses have been identified as agents of human infection, causing disease ranging from mild common cold to severe respiratory failure.1

SARS-CoV-2 is transmitted primarily from person-to-person through respiratory droplets and aerosols.2,3 The incubation period from infection to detectable viral load in the host commonly ranges from two to 14 days.4,5 Detection of viral load can be associated with the onset of clinical signs and symptoms, although a considerable proportion of individuals remain asymptomatic or mildly symptomatic.6-8 The interval during which an individual with COVID-19 is infectious has not yet been clearly established, however, transmission from symptomatic, asymptomatic, and pre-symptomatic individuals has been well described.9-11

 

Coronavirus genomes encode 4 main structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N). The S protein is a very large transmembrane protein that assembles into trimers to form the distinctive surface spikes of coronaviruses. Each S monomer consists of an N-terminal S1 subunit and a membrane-proximal S2 subunit. The virus gains entry to the host cell through binding of the S protein to the angiotensin-converting enzyme 2 (ACE2) receptor, which is present on the surface of numerous cell types including the alveolar type II cells of the lung and epithelial cells of the oral mucosa.12,13 Mechanistically, ACE2 is engaged by the receptor-binding domain (RBD) on the S1 subunit.14,15

Upon infection with SARS-CoV-2, the host usually mounts an immune response against the virus, typically including production of specific antibodies against viral antigens. IgM and IgG antibodies against SARS-CoV-2 appear to arise nearly simultaneously in blood.16 There is significant inter-individual difference in the levels and chronological appearance of antibodies in COVID-19 patients, but median seroconversion has been observed at approximately two weeks.17-20

After infection or vaccination, the binding strength of antibodies to antigens increases over time - a process called affinity maturation21. High‑affinity antibodies can elicit neutralization by recognizing and binding specific viral epitopes22,23. Antibodies against SARS‑CoV‑2 with strong neutralizing capacity, especially potent if directed against the RBD, have been identified.24-27 Numerous vaccines for COVID-19 are in development, many of which focus on eliciting an immune response to the RBD.28-30

Structure of the SARS-CoV-2 spike protein and binding to host receptor

Illustration of the structure of the spike protein

Clinical sensitivity31

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Clinical sensitivity31

A total of 1,610 samples from 402 symptomatic patients (including 297 samples from 243 hospitalized patients) with a PCR confirmed SARS-CoV-2 infection were tested with the Elecsys® Anti-SARS-CoV-2 S assay. One or more sequential samples from these patients were collected at various time points after PCR confirmation.

1,423 of the tested samples had a sampling date of 14 days or later after diagnosis with PCR. 1,406 of these 1,423 samples were determined with ≥0.8 U/mL in the Elecsys® Anti‑SARS‑CoV‑2 S assay and hence considered positive, resulting in a sensitivity of 98.8 % (95 % CI: 98.1 – 99.3 %) in this sample cohort.

Days post PCR confirmation

Days post PCR confirmation N Non-reactive Sensitivity (95 % CI*)
0-6 days 35 4 88.6 % (73.3 – 96.8 %)
7-13 days 152 22 85.5 % (78.9 – 90.7 %)
14-20 days 130 14 89.2 % (82.6 – 94.0 %)
21-27 days 176 3 98.3 % (95.1 – 99.7 %)
28-34 days 197 0 100 % (98.1 – 100 %)
≥35 days 920 0 100 % (99.6 – 100 %)
* confidence interval
Graph Days post PCR confirmation

Analytical specificity 31

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Analytical specificity 31

A total of 1,100 potentially cross-reactive samples collected before October 2019, including anti-MERS-CoV positive samples, samples from individuals with common cold symptoms, and samples from individuals confirmed to be infected with one of the four common cold coronaviruses were tested with the Elecsys® Anti-SARS-CoV-2 S assay. Overall specificity in this cohort of potentially cross-reactive samples was 100 % (95 % CI: 99.7 – 100 %).

Cohort N Reactive Specificity (95% CI)
MERS-CoV* 7 0 100 % (59.0 – 100 %)
Common cold panel** 21 0 100 % (83.4 – 100 %)
Coronavirus panel*** 94 0 100 % (96.2 – 100 %)
Other potentially cross-reactive samples****   978 0 100 % (99.6 – 100 %)
Overall 1.100   0 100 % (99.7 – 100 %)  
* positive for IgG antibodies against the Middle East respiratory syndrome-related coronavirus (MERS-CoV) spike protein subunit S1
** 40 samples from individuals with common cold symptoms, collected before October 2019
*** from individuals with past infection with coronavirus HKU1, NL63, 229E, or OC43, confirmed by antigen testing
**** pre-pandemic samples with reactivity for various other indications, which could have an elevated potential for unspecific interference

Clinical specificity 31

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Clinical specificity 31

A total of 5,991 samples from diagnostic routine and blood donors drawn before October 2019 were tested with the Elecsys® Anti-SARSCoV-2 S assay. Overall specificity in this cohort of pre-pandemic samples was 99.98 % (95 % CI: 99.91 – 100 %).

Cohort N Reactive Specificity (95 % CI)
Diagnostic routine  2,528  0 100 % (99.85 – 100 %)
US blood donors 2,713 1 99.96 % (99.79 – 100 %)
African blood donors 750 0 100 % (99.51 – 100 %)
Overall 5,991 1 99.98 % (99.91 – 100 %) 

Correlation to serum neutralization 31

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Correlation to serum neutralization 31

The Elecsys® Anti-SARS-CoV-2 S assay was compared to a VSV-based pseudo-neutralization assay32 in 15 clinical samples from individual patients.

       Pseudo-Neutralization       
  Positive Inderterminate Negative Total
Elecsys® Anti-SARS-CoV-2 S ≥0.8 U/mL 12 0 0 12
<0.8 U/mL 1 1 1 3
Total 13 1 1 15
Percent Positive Agreement 92.3 % (95 % CI 63.97 – 99.81 %)
Estimated course of markers in SARS-CoV-2 infection33

References

 

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  2. Word Health Organization (2020). Available from: https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions.
  3. Zhu, N. et al. (2020). N Engl J Med. 20, 382(8), 727-33.
  4. Chan, J.F.-W. et al. (2020). Lancet. 15, 395(10223), 514-23.
  5. Lauer, S.A. et al. (2020). Ann Intern Med. 172(9), 577-582.
  6. Zhou, R. et al. (2020). Int J Inf Dis. 96, 288-90.
  7. He, X. et al. (2020). Nat Med. 26(5), 672-5.
  8. Mizumoto, K. et al. (2020). Euro Surveill. 25(10), pii=2000180.
  9. Gao, M. et al. (2020). Respir Med. 169, 106026.
  10. Yu, P. et al. (2020). J Infect Dis. 221(11), 1757-61.
  11. Liu, Z. et al. (2020). Int J Inf Dis. https://doi.org/10.1016/j.ijid.2020.06.036.
  12. Letko, M. et al. (2020). Nat Microbiol. 5(4), 562-9.
  13. Xu, H. et al. (2020). Int J Oral Sci. 24, 12(1), 1-5.
  14. Wrapp, D. et al. (2020). Science. 13, 367(6483), 1260-3.
  15. Hoffmann, M. et al. (2020). Cell. 16, 181(2), 271-280.e8.
  16. Centers for Disease Control and Prevention (2020). Available from: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html.
  17. Long, Q. et al. (2020). medRxiv. https://doi.org/10.1101/2020.03.18.20038018.
  18. Lou, B. et al. (2020). Eur Respir J. 19, 2000763.
  19. Zhao, J. et al. Clin Infect Dis. ciaa344. https://doi.org/10.1093/cid/ciaa344.
  20. Tuaillon, E. et al. (2020). J Inf. 81(2), e39-e45.
  21. Klasse, P.J. (2016). Expert Rev Vaccines 15(3), 295-311.
  22. Payne, S. (2017). Viruses: Chapter 6 – Immunity and Resistance to Viruses, Editor(s): Susan Payne, Academic Press, Pages 61-71, ISBN 9780128031094.
  23. Iwasaki, A. and Yang, Y. (2020). Nat Rev Immunol. https://doi.org/10.1038/s41577-020-0321-6.
  24. Salazar, E. et al. (2020). bioRxiv 2020.06.08.138990; https://doi.org/10.1101/2020.06.08.138990.
  25. Klasse, P. and Moore, J.P. (2020). Elife. 2020, 9:e57877. doi:10.7554/eLife.57877.
  26. Premkumar, L. et al. (2020). Sci Immunol. 11, 5(48).
  27. Luchsinger, L.L. et al. (2020). medRxiv. https://doi.org/10.1101/2020.06.08.20124792
  28. Mukherjee, R. (2020). J Biosci. 45, 68. https://doi.org/10.1007/s12038-020-00040-7.
  29. Graham, B.S. (2020). Science. 368(6494), 945-6.
  30. Hotez, P.J. et al. (2020). Nat Rev Immunol. 20(6), 347-8.
  31. Elecsys® Anti-SARS-CoV-2 S. Package Insert 2020-09, V1.0; Material Numbers 09289267190 and 09289275190.
  32. Meyer, B. et al. medRxiv. https://doi.org/10.1101/2020.05.02.20080879.
  33. Sethuraman, N. et al. (2020). JAMA. Published online May 06, 2020. doi:10.1001/jama.2020.8259.
  34. To, K. et al. (2020). Lancet Infect Dis. 20(5), 565-74.
  35. Xiang, F. et al. (2020). Clin Infect Dis. pii: ciaa46. https://doi.org/10.1093/cid/ciaa461.

System specifications

  • Testing time

    18 minutes

  • Test principle

    One-step double antigen sandwich assay

  • Traceability

    Internal Roche standard for anti-SARS-CoV-2-S consisting of monoclonal antibodies. 1 nM of these antibodies correspond to 20 U/mL of the Elecsys® Anti-SARS-CoV-2 S assay

  • Linear range

    0.4 to 250 U/mL

  • Calibration

    2-point (separate CalSet)

  • Interpretation

    <0.8 U/mL = non-reactive, ≥0.8 U/mL = reactive

  • Specimen types

    Serum collected using standard sampling tubes; Li-heparin, K2-EDTA-, K3-EDTA-, and sodium citrate plasma

  • Sample volume

    20 μL cobas e 411 analyzer, cobas e 601 / cobas e 602 modules
    12 μL cobas e 801 module

  • Onboard stability

    14 days

  • Intermediate precision in positive samples

    cobas e 411 analyzer: CV* 1.9 – 2.9 %
    cobas e 601 / cobas e 602 modules: CV 2.7 – 3.6 
    cobas e 801 module: CV 1.4 – 2.4 %