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Elecsys® Anti-SARS-CoV-2 S

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

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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 total antibodies to the SARS-CoV-2 S protein RBD in human serum and plasma. The assay uses a recombinant RBD protein in a double-antigen sandwich assay format, which favors the quantitative determination of high affinity antibodies against SARS-CoV-2. The test is intended as an aid to assess the adaptive humoral immune response, including neutralizing antibodies, to the SARS-CoV-2 S protein after natural infection with SARS-CoV-2 or in vaccine recipients.

Elecsys® Anti-SARS-CoV-2 S

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

Severe acute respiratory syndrome coronavirus 2 (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

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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.

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 – 100.0 %)
≥35 days 920 0 100 % (99.6 – 100.0 %)
* confidence interval

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 – 100.0 %).

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 – 100.0 %).

Cohort N Reactive Specificity (95% CI)
MERS-CoV* 7 0 100 % (59.0 – 100.0 %)
Common cold panel** 21 0 100 % (83.4 – 100.0 %)
Coronavirus panel*** 94 0 100 % (96.2 – 100.0 %)
Other potentially cross-reactive samples****   978 0 100 % (99.6 – 100.0 %)
Overall 1,100   0 100 % (99.7 – 100.0 %)  
* 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 %).

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 – 100.0 %)
US blood donors 2,713 1 99.96 % (99.79 – 100.0 %)
African blood donors 750 0 100 % (99.51 – 100.0 %)
Overall 5,991 1 99.98 % (99.91 – 100.0 %) 

Detection of antibodies induced by active immunization with vaccines against SARS-CoV-2 31

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Detection of antibodies induced by active immunization with vaccines against SARS-CoV-2 31

After vaccination with the Moderna vaccine Spikevax® (mRNA‑1273) and the Pfizer‑BioNTech vaccine Comirnaty® (BNT162b2), applying the respectively approved 2-dose vaccination scheme, the antibody response in vaccinated, infection-naïve individuals was assessed using the Elecsys® Anti-SARS-CoV-2 S assay at three time-points: pre-vaccination (baseline), 21 days post 1st vaccination dose, and 14 days post 2nd vaccination dose. Following vaccination, rapidly rising antibody titers, indicating a strong humoral immune response to vaccination, were observed. All individuals that had been seronegative at baseline seroconverted after vaccination.

After vaccination with the Moderna vaccine Spikevax® (mRNA‑1273) and the Pfizer‑BioNTech vaccine Comirnaty® (BNT162b2), applying the respectively approved 2-dose vaccination scheme, the antibody response in vaccinated, infection-naïve individuals was assessed using the Elecsys® Anti-SARS-CoV-2 S assay at three time-points: pre-vaccination (baseline), 21 days post 1st vaccination dose, and 14 days post 2nd vaccination dose. Following vaccination, rapidly rising antibody titers, indicating a strong humoral immune response to vaccination, were observed. All individuals that had been seronegative at baseline seroconverted after vaccination.

Correlation of assay results to serum neutralization capacity 31

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Correlation of assay results to serum neutralization capacity 31

In a study investigating COVID‑19 convalescent plasma for virus neutralization capacity, plasma donations from convalescent donors after a SARS‑CoV‑2 infection were analyzed for whole virus neutralizing potential using an in vitro plaque reducing neutralization (PRNT) assay (BROAD Institute, USA). Presence of 50 % neutralization (NT50) at a sample dilution of >1:20 identified functional virus neutralization in vitro. 390 donations, including cross-sectional and longitudinal sample panels, were analyzed by PRNT and compared to Elecsys® Anti‑SARS‑CoV‑2 S assay results by applying two different thresholds: one representing the assay’s cutoff for detecting presence of RBD-specific antibodies (0.8 U/mL), and one based on optimized correlation with detection of virus neutralizing effects (15 U/mL).

In a study investigating COVID‑19 convalescent plasma for virus neutralization capacity, plasma donations from convalescent donors after a SARS‑CoV‑2 infection were analyzed for whole virus neutralizing potential using an in vitro plaque reducing neutralization (PRNT) assay (BROAD Institute, USA). Presence of 50 % neutralization (NT50) at a sample dilution of >1:20 identified functional virus neutralization in vitro. 390 donations, including cross-sectional and longitudinal sample panels, were analyzed by PRNT and compared to Elecsys® Anti‑SARS‑CoV‑2 S assay results by applying two different thresholds: one representing the assay’s cutoff for detecting presence of RBD-specific antibodies (0.8 U/mL), and one based on optimized correlation with detection of virus neutralizing effects (15 U/mL).

       Virus Neutralization Test (PRNT)     
  Neutralizing
(NT50 ≥ 1:20)		 Non-neutralizing Total
Elecsys® Anti‑SARS‑CoV‑2 S ≥0.8 U/mL 356 4 360
<0.8 U/mL 2 28 30
Total 358 32 390
Percent Positive Agreement 99.4 % (98.0 – 99.9 %)
Percent Negative Agreement 87.5 % (71.0 – 96.5 %)
Positive Predictive Value 98.9 % (97.2 – 99.7 %)
 
Elecsys® Anti‑SARS‑CoV‑2 S ≥15 U/mL 331 0 331
<15 U/mL 27 32 59
Total 358 32 390
Percent Positive Agreement 92.5 % (89.2 – 95.0 %)
Percent Negative Agreement
 100 % (89.1 – 100.0 %)
Positive Predictive Value 100 % (98.9 – 100.0 %)
Estimated course of markers in SARS-CoV-2 infection32
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References

  1. Ye, Z.-W. (2020). Int J Biol Sci. 16(10), 1686-97.
  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. 99, 325-27.
  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. 367(6483), 1260-3.
  15. Hoffmann, M. et al. (2020). Cell. 181(2), 271-280.e8.
  16. U.S. 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. 56, 2000763.
  19. Zhao, J. et al. Clin Infect Dis. 71(16):2027-2034.
  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). J Clin Invest. 130(12), 6728-38.
  25. Klasse, P. and Moore, J.P. (2020). Elife. 2020, 9:e57877.
  26. Premkumar, L. et al. (2020). Sci Immunol. 11, 5(48).
  27. Luchsinger, L.L. et al. (2020). J Clin Microbiol. 58:e02005-20.
  28. Mukherjee, R. (2020). J Biosci. 45, 68.
  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. Material Number 09289267190, Method Sheet 2022-07, V3.0. Material Number 09289275190, Method Sheet 2022-06, V4.0.
  32. Koch, T. et al. (2021). Vaccines 9(3):238.

System specifications

  • Testing time

    18 minutes

  • Test principle

    One-step double antigen sandwich assay

  • Traceability

    Standardized against the internal Roche standard for anti‑SARS‑CoV‑2‑S. The 1st WHO International Standard for anti‑SARS‑CoV 2 immunoglobulin (NIBSC code: 20/136) behaves identically to the internal Roche standard (Pearson r = 0.9996 between Limit of Quantitation and 1000 BAU/mL). Hence, the numeric results in U/mL of the Elecsys® Anti‑SARS‑CoV‑2 S assay and WHO BAU/mL are equivalent.

  • 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 or tubes containing separating gel; Li-heparin, K2-EDTA-, K3-EDTA-, and sodium citrate plasma. Li‑heparin and K2‑EDTA plasma tubes containing separating gel can be used. Capillary blood collected in serum, Li‑heparin plasma or K2‑EDTA plasma sampling tubes.

     

  • Sample volume

    20 μL cobas e 411 analyzer, cobas e 601 / cobas e 602 modules
    12 μL cobas e 402 / cobas e 801 analytical units

     

  • Onboard stability

    28 days cobas e 411 analyzer, cobas e 601 / cobas e 602 modules
    16 weeks cobas e 402 / cobas e 801 analytical units

  • 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 402 / cobas e 801 analytical units: CV 1.4 – 2.4 %

     

    * coefficient of variation

     
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