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Elecsys® Anti-SARS-CoV-2

Immunoassay for the qualitative detection of antibodies (incl. IgG) against SARS-CoV-2
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Immunoassay to qualitatively detect antibodies (including IgG) against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)

Elecsys® Anti-SARS-CoV-2 is an immunoassay for the in vitro qualitative detection of antibodies (including IgG) to SARS-CoV-2 in human serum and plasma. The assay uses a recombinant protein representing the nucleocapsid (N) antigen in a double-antigen sandwich assay format, which favors detection of high affinity antibodies against SARS-CoV-2. Elecsys® Anti-SARS-CoV-2 detects antibody titers, which have been shown to positively correlate with neutralising antibodies in neutralisation assays43, 44. The test is intended as an aid in the determination of the immune reaction to SARS-CoV-2.45

Elecsys® Anti-SARS-CoV-2 Factsheet

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

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is an enveloped, single-stranded RNA virus of the family Coronaviridae. Coronaviruses share structural similarities and are composed of 16 nonstructural proteins and 4 structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N). Coronaviruses cause diseases with symptoms ranging from those of a mild common cold to more severe ones such as Coronavirus Disease 2019 (COVID-19) caused by SARS-CoV-2 1,2. 

SARS-CoV-2 is transmitted from person-to-person primarily via respiratory droplets, while indirect transmission through contaminated surfaces is also possible3-6. The virus accesses host cells via the angiotensin-converting enzyme 2 (ACE2), which is most abundant in the lungs7,8.

The incubation period for COVID-19 ranges from 2 - 14 days following exposure, with most cases showing symptoms approximately 4 - 5 days after exposure3,9,10. The spectrum of symptomatic infection ranges from mild (fever, cough, fatigue, loss of smell and taste, shortness of breath) to critical11,12. While most symptomatic cases are not severe, severe illness occurs predominantly in adults with advanced age or underlying medical comorbidities and requires intensive care. Acute respiratory distress syndrome (ARDS) is a major complication in patients with severe disease. Critical cases are characterised by e.g., respiratory failure, shock and/or multiple organ dysfunction, or failure11,13,14. 

Definite COVID-19 diagnosis entails direct detection of SARS-CoV-2 RNA by nucleic acid amplification technology (NAAT)21-23. Serological assays, which detect antibodies against SARS-CoV-2, can contribute to identify individuals, which were previously infected by the virus, and to assess the extent of exposure of a population. They might thereby help to decide on application, enforcement, or relaxation of containment measures24.

Upon infection with SARS-CoV-2, the host mounts an immune response against the virus, including production of specific antibodies against viral antigens. Both IgM and IgG have been detected as early as day 5 after symptom onset25,26. Median seroconversion has been observed at day 10 - 13 for IgM and day 12 - 14 for IgG27-29, while maximum levels have been reported at week 2 - 3 for IgM, week 3 - 6 for IgG and week 2 for total antibody25-31. Whereas IgM seems to vanish around week 6 - 732,33, high IgG seropositivity is seen at that time25,32,33. While IgM is typically the major antibody class secreted to blood in the early stages of a primary antibody response, levels and chronological order of IgM and IgG antibody appearance seem to be highly variable for SARS-CoV-2. Anti-SARS-CoV-2 IgM and IgG often appear simultaneously, and some cases have been reported where IgG appears before IgM, limiting its diagnostic utility26,27,29,34,35. 

After infection or vaccination, the binding strength of antibodies to antigens increases over time - a process called affinity maturation36. High-affinity antibodies can elicit neutralisation by recognising and binding specific viral epitopes37,38. In SARS-CoV-2 infection, antibodies targeting both the spike and nucleocapsid proteins, which correlate with a strong neutralising response, are formed as early as day 9 onwards, suggesting seroconversion may lead to protection for at least a limited time34,39-42.

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Structure of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)

 

  • Nucleocapsid protein (N)
  • Envelope protein (E)
  • Spike protein (S)
  • Membrane glycoprotein (M)
  • RNA
Coronavirus illustration

Elecsys® Anti-SARS-CoV-2 assay characteristics

  • Systems

    cobas® e 411 analyser, cobas® e 601 / cobas® e 602 modules, cobas® e 402 / cobas® e 801 analytical units

  • Testing time

    18 minutes

  • Calibration

    2-point

  • Interpretation

    COI* < 1.0 = non-reactive
    COI ≥ 1.0 = reactive

  • Sample material

    Serum collected using standard sampling tubes or tubes containing separating gel.

    Li-heparin, K2-EDTA and K3-EDTA plasma as well as Li-heparin and K2‑EDTA plasma tubes containing separating gel.

    Capillary blood collected in serum, Li‑heparin or K2‑EDTA sampling tubes.

  • Sample volume

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

  • Onboard stability

    14 days

  • Intermediate precision in positive samples

    cobas® e 411 analyser: 2.2 - 5.0%
    cobas® e 601 / cobas® e 602 modules: 2.3 - 2.7 %
    cobas® e 402 / cobas® e 801 analytical units: 2.1 - 2.6 %

* COI: cutoff index

** CV: coefficient of variation

Seroconversion sensitivity45

After recovery from infection, confirmed by a negative PCR result, 26 sequential samples from 5 individuals were tested with the Elecsys® Anti-SARS-CoV-2 assay.

Seroconversion sensitivity

* Day 0 represents initial positive PCR

Clinical sensitivity45

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Clinical sensitivity45

A total of 496 samples from 102 symptomatic patients with a PCR confirmed SARS-CoV-2 infection were tested with the Elecsys® Anti-SARS-CoV-2 assay. One or more sequential specimens from these patients were collected after PCR confirmation at various time points.

A total of 496 samples from 102 symptomatic patients with a PCR confirmed SARS-CoV-2 infection were tested with the Elecsys® Anti-SARS-CoV-2 assay. One or more sequential specimens from these patients were collected after PCR confirmation at various time points.

Days post PCR confirmation N Non-reactive Sensitivity (95 % CI**)
0 – 6 days 161 64 60.2 % (52.3 – 67.8 %)
7 – 13 days 150 22 85.3 % (78.6 – 90.6 %)
≥14 days 185 1* 99.5 % (97.0 – 100 %)
* 1 patient was non-reactive at day 14 (0.696 COI) but reactive at day 16 (4.48 COI); ** confidence interval

Analytical specificity45

Overall specificity in a cohort of 792 potentially cross-reactive samples was 99.5 % (95 % CI: 98.63 – 99.85 %).

* 40 samples from individuals with common cold symptoms, collected before Dec 2019

** 40 samples from individuals following an infection with Coronavirus HKU1, NL63, 229E or OC43, confirmed by PCR

*** N=712

Common cold panel*

100 % specificity

Coronavirus panel**

100 % specificity

Other potential cross-reactivity***

99.44 % specificity

Clinical specificity45

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Clinical specificity45

A total of 10,453 samples from diagnostic routine and blood donors obtained before December 2019 were tested with the Elecsys® Anti-SARS-CoV-2 assay.

A total of 10,453 samples from diagnostic routine and blood donors obtained before December 2019 were tested with the Elecsys® Anti-SARS-CoV-2 assay.

Cohort N Reactive Specificity % (95 % CI)
Diagnostic routine 6305 12 99.81 % (99.67 – 99.90 %)
Blood donors 4148 9 99.78 % (99.59 – 99.90 %)
Overall 10453 21 99.80 % (99.69 – 99.88 %)

Correlation to serum neutralisation45

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Correlation to serum neutralisation45

The Elecsys® Anti-SARS-CoV-2 assay was compared to a VSV-based pseudo-neutralisation assay46 in 46 clinical samples from individual patients.

The Elecsys® Anti-SARS-CoV-2 assay was compared to a VSV-based pseudo-neutralisation assay46 in 46 clinical samples from individual patients.

    Pseudo-NT*
Positive Negative
Elecsys® Anti-SARS-CoV-2 Reactive 38 0
Non-reactive 6 2
Percent Positive Agreement 86.4 % (95 % CI 73.3 % – 93.6 %)  
Percent Negative Agreement 100 % (95 % CI: 34.2 – 100 %)  
Percent Overall Agreement 87.0 % (95 % CI 74.3 % – 93.9 %)  
* A titer of 1:20 was used as the positive cut-off for the pseudo-NT assay.

Estimated course of markers in SARS-CoV-2 infection47

Estimated course of markers in SARS-CoV-2 infection

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