Within the first week after the onset of symptoms, qRT-PCR is used as a reliable method for detecting SARS-CoV-2 infection. As the infection progresses, the combination of qRT-PCR and antibody tests is optimal for accurate diagnosis ^{[1, 2, 3, 4, 5]} . The diagnostic sensitivity is significantly increased to 98.6% by the combination of IgM ELISA with qRT-PCR compared with a single qRT-PCR test (51.9%)^[3].
The sole use of antibody detection is particularly important in the later stages of infection, when the virus has already been eliminated by the host’s immune response ^[6]. According to current recommendations of the German National Association of Statutory Health Insurance, it is reasonable to use antibody tests from two weeks after the onset of symptoms ^[7]
In addition to the diagnostic value of antibody tests, they can be used for epidemiological purposes to identify individuals who have developed an immunity after infection that may protect against subsequent re-infection ^[8], and to define and monitor the extent of virus spread and the population immunity at the societal level.

^[1] To KK et al. Articles Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2 : an observational cohort study. Lancet Infect Dis 2020;3099:1–10.

^[2] Jia X. et al., “Clinical Significance of IgM and IgG Test for Diagnosis of Highly Suspected COVID-19 Infection“, preprint (Infectious Diseases (except HIV/AIDS), March 3rd 2020)

^[3] Guo L. et al., “Profiling Early Humoral Response to Diagnose Novel Coronavirus Disease (COVID-19)“, Clinical Infectious Diseases, March 21st 2020,

^[4] Nisreen M.A. Okba et al., “SARS-CoV-2 Specific Antibody Responses in COVID-19 Patients“, preprint (Infectious Diseases (except HIV/AIDS), March 20th 2020)

^[5] Amanat F. et al., “A Serological Assay to Detect SARS-CoV-2 Seroconversion in Humans“, preprint (Allergy and Immunology, March 18th 2020)

^[6] Zou L, et al. SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients. N Engl J Med 2020.

^[7] kassenärztlichen Bundesvereinigung https://www.kbv.de/html/1150_46092.php

^[8] Bao L, et al. Reinfection could not occur in SARS-CoV-2 infected rhesus macaques. bioRxiv 2020:2020.03.13.990226.

At present, experts assume that recovered patients have only low risk of getting COVID-19 a second time. Initial studies have shown that people who experienced SARS-CoV-2 infection develop specific antibodies that can neutralize the virus in laboratory tests. However, at this stage it is unclear how robust and sustainable this immune status is built up and whether there may be differences from person to person. Experience with other coronavirus diseases such as SARS and MERS suggests that an at least partial immune status could last up to three years. To accurately assess this aspect of COVID-19 disease, longitudinal serological studies are needed to monitor the immune status of convalescents over a longer period of time ^[1].

^[1] RKI https://www.rki.de/SharedDocs/FAQ/NCOV2019/gesamt.html

In order to achieve high sensitivities, a mixture of recombinant highly purified nucleoprotein and S-protein of SARS-CoV-2 is used for the IgA and IgM tests. To obtain high specificities, the IgG test is based solely on the recombinant highly purified S-protein.

Recent studies have shown that the nucleoprotein enables early detection of antibodies, especially in mild infection courses, while the S protein, due to its lower conservation within the virus family coronaviridae, allows very specific diagnostics. By combining both antigens, Virion\Serion has succeeded in developing ELISA tests for the detection IgA and IgM antibodies that meet our demand for the best possible performance in terms of sensitivity and specificity.

The S protein on the surface of SARS-CoV-2 is a promising target for a vaccine. As a surface antigen it is easily accessible and its function is crucial for the interaction with the human cell. By binding to the human ACE2 receptor, the virus gains access to the cell. Furthermore, it is known that neutralizing antibodies are formed mainly against components of the S protein ^[1]. Therefore, a large part of the vaccines currently in clinical trials is based on SARS-CoV-2 S proteins.

Since the SERION ELISA agile SARS-CoV-2 IgG uses recombinant S-protein of SARS-CoV-2, the basis for detecting antibodies produced as a result of vaccination has been laid. Whether these detected antibodies have a neutralizing effect and thus cause immune protection is generally difficult to answer with serological test systems, even for other pathogens, due to the methodology involved. For this purpose, sufficient scientific studies as well as standardized reference materials and recommendations of independent institutions must be available.

^[1] Meyer B., Drosten C., and Müller M., “Serological Assays for Emerging Coronaviruses: Challenges and Pitfalls”, Virus Research 194 (December 2014)

SERION ELISA control SARS-CoV-2 IgA and IgG are already available.

Serum or plasma samples can be used for the SERION ELISA agile SARS-CoV-2. Sputum, faeces, urine is not suitable.

Learn more here about the available SARS-Cov-2 antigens as raw material for the development of IVD test systems.

SERION Immunologics offers a wide range of raw materials for in-vitro diagnostics. The recombinant SARS-CoV-2 antigens are tested for the development of ELISA and particle-based test systems such as CLIA but are also suitable for the development of lateral flow (LF) tests.