No test is 100% reliable, so some margin should always be taken. This test is new, and as more tests are used, more insight will be gained into its sensitivity and specificity.
The EUROIMUUN test results showed the following: from 10 days after the onset of symptoms a 100% sensitivity could be demonstrated with the combined use of the Anti-SARS-CoV-2-ELISA (IgA) and (IgG). This concerns the percentage of rightly positive results. With the Anti-SARS-CoV-2-ELISA (IgG) alone a sensitivity of 80% was achieved. The specificity (the percentage of rightly negative test results among the non-patients) was determined with samples from patients with other diseases and healthy blood donors and is >98.5%.
A study by Okba et al. demonstrated sensitivities of 89 - 100% for the EUROIMMUN Anti-SARS-CoV-2ELISAs(IgA) and (IgG), which had not received CE certification at the time of the study. The specificity was 87.5 - 95.5% for IgA and 83.5 - 97.5% for IgG detection, depending on the cohort studied. Based on the data collected, EUROIMMUN performed a design transfer of the IgG kits, which helped optimize the signal-to-noise ratio. This improved version of the EUROIMMUN Anti-SARS-CoV-2-ELISA (IgG) is CE-registered.
The microtiter plates of the Anti-SARS-CoV-2 ELISA were coated with recombinant S1 structural protein of SARS-CoV-2 to which antibodies against SARS-CoV-2 in the patient samples bind. This antigen was selected because of its relatively low homology to other coronaviruses, particularly SARS-COV. The genetic homology of SARS-COV and SARS-COV-2 is approximately 86% according to Lu et al. but is significantly lower for the structural S1 protein.
Relevance of IgA/Why not IgM?
To reduce or close a potential diagnostic gap between direct detection and serology, an anti-SARS-CoV-2-ELISA (IgA) was developed. According to the MIQ, secretory antibodies of the IgA class are an "essential carrier of the humoral immune response at the mucosal and body fluid levels," making detection of diagnostic importance to support diagnosis in suspected respiratory infections. The Anti-SARS-CoV-2-ELISA (IgA) is based on recombinant S1 structural protein, analogous to IgG detection, and thus allows for a significantly more specific detection than with IgM antibody detection, which primarily targets the highly conserved (N) protein. Thus, the risk of detecting cross-reactive antibodies to other coronaviruses with the Anti-SARS-CoV-2-ELISA (IgA) is significantly lower than with the IgM detection. EUROIMMUN does not currently offer such IgM detection.
Importance of serology
For the detection of acute COVID-19 infections, direct detection of pathogens by nucleic acid amplification is the method of choice. However, the sensitivity of direct detection decreases with the onset of the immune response and the associated reduction in viral load. The pathogen cannot be reliably detected after approximately 10-14 days (Liu et al. (2020), so serological methods can be used as a support from this stage onwards. The following applications for serology have been described:
- Detection of infection chains to identify previously undetected infections that are long-standing or have occurred in the past and to enable action to be taken if necessary (source: European Centers for Disease Control and Prevention (ECDC) - Technical report - New coronavirus [SARS-CoV-2]).
- Decision support for discharge of patients from medical care (Source: European Centres for Disease Control and Prevention (ECDC) - Technical report - New coronavirus [SARS-CoV-2])
- Identification of contact with the pathogen or previous infection with SARS-COV-2 (Source: Bao et al, Reinfection could not occur in SARS-CoV-2 infected rhesus monkeys, bioRxiv 2020.03.13.990226
- Epidemiological studies in the population (Source: Robert Koch Institute (RKI) - SARS-CoV-2 Coronavirus Disease Profile-2019 [COVID-19])
RKI President Lothar Wieler assumes that in the long term about 2/3 of the population will have contact with SARS-CoV-2 and that infections will not spread further when this threshold is reached. In the meantime, however, it is very important to identify people who have already had contact with this virus (even undetected). Under current assumptions, there is no risk of these individuals spreading the virus or becoming ill themselves (see Bao et al.)
Unfortunately, ELISA techniques cannot be used to determine whether the detected antibodies have a neutralizing effect on a pathogen. Such detection is usually only possible in the neutralization test. ELISA generally detects all antibodies that bind to the coated antigen, regardless of their functionality. For some pathogens, certain protection statements can be made by selecting the antigen. However, for SARS-CoV-2, no epitope has yet been described that initiates the synthesis of protective antibodies that mediate reliable immunity. In a recently published study (Bao et al, Reinfection could not occur in SARS-CoV-2 infected rhesus macaques, bioRxiv 2020.03.13.990226; doi:https://doi.org/10.1101/2020.03.13.990226) it has been shown that rhesus macaques are not reinfected after infection, so these data suggest that immunity exists. However, the evidence in humans has not yet been demonstrated. However, in an initial study using the EUROIMMUN Anti-SARS-CoV-2 ELISA (IgG), a good correlation with a neutralization test could be demonstrated (see Okba et al, SARS-CoV-2 specific antibody responses in COVID-19 patients, medRxiv 10.1101/2020.03.18.20038059). These data suggest that the detected antibodies may mediate protection. Based on the data collected by Okba et al, EUROIMMUN performed a design transfer of the IgG kits, which contributed to an optimization of the signal-to-noise ratio. This improved version of the EUROIMMUN Anti-SARS-CoV-2-ELISA (IgG) is CE-registered.
Should the RIVM require data regarding the results, we are at all times happy to supply these within the possibilities of the privacy legislation.