A team of scientists in Germany recently evaluated the specificities and sensitivities of different serological assays for detecting antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The study is currently available on the medRxiv* preprint server.
In the context of the ongoing coronavirus disease 2019 (COVID-19) pandemic, the importance of non-pharmacological control measures, such as mask wearing, physical distancing, and hand sanitizing, in curbing the spread of SARS-CoV-2 has already been established. According to epidemiological experts, rapid testing, contact tracing, and isolation are the key steps in controlling the trajectory of any pandemic situation.
While reverse transcription polymerase chain reaction (RT-PCR) or rapid antigen testing is conducted to diagnose the status of an individual’s current SARS-CoV-2 infection, serological testing of anti-SARS-CoV-2 antibodies primarily provides information about both ongoing and previous infection. Thus, large-scale serosurveillance is more effective than RT-PCR/antigen testing in estimating the actual prevalence of SARS-CoV-2 infection in the general population, which also includes asymptomatic individuals.
Regarding antibody kinetics, studies have shown that most SARS-CoV-2-infected individuals develop antibodies within 2 weeks of infection. Moreover, IgA- and IgM-specific antibodies are developed earlier than IgG-specific antibodies. Because of high immunogenicity, the spike protein of SARS-CoV-2 is the major target of neutralizing antibodies. Similarly, because of the robust immune dominance among all human coronaviruses, the nucleocapsid protein of SARS-CoV-2 is another important target of neutralizing antibodies.
In the current study, the scientists have aimed to analyze the kinetics of anti- SARS-CoV-2 antibodies in patient sera, as well as to examine the diagnostic performance of 8 serological assays.
The scientists collected a total of 129 serum samples from 42 COVID-19 patients to examine the anti- SARS-CoV-2 antibody kinetics. The samples were collected on known days after the symptom onset. Specifically, the samples were collected at different phases of the infection: 11 – 20 days, 21 – 60 days, and more than 60 days after the symptom onset.
They determined the presence of IgG, IgM, and IgA antibodies in SARS-CoV-2-infected cells using indirect immunofluorescence testing. Moreover, using five different enzyme-linked immunosorbent assays (ELISA), they determined the presence of IgG and IgA specific antibodies against the S1 domain of viral spike protein and IgG and IgM specific antibodies against the viral nucleocapsid protein.
The study revealed significant variations between assay sensitivities. Specifically, the estimated sensitivities of indirect immunofluorescence testing for IgG, IgA, and IgM specific antibodies were found to be 94%, 72%, and 65%, respectively. Similarly, ELISA showed 75% and 80% sensitivities for IgG and IgA specific anti-spike S1 antibodies and 82% and 19% sensitivities for IgG and IgM specific anti-nucleocapsid antibodies, respectively. Overall, the highest sensitivity of IgA and IgM assays was observed at the initial phase of the infection (11 – 20 days post-infection), whereas IgG detection assays showed the highest sensitivity during the intermediate phase of infection (20 – 60 days post-infection).
In the initial phase of infection, ELISA detected IgG and IgA specific anti-spike S1 antibodies in 70% and 88% of samples, and IgG and IgM specific anti-nucleocapsid antibodies in 86% and 50% of samples, respectively. Similarly, the indirect immunofluorescence testing detected IgG, IgA, and IgM specific anti-SARS-CoV-2 antibodies in 96%, 93%, and 96% of samples, respectively.
During the intermediate phase of infection, ELISA detected IgG and IgA specific anti-spike S1 antibodies in 93% and 82% of samples, and IgG and IgM specific anti-nucleocapsid antibodies in 96% and 12% of samples, respectively. Similarly, the indirect immunofluorescence testing detected IgG, IgM, and IgA specific anti-SARS-CoV-2 antibodies in 100%, 87%, and 100% of samples, respectively.
In the late phase of infection, ELISA detected IgG and IgA specific anti-spike S1 antibodies in 85% and 80% of samples, and IgG and IgM specific anti-nucleocapsid antibodies in 81% and 0% of samples, respectively. Similarly, the indirect immunofluorescence testing detected IgG, IgA, and IgM specific anti-SARS-CoV-2 antibodies in 98%, 44%, and 30% of samples, respectively.
Importantly, the agreement analysis between anti-SARS-CoV-2 QuantiVac ELISA (an assay used to quantify antibodies) and anti-SARS-CoV-2 ELISA (qualitative) revealed a strong correlation between results obtained from these two methods.
Overall, the study reveals that the sensitivities of serological assays may vary depending on the phase of infection. Specifically, the findings reveal that indirect immunofluorescence testing is almost 95% sensitive in detecting IgG specific anti-SARS-CoV-2 antibodies in serum samples obtained from PCR-confirmed COVID-19 patients. In contrast, ELISA is particularly useful in determining the prevalence and quantity of IgG, IgA, and IgM specific antibodies targeting selected SARS-CoV-2 antigens.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.