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SARS-CoV-2 antibodies in COVID-19 patients

A recent research paper published in in the BSI’s official journal Clinical & Experimental Immunology suggests that testing for SARS-CoV-2 specific antibodies is useful in diagnosing patients with COVID-19 and that certain antibody types correspond to disease severity.

Antibody illustration

Testing for SARS-CoV-2 genetic material is routinely used to diagnose COVID-19 but the PCR laboratory test, which uses swabs from the throat and nose of patients, has a high false negative rate. Therefore, doctors use a combination of the PCR test result and clinical symptoms to diagnose COVID-19. However, with large numbers of asymptomatic infections, this method has flaws. This study looked at the levels of different SARS-CoV-2 specific antibodies in patients’ blood and how they could be used to help improve diagnosis.

What experiments were carried out in this study?

Researchers from the National Clinical Research Centre for Respiratory Disease in Guangzhou, China recruited 43 hospitalised patients with moderate, severe or critical COVID-19 who had been diagnosed by PCR test. There were also 61 patients with non-COVID-19 respiratory diseases recruited as controls. Blood serum samples were collected up to 49 days after symptom onset in the patients with COVID-19 whilst they were in hospital. Samples from patients without COVID-19 were collected during their first visit to the outpatient respiratory clinic. All samples were tested for SARS-CoV-2 specific IgM, IgA, and IgG antibodies levels. The SARS-CoV-2 specific IgM and IgA antibodies appear during the initial innate immune response and indicate a recent infection, whilst IgG is the main antibody produced by the adaptive immune response.

The researchers performed combined antibody tests looking at levels of IgA-IgM, IgA-IgG and IgM-IgG in blood samples. The study also tracked the level of these different antibodies in COVID-19 patients’ blood for several weeks to understand how antibody levels changed with disease progression.

What did the results of this study suggest?

Of the patients with COVID-19, 100% of the severe and critical patients were correctly diagnosed by IgA-IgM, IgA-IgG and IgM-IgG combined antibody testing. However, 16.4% of patients without COVID-19 were diagnosed incorrectly as positive for SARS-CoV-2 by the IgA-IgM and IgM-IgG combined tests. Using the IgA-IgG combined antibody test, only 3.3% of non-COVID-19 patients tested positive. This suggests that the SARS-CoV-2 IgA-IgG combined antibody test is a more useful and accurate test for diagnosing COVID-19.

Higher levels of IgA and IgG SARS-CoV-2 antibodies were detected in the blood of severe and critical COVID-19 patients when compared to patients with moderate disease. This implies that IgA and IgG antibody levels in the blood could be associated with disease severity.

In all COVID-19 patients, IgA antibody levels were detected in their blood by the first week of symptom onset and reached a peak in the third week before gradually declining. Therefore, IgA antibody testing may be useful for early diagnosis of SARS-CoV-2 infection and in asymptomatic individuals.

What can we conclude from this study?

Testing for certain SARS-CoV-2 antibodies in blood samples could potentially be used to diagnose COVID-19 alongside PCR testing and clinical symptoms. However, the study sample size was too small to make any conclusions about how SARS-CoV-2 specific antibodies correlate to disease severity. Additionally, the study was carried out over a short period of time, so researchers cannot know what happens to antibody levels after 49 days. More research is needed to better understand how levels of SARS-CoV-2 antibodies can be used to inform diagnosis and predict COVID-19 progression. 

Huang, Z., Chen, H., Xue, M., Huang, H., Zheng, P., Luo, W., Liang, X., Sun, B. and Zhong, N. (2020), Characteristics and roles of severe acute respiratory syndrome coronavirus 2‐specific antibodies in patients with different severities of coronavirus 19. Clinical & Experimental Immunology 202: 210-219.

Paper first published 24 July 2020

Summary author Erika Aquino, BSI Public Engagement Manager