Antibody Seroconversion Response in COVID-19

Antibody Responses in COVID-19:
Seroconversion Implications in SARS-CoV-2 Diagnostic Testing

Key Points:

  • COVID-19, the pandemic coronavirus disease caused by SARS-CoV-2, has been shown to follow a typical seroconversion and immunoglobulin class (isotype) switching time course, according to preliminary data.
  • A strong positive correlation between clinical severity and antibody titer two weeks after symptom onset was reported.

COVID-19 Background: SARS-CoV-2 Structure & Infiltration

SARS-CoV-2, the causative viral agent of the disease COVID-19, is a coronavirus which bears the transmembrane glycoprotein spikes (S protein) typical of viruses in its clade. These spikes are a prominent target of human immune responses and have been found to be highly immunogenic. The receptor-binding domain (RBD) of the S protein is particularly targeted by neutralising antibodies.

The spikes on SARS-CoV-2 allows the virus to enter host cells through the human receptor angiotensin converting enzyme 2 (ACE2), present in alveolar epithelial cells. The S protein RBD and ACE2 interaction is explained in great detail in this article.

The time between initial viral exposure and symptom onset is known as the incubation period. For COVID-19, the average incubation period has been reported to be between five and six days. However, there is considerable variation in incubation time, with some studies suggesting symptoms can appear as soon as three days post-exposure or as late as thirteen days post-exposure.

Immune Response to SARS-CoV-2: Implications for Diagnostic Testing

Seroconversion is the transition from a seronegative condition—where no antibodies are in the serum, or they are present but below the limit of detection— to a seropositive condition, in which antibodies can be detected in serum samples. This is the definition set forth by The World Health Organisation (WHO).

Understanding the timing of antibody production and seroconversion is key. Determining the optimal timepoints for the collection of patient specimens increases the efficacy of diagnostic antibody testing. Moreover, knowledge of timing informs the choice of when to obtain peripheral B cells for the development of monoclonal antibody therapeutics.

Isotype switching, also called immunoglobulin class switching, is the conversion of B cell’s production of antibodies from one type to another. Of particular interest is the transition from IgM isotype antibodies, which are the first to be generated to a novel antigen, to IgG antibodies, which are more prolific effector antibodies.

Class switching does not change the antibody’s specificity for an antigen, but instead allows it to exert different biological effects. The antibody isotypes present in a patient specimen can give important information about the timing of initial exposure, as well as provide insight on the progression of the disease and prognosis.

Antibody tests boast a shorter turnaround time (as in the case of this test, which provides results in 15 minutes), a reduced workload on healthcare professionals, and are more portable given their self-contained nature and reduced need for specialised laboratory equipment.  

Serological testing plays a vital role in understanding and ultimately combating viral outbreaks. Their relevant applications include providing robust epidemiological data— invaluable in the determination of rates of infection and thus fatality metrics— as well as aiding in screening for patients who mounted strong antibody responses who could be donors for the engineering of serum therapeutics. Similarly, serological tests can help identify healthcare workers who may have already developed immunity to the disease. This information could inform staffing decisions and limit the risk of inadvertently spreading the virus in hospital settings.

Seroconversion in COVID-19: Preliminary Data

Seroconversion for IgG IgM and total antibodies over time.

Fig 1. Cumulative seroconversion percentage over time. *

Researchers in China have recently published data indicating that COVID-19 patients show typical antibody production times in response to acute viral infection with SARS-CoV-2. Overall, the data suggest that SARS-CoV-2 infection follows a seroconversion timeline similar to other viral infections.

The reported data is as follows:

173 hospitalised patients with confirmed SARS-CoV-2 infection were enrolled in the study. In total, 535 plasma samples were collected from them and analysed for the presence of total anti-SARS-CoV-2 antibodies (IgM, IgG, IgA, etc.), IgM antibodies, and IgG antibodies.

Seroconversion Rates

Antibody Type Percentage

Total Anti-SARS-CoV-2 antibodies

93.1% (161/173)

Anti-SARS-CoV-2 IgM antibodies

82.7% (143/173)

Anti-SARS-CoV-2 IgG antibodies

64.7% (112/173)


Seroconversion Timing in COVID-19

Median Seroconversion Time

Antibody Type Days (post-symptom onset)

Total Anti-SARS-CoV-2 antibodies

11 days

Anti-SARS-CoV-2 IgM antibodies

12 days

Anti-SARS-CoV-2 IgG antibodies

15 days

Antibody binding signal for IgG IgM and total antibody levels over time (days post-onset) for SARS-CoV-2 COVID-19.

Fig 2. Relative antibody binding signal over time. *

Seroconverted Patients 15 Days after Symptom Onset

Antibody Type Percentage of Patients

Total Anti-SARS-CoV-2 antibodies

100%

Anti-SARS-CoV-2 IgM antibodies

94.3%

Anti-SARS-CoV-2 IgG antibodies

79.8%

Seroconversion curves were constructed from the data and showed that total antibodies and IgM isotype antibodies were 100% detectable approximately one month after symptom onset. However, in the first week after developing signs of illness, antibodies were present in less than 40% of patients tested. The seroconversion rate and antibody levels rose quickly during the fortnight after symptom onset, and the cumulative seropositive rate was 50% on day 11 and 100% on day 39.

Antibody vs. Viral RNA Detection of SARS-CoV-2

Combining PCR and serological testing was reported to significantly increase the sensitivity of COVID-19 diagnosis, even in the early stage of infection. Detection of RNA was 66.7% in specimens collected prior to day 7, but 45.5% during days 15 to 39. Antibody-based detection was more robust from day 8 onwards, and was over 90% sensitive after day 12 post-onset.

Performance of Detection Methods Over Time (Sensitivity Scores, Days Post-Symptom Onset)

Day 1-7 Day 8-14 Day 15-39

RNA

66.7

54.0

45.5

Total Ab

38.3

89.6

100.0

IgM

28.7

73.3

94.3

IgG

19.1

54.1

79.8

RNA + Ab

78.7

97.0

100.0

Clinical Severity and Antibody Titres in COVID-19

A higher titer of total antibodies was independently associated with a more severe clinical classification. Quantitative total antibody titer data was reported to show a significant difference between patients in critical and non-critical groups . Age, gender, and total antibody titer were the independent factors found to be most strongly linked with clinical disease severity.

Critical vs. non-critical antibody titres for COVID-19 patients with sever or non-severe cases of the disease.

Fig 3. Critical vs. non-critical antibody titers over time. *

Significance for Future Testing Methodology

The data presented in this study provide evidence that SARS-CoV-2 infection follows a similar antibody pattern and time course to other viral infections. It also implies that serological testing, in conjunction with other methods of detection, can provide a robust method of diagnosing COVID-19 across early and late stages of infection. Of the types of antibodies measured, total antibody metrics were the most sensitive in detecting SARS-CoV-2 versus IgM or IgG alone.

The strong positive correlation between antibody titer levels after two weeks of symptom onset and clinical severity is also noteworthy. High titers of SARS-CoV-2 total antibodies could be an important risk factor for critical illness, distinct from the previously established risk factors of age, gender, and underlying conditions.

Further Tests for COVID-19

Testing for antibodies is one method for detecting the presence of SARS-CoV-2. Other tests, such as reverse-transcriptase polymerase chain reaction (RT-PCR), can be employed as well. Fore more information about the principles and methods of RT-PCR testing in COVID-19, please see this article.

Referenced Study

Original data available here,

* All figures adapted from the original paper below.

Juanjuan Zhao Jr. et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Medrxiv (pre-print). March 2019. doi: https://doi.org/10.1101/2020.03.02.20030189

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6th Nov 2020 Paige Dougherty MSc

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