Journal of Association of Pulmonologist of Tamil Nadu

: 2020  |  Volume : 3  |  Issue : 3  |  Page : 124--127

Seroprevalence of SARS-CoV-2-specific neutralizing antibodies in COVID-19 pneumonia patients

Sivanthi Sapna Rajendran1, R Sridhar2, R Narasimhan1,  
1 Department of Respiratory Medicine, Apollo Main Hospitals, Chennai, Tamil Nadu, India
2 Department of Respiratory Medicine, Sundaram Medical Foundation Hospitals, Chennai, Tamil Nadu, India

Correspondence Address:
Sivanthi Sapna Rajendran
Department of Respiratory Medicine, Apollo Hospitals, Greams Road, Chennai, Tamil Nadu


Aim: The aim is to know the seroprevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific neutralizing antibody responses in COVID-19 pneumonia patients and correlation of antibody titer values with COVID-19 pneumonia Category, cyclic threshold values of COVID-19 reverse transcription polymerase chain reaction (RT-PCR) test, and computed tomography (CT) chest severity score. Material and Methods: Using serologic assay, SARS-CoV-2 antibody titers were measured in 65 patients with COVID-19 pneumonia symptoms with either positive or negative COVID-19 RT-PCR tests and radiological changes in CT chest during COVID-19 Pandemic period. About 3–5 ml of venous blood was collected and blood samples were tested using Abbott SARS-COV 2 immunoglobulin G (IgG) assay. This assay detects IgG Antibody against SARS-CoV-2 nucleocapsid protein and has a sensitivity of 100% and a specificity of 99.6%. Sixty-five patients were enrolled for this study and their clinical data, comorbid conditions, laboratory RT-PCR test reports, SARS-COV-2 antibody titers, and CT chest severity score were assessed cross-sectionally. Results: The study was done on 65 hospitalized patients. Among them, 39 were male (60.9%) and 25 were female (39.1%). Most common comorbidities among them were systemic hypertension (16.7%), chronic obstructive pulmonary disease (36.1%), diabetes mellitus (5.6%), coronary artery disease (22.2%), and bronchial asthma (27.8%). Among 65 patients, 36 (56.3%) tested COVID-19 RT-PCR positive and 28 (43.8%) tested COVID-19 RT-PCR negative. 24 patients belong to category B1 (55.8%), 11 patients belong to category B2 (25.6%), and 8 patients belong to category C (18.6%). A high seroprevalence of SARS-CoV-2 IgG antibody after 2 weeks were observed-17% among Category B1 patients, 26% among Category B2 patients, and 43% among category C patients. CT chest severity score was correlated with serum antibody titers by Pearson correlation which revealed a positive correlation that is statistically significant (0.01). Cycle Threshold value of COVID-19 RT-PCR reports was correlated with serum antibody titers by Pearson correlation which revealed a negative correlation that is statistically insignificant. Conclusion: This study found a high seroprevalence of SARS-CoV-2 specific IgG antibodies among COVID-19 pneumonia patients and positive correlation of antibody titers with clinical COVID-19 pneumonia category and with CT Chest Severity score.

How to cite this article:
Rajendran SS, Sridhar R, Narasimhan R. Seroprevalence of SARS-CoV-2-specific neutralizing antibodies in COVID-19 pneumonia patients.J Assoc Pulmonologist Tamilnadu 2020;3:124-127

How to cite this URL:
Rajendran SS, Sridhar R, Narasimhan R. Seroprevalence of SARS-CoV-2-specific neutralizing antibodies in COVID-19 pneumonia patients. J Assoc Pulmonologist Tamilnadu [serial online] 2020 [cited 2021 Oct 27 ];3:124-127
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Full Text


COVID-19 Pandemic maimed millions of people globally. India reports largest population at risk of COVID-19 Pneumonia. Herd immunity at population level can curb this global COVID-19 pandemic. About 60%–80% population need immunity for viral replication rate to drop below 1, which can be achieved by recovery of patients from COVID-19 Pneumonia or via safe and effective vaccines. Patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection are protected against future infection irrespective of their antibody status. Immunoglobulin G (IgG) titers rise during weeks following infection as active plasma cells secrete Antibody into systemic circulation and then wane as plasma cells senesce. However, Memory B-cells and T-lymphocytes mediate long-term immunity against COVID-19 infections. This seroprevalence study of severe acute respiratory syndrome coronavirus 2 antibody helps us to understand the transmission dynamics of virus, estimate total infections including asymptomatic, mild cases, and COVID-19 reverse transcription polymerase chain reaction (RT-PCR) tested negative patients and also assess whether seroconversion is associated with robust immunity.

Study design

This cross-sectional study was done including 65 patients who were hospitalized during COVID-19 pandemic in Apollo Main Hospital, Greams road, Chennai, after getting informed consent. The privacy and confidentiality of patients were observed as per norms.

Data collection

The clinical data of 65 hospitalized patients-age, gender, comorbidities, date of onset of symptoms, COVID-19 pneumonia clinical category, cycle threshold value of COVID-19 RT-PCR reports, computed tomography (CT) chest severity score, serum IgG antibody titer values, clinical outcome were collected, compiled, and tabulated. Patients with positive serum IgG antibody titers against SARS-CoV-2 measured after 2 weeks of recovery were included in the study.

Statistical analysis plan

Categorical variables were represented by percentages. Normally distributed continuous variables were expressed as means +- standard deviation. Comparison of categorical variables was made by Chi-square test. Comparison of continuous variables was carried out by either Independent sample t-test or ANOVA, based on the categories available. Pearson correlation coefficient was computed to know the association between CT chest severity score and antibody titer values. Data entry was done using Microsoft Excel 2007. Data analysis was performed by IBM SPSS statistics for windows Armonk “NY, USA, IBM Corp version 25.0.” All P < 0.05 was considered statistically significant.


In this study, 65 patients with COVID-19 Pneumonia were included. Among them, 39 were male (60.9%), and 25 were female (39.1%).

COVID-19 RT-PCR was positive in 36 patients (56.3%) and negative in 28 patients (43.8%) as shown by [Figure 1].{Figure 1}

In terms of clinical COVID-19 category, 55.8% patients were category B1, 25.6% patients were category B2 and 18.6% patients were category C as shown by [Figure 2].{Figure 2}

Comorbid conditions like diabetes existed among 5.6% of patients, hypertension among 16.7% of patients, coronary artery disease among 22.2% of patients, chronic obstructive pulmonary disease (COPD) among 36.1% of patients and bronchial asthma among 27.8% of patients.

By Pearson correlation, serum sntibody titer values had a negative correlation (r = −0.281) with cyclic threshold values of COVID-19 RT-PCR test though statistically insignificant (P = 0.096) as shown in [Table 1].{Table 1}

A high seroprevalence of SARS-CoV-2 IgG antibody were observed after 2 weeks of infection-17% among Category B1 patients, 26% among Category B2 patients, and 43% among Category C patients, as shown in [Figure 3].{Figure 3}

By Pearson correlation, CT chest severity score had a positive correlation (r = 0.538) with serum antibody titers and it is statistically significant (P = 0.0001) as demonstrated in [Table 2] and [Figure 4].{Figure 4}{Table 2}

SARS-CoV-2 antibody titers was analyzed and compared among different Categories of COVID-19 pneumonia (Category B1/B2/C) patients by one-way ANOVA (Analysis of variance) test which showed significant mean difference value (0.05) as shown in [Table 3] and [Figure 5].{Figure 5}{Table 3}


Serological diagnosis is important for patients who present late with a very low viral load, below detection limit of RT-PCR assays. To know whether serology testing could detect COVID-19 Pneumonia, 28 patients with negative COVID-19 RT-PCR reports who had clinical symptoms of COVID-19 Pneumonia or abnormal radiological findings were included in the study and screened. Among these 28 suspected cases, all had virus specific neutralizing IgG Antibodies against SARS-CoV-2, tested 2 weeks after onset of symptoms suggesting that all these patients were infected with SARS-CoV-2. This kind of similar results were observed in study by Quan-Xin Long et al.,[1] but antibody titers were measured even in asymptomatic patients and contacts of COVID-19 Pneumonia patients.

Among 65 patients, 28 (48.3%) tested COVID-19 RT-PCR negative as seen in study by Arevalo-Rodriguez et al.[2] suggesting false-negative RT-PCR reports. Clinical suspicion of COVID-19 Pneumonia is based on respiratory symptoms such as fever, cough and shortness of breath as primary manifestations. Cases are then confirmed by detection of viral ribonucleic acid sequences by RT-PCR. A false-negative RT-PCR report could be related to insufficient viral specimens and viral load, incubation period of disease, presence of mutations that escape detection and improper swab collection methods.

Most patients with COVID-19 pneumonia were males (60.9%) as seen in study by Kai-Wang et al.[3] suggesting a fact that males have higher expression of angiotensin-converting-enzyme-2 (ACE-2) receptors for coronavirus as compared to females and also due to lifestyle changes like higher levels of smoking and drinking in men.

Most common underlying comorbid conditions were COPD (36.1%) which is similar to study by Leung et al.[4] and higher than studies conducted by Wai-Shing et al.[5] and Ling Ni et al.[6] suggesting that COPD may be a risk factor for more severe COVID-19 disease. This is attributed to fact that smoking was associated with higher expression of ACE-2 receptors in COPD patients.

Eight patients in Category C (all required oxygen supplementation) had a greater than fourfold increase in IgG titer. This finding is consistent with previous studies by Bai Zhong et al.[7] and Fatima Amanat et al.[8] suggesting that patients with more severe disease contain relatively higher levels of antibodies against SARS-CoV-2 infection.

Main hallmark of COVID-19 pneumonia is the presence of bilateral ground glass opacities with or without consolidative areas with a predominant peripheral, posterior, and lower lobe distribution which is assessed as validated CT severity score based on lobar extent of disease. This CT Chest Severity Score correlation with clinical symptoms and serum IgG Antibody titers helps in starting symptomatic treatment early in course of disease although COVID-19 RT-PCR test is negative. CT chest severity score had a strong positive correlation (r = 0.538) with serum antibody titers and statistically significant P value of 0.01 proving that chest CT score is a good indicator of extent of systemic inflammation due to coronavirus as seen in studies by Jie Zhang et al.[9]


This study found a high seroprevalence of SARS-CoV-2 specific IgG antibodies among COVID-19 pneumonia patients and positive correlation of antibody titers with clinical COVID-19 pneumonia category and with CT chest severity score. This seroprevalence study (percentage of population with serum containing antibodies that recognize virus) of SARS-CoV-2 proves that Virus specific antibody detection against SARS-CoV-2 could be considered as a complement to nucleic acid testing by RT-PCR for diagnosing suspected cases with negative RT-PCR results. This study highlights the importance of serological testing to achieve more accurate estimates of extent of COVID-19 pandemic.


Thanks to the Department of Radiology, Microbiology and Balasubramaniam Ramakrishnan, Senior Biostatistician, Department of Biostatistics, DME Apollo Hospitals, Greams Road, Chennai, India.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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