key: cord-0872794-tozxcflr
authors: Bayram, Ayşen; Demirbakan, Hadiye; Günel Karadeniz, Pınar; Erdoğan, Merve; Koçer, Ipek
title: Quantitation of antibodies against SARS‐CoV‐2 spike protein after two doses of CoronaVac in healthcare workers
date: 2021-05-31
journal: J Med Virol
DOI: 10.1002/jmv.27098
sha: fe508fc202e536354a32477c57bee6580c954a33
doc_id: 872794
cord_uid: tozxcflr

Quantitation of antibodies to the spike protein of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) was performed for the detection of adaptive immune response in healthcare workers (HCWs) vaccinated with CorovaVac. We prospectively recruited HCWs from a university hospital in Turkey. Serum samples from 1072 HCWs were obtained following 28 days of the first, and 21 days of the second dose. Detection and quantitation of SARS‐CoV‐2 antispike antibodies were performed by the chemiluminescent microparticle immunoassay (SARS‐CoV‐2 IgG II Quant; Abbott). Results greater than or equal to the cutoff value 50.0 AU/ml were reported as positive. After the first dose, antispike antibodies were detected in 834 of 1072 (77.8%) HCWs. Seropositivity was higher among females (84.6%) than males (70.6%) (p < 0.001) and was found to be highest in both women and men between the ages of 18–34. After the second dose, antibodies were detected in 1008 of 1012 (99.6%) HCWs. Antibody titers were significantly higher in those who had coronavirus disease‐2019 before vaccination than those who did not (p < 0.001). Antibody positivity and median antibody titers were significantly less in HCWs with chronic diseases compared to those without (p < 0.05 and p < 0.001, respectively). In conclusion, our findings indicated that a relatively high frequency (99.6%) of humoral immunity was produced in HCWs aged 18–59 after two doses of CoronaVac. Quantitation of antibodies may help facilitate longitudinal monitoring of the antibody response, which will be especially useful in deciding the dose of the vaccine in vulnerable groups such as those over 60 years of age and those with chronic diseases.

Therefore, prioritizing HCWs for vaccination has been at the forefront of SARS-CoV-2 vaccination programs internationally.

Within the scope of combating the COVID-19 pandemic, Turkey had given emergency use approval (EUA) for the use of CoronoVac and vaccination started with HCWs on January 14, 2021 in Turkey.

CoronaVac is a chemically inactivated whole virus vaccine for COVID-19 developed by Chinese biopharmaceutical company Sinovac Life Sciences and is created from African green monkey kidney cells (Vero cells) that have been inoculated with SARS-CoV-2 CN02 strain. It has shown good immunogenicity in mice, rats, and nonhuman primates with vaccine-induced neutralizing antibodies, which could neutralize ten representative strains of SARS-CoV-2. 1 Antibodies serve as biomarkers of immunity; detection of specific antibodies can provide information on adaptive immunity against SARS-CoV-2. Neutralization tests seen as the gold standard for assessing specific immunity and a benchmark for other antibody assays requires individual tests with incubation times of 5-7 days. This complexity and the need for increased biosafety level 3 precautions make it difficult for routine testing on a large scale. 2, 3 Quantitative assays detecting anti-SARS-CoV-2 antibodies may help determine specific antibody response to vaccines, individual antibody titer, and longitudinal monitoring of the antibody response. 4 They also may assess whether a person's antibody levels are a result of the adaptive immune response induced by infection, versus a vaccine-induced response. 5 Most serologic assays are qualitative and use either full-length or truncated versions of the nucleocapsid (N) or spike (S) SARS-CoV-2 protein as the target for antibody detection. SARS-CoV-2 spike protein is highly conserved among all human coronaviruses and is involved in receptor recognition, viral attachment, and entry into host cells. Due to its indispensable functions, it represents one of the most important targets for the COVID-19 vaccine and therapeutic research. 6 Abbott recently developed a quantitative immunoassay that measures antibodies against the receptor-binding domain (RBD) of the S1-subunit of the SARS-CoV-2 S protein, the target of vaccines in development and in use. 7 To determine the immunogenicity of CoronaVac against SARS-CoV-2, this study aimed to quantify the humoral immune response induced in HCWs after the first and second doses of vaccination. Furthermore, we planned to evaluate the longitudinal dynamics of the antibody response to SARS-CoV-2 after 4 and 6 months following the initial dose. Here, we report the preliminary results of the study.

HCWs of both genders, 18 years of age or older, who agreed to participate in this prospective study and those who underwent two-dose (28-day interval) SARS-CoV-2 vaccination with 

The SARS-CoV-2 IgG II Quant assay (Abbott) is designed to detect IgG antibodies to the RBD of the S1 subunit of the spike protein of SARS-CoV-2 in serum and plasma from individuals who are suspected to have been infected by SARS-CoV-2. The assay is also to be used as an aid in evaluating the immune status of individuals with quantitative measurement of IgG antibodies induced by vaccination.

This assay is an automated, two-step immunoassay for the qualitative and quantitative determination of IgG antibodies to SARS-CoV-2 in human serum and plasma using chemiluminescent microparticle immunoassay technology. Sample, SARS-CoV-2 antigen-coated paramagnetic microparticles, and assay diluent are combined and incubated according to the manufacturer's instructions. The IgG antibodies to SARS-CoV-2 present in the sample bind to the SARS-CoV-2 antigen-coated microparticles. The mixture is washed.

Anti-human IgG acridinium-labeled conjugate is added to create a reaction mixture and incubated. Following a wash cycle, Pre-Trigger and Trigger Solutions are added.

The resulting chemiluminescent reaction is measured as a relative light unit (RLU). There is a direct relationship between the amount of IgG antibodies to SARS-CoV-2 in the sample and the RLU detected by the system optics. Detection was carried on with Architect i2000SR instrument (Abbott). Test results greater than or equal to the cutoff value stated in the assay's package insert, that is, 50 arbitrary units per milliliter (50 AU/ml) were reported as reactive and interpreted as positive for SARS-CoV-2 antispike IgG antibodies.

Results below the cutoff value are reported as nonreactive and interpreted as negative. This assay has an analytical measuring interval of 21-40 000 AU/ml (up to 80 000 AU/ml with on-board 1:2 dilution). The assay presented a positive predictive agreement of 99.4% (95% confidence interval [95% CI]: 96.50%-99.97%) and a negative predictive agreement of 99.6% (95% CI: 99.15%-99.37%), and was in agreement with a neutralization method (positive agreement, 100.0%; 95% CI: 95.72%-100.00%). 8 

As descriptive statistics, median and minimum-maximum values for continuous variables, and frequency and percentage values for qualitative variables were given. In group comparisons, oneway analysis of variance or independent samples t test was used for continuous variables and χ 2 test was used for qualitative variables. In all evaluations, p < 0.05 was considered statistically significant.

A total of 1072 voluntary HCWs gave written informed consent and completed the questionnaire at the beginning of the study. The median age of the participants was 33.2 years (95% CI-0.67: 32.6-33.9 years). The cohort had a slightly greater representation from female individuals, with 51.5% female and 48.5% male. The age distribution of this cohort was as follows: 18-34 years old, 642 (59.9%); 35-59 years old, 406 (37.9%); 60 years and older, 24 (2.2%) ( Table 1 ).

After 28 days of the first dose of CoronaVac, antispike IgG antibodies were detectable in 834 of 1072 (77.8%; 95% CI-0.025: 75.44%-80.4%) HCWs. Seropositivity was higher among females (467/552; 84.6%) than males (367/520; 70.6%) (p < 0.001) and was found to be highest in both women and men between the ages of 18-34 (88.9% and 79.5%, respectively). Among HCWs between 35 and 59 years, antispike IgG antibodies in females and males were 75.3% and 64.2%, respectively, and among those more than 60 years 37.5% in both genders. There was no statistically significant difference between age groups of both genders in terms of antibody positivity (p < 0.05). After the first vaccine, antibody titers were found 3-4 times higher in those who had COVID-19 than those who did not, and the difference was statistically significant (p < 0.001). Though antibody titers were not significantly different between the age groups in workers with a history of COVID-19, there were statistically significant differences in antibody titers between age groups both in men and women who have not had COVID-19 (p < 0.001 for both).

The highest antibody titers were found between ages 18 and 34. patients in the last 12 months, are provided in Table 3 .

Of 1072 HCWs, 225 (21%) informed that they had at least one chronic disease; hypertension (59.6%) was the most commonly reported clinical complaint. Antibody positivity rate was lower in HCWs who did not have COVID-19 and had at least one chronic disease (59.6%) compared to those without the chronic disease Table 4 .

This study aimed to determine and quantitate the level of antibodies After the first vaccine, the rate of antibody positivity and the amount of antibody titers were found higher in those who had COVID-19 than those who did not, and the differences were statistically significant (p < 0.001, p < 0.001, respectively). This result showed that people who had COVID-19 can generate high antibody levels even with a single dose of vaccination, thus they could undergo a different vaccination schedule.

People older than 60 years have an increased risk of severe illness and death from COVID-19, especially those with underlying chronic conditions. The response to vaccines is usually reduced in older adults due to immune senescence. Zhiwei et al. 11 reported in their clinical trial that CoronaVac was well tolerated and immunogenic in healthy adults aged 60 years and older and neutralizing antibody responses to live SARS-CoV-2 were not reduced in that population. Our findings showed that the antispike antibody response in HCWs ≥60 years old (n = 24) after the first dose was relatively low (37.5%); however, immunogenicity reached a level close to that in the 18-59 age group after the second dose (95.7%). A CoronaVac study from Chile reported the seroconversion rate for the ≥60 years old group 18.1% after 14 days of the first dose, and 100% after 28 days of the second dose. 12 As mentioned by Grupper et al. 13 age is an important factor in the humoral response induced after vaccination regardless of chronic medical conditions. We conclude that two doses of vaccination with CoronaVac were capable of induction humoral response in people over 60 years of age.

Patients with comorbidities tend to have a reduced immune response to infection or vaccination, and consequently, there is often a need for higher vaccine dosage or scheduling changes in these patients. 13 Geisen et al. 14 quantitative determination of anti-SARS-CoV-2 antibodies may help facilitate longitudinal monitoring of the antibody response in individual patients and specifically monitor antibody response to vaccines. 15 Although the induction of the humoral response after two consecutive doses of CoronaVac was considered positive for most HCWs aged 18-59 years, lower rates of antibody production and lower median antibody titers were detected in participants aged more than 60 years and those with comorbidities. Although our findings are preliminary, additional data obtained in antibody titers at the end of the 4th and 6th months following the first dose will prompt consideration for changing the dose/schedule of vaccinations in vulnerable patient groups. 

Development of an inactivated vaccine candidate for SARS-CoV-2

Clinical performance of different SARS-CoV-2 IgG antibody tests

Serological assays for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor

SARS-CoV-2 Serology Status Detected by Commercialized Platforms Distinguishes Previous Infection and Vaccination Adaptive Immune Responses. medRxiv

Structural and functional properties of SARS-CoV-2 spike protein: potential antivirus drug development for COVID-19

Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial

SARS-CoV-2 Immunoassays: Advancing diagnostics of COVID-19

Discrete SARS-CoV-2 antibody titers track with functional humoral stability

Vaccines: correlates of vaccine-induced immunity

Safety, tolerability, and immunogenicity of an inactivated SARS-CoV-2 vaccine (CoronaVac) in healthy adults aged 60 years and older: a randomised, double-blind, placebocontrolled, phase 1/2 clinical trial

Interim report: safety and immunogenicity of an inactivated vaccine against SARS-CoV-2 in healthy chilean adults in a phase 3 clinical trial. medRxiv

Humoral response to the Pfizer BNT162b2 vaccine in patients undergoing maintenance hemodialysis

Immunogenicity and safety of anti-SARS-CoV-2 mRNA vaccines in patients with chronic inflammatory conditions and immunosuppressive therapy in a monocentric cohort

A high-throughput neutralizing antibody assay for COVID-19 diagnosis and vaccine evaluation

How to cite this article

Quantitation of antibodies against SARS-CoV-2 spike protein after two doses of CoronaVac in healthcare workers

This work was funded by the Scientific Research Projects Unit of Sanko University. The funder of the study had no role in study design, data collection, data analysis, or data interpretation, or writing of the report. The authors would like to thank the laboratory staff at Sanko Hospital for completing the antibody testing.

The authors declare that there are no conflict of interests.

Ayşen Bayram and Hadiye Demirbakan were responsible for the project administration, methodology, and writing of the study. Ipek Koçer and Merve Erdoğan were responsible for the acquisition, analysis, and interpretation of the results. Pınar Günel Karadeniz was responsible for statistical analysis.

The peer review history for this article is available at https://publons. Merve Erdoğan https://orcid.org/0000-0003-3845-9100Ipek Koçer https://orcid.org/0000-0002-0631-6415