Summary

Introduction: The Coronavirus disease-2019 (COVID-19) pandemic that started over two years ago has led to high mortality and morbidity. Vaccine studies have been initiated worldwide to end the pandemic, and the CoronaVac^® vaccine was first administered to healthcare workers at high risk of COVID-19 in Turkey. In our study, we aimed to investigate serum antibody levels after vaccination.
Materials and Methods: Volunteer healthcare workers without COVID-19 disease who received two doses of CoronaVac^® vaccine 28 days apart and were at least 14 days after the last dose of vaccine were included in this study. Assessment of antibodies against Severe acute respiratory syndrome-Coronavirus-2 (SARS-CoV-2) in blood samples from participants was performed using the Elecsys^® anti-SARS-CoV-2 electrochemiluminescence immunoassay. Samples with a cut-off index (COI) (COI; signal sample/cut-off) <1.0 were considered negative; whereas samples with COI≥1.0 were deemed positive.
Results: A total of 269 healthcare workers, 168 women (62.5%), were included in our study. The mean age of the participants was 37.7±8.6 (minimum-maximum: 21-62). Antibody levels were positive in 188 (69.9%) of the participants. The median antibody level was 9.2 COI (interquartile ranges=3-34.7). In terms of mean age, the mean age of participants with negative antibodies was higher with a statistically significant difference (p=0.001). The antibody positivity rate of women was higher than that of men (p<0.001). No statistically significant association was found between the time elapsed after vaccination, presence of comorbidities, development of post-vaccine side effects, and antibody levels. It was found that one or more side effects developed in 45.7% of the participants after vaccination.
Conclusion: Our study showed that seropositivity developed significantly in healthcare workers after the CoronaVac^® vaccine. It emphasizes the importance of maintaining infection prevention and control measures and administering the SARS-CoV-2 vaccine for healthcare workers at high risk.

Introduction

The Coronavirus disease-2019 (COVID-19) pandemic, which has been ongoing for over a year, has led to high mortality and morbidity. As of July 12, 2021, there were 186 million confirmed cases worldwide, resulting in approximately 4 million deaths^[1].

Vaccine studies have been started worldwide to end the pandemic. According to their technological features, inactivated virus vaccines, live virus vaccines, vaccines containing recombinant viral carriers, nucleic acid-based vaccines, protein subunit vaccines, and many vaccines containing virus-like particles are under development^[2]. At least 13 different vaccines have already been developed, in addition to vaccines that are still undergoing clinical trials^[3]. In a meta-analysis of 51 studies, various COVID-19 vaccines were highly protective against Severe acute respiratory syndrome-Coronavirus-2 (SARS-CoV-2) related mortality and morbidity worldwide.

CoronaVac^® is an inactivated vaccine obtained from African green monkey kidney cells inoculated with the SARS-CoV-2 strain CN02. After the incubation period in the cell, the virus is collected, inactivated with p-propiolactone, concentrated, purified, and finally absorbed onto aluminum hydroxide to complete its production^[4].

In Turkey, the CoronaVac^® vaccine, which we are involved in its phase-3 studies and is approved for emergency use by the Turkish Medicines and Medical Devices Agency, was first administered to high risk healthcare workers (HCWs). The vaccine was administered in two doses, 28 days apart.

Our study aimed to investigate serum antibody levels at least 14 days after the last dose of vaccine, which indicates post-vaccination humoral immunity.

Methods

This study was conducted at Kayseri City Hospital from March 1 to March 15, 2021. All volunteer HCWs without COVID-19 disease, who received two doses of CoronaVac^® vaccine with an interval of 28 days, and who received their last dose of vaccine at least 14 days prior were included in the study. Participants were asked to complete a questionnaire that included medical history and recent or current symptoms. The Bio-speedy SARS-CoV-2 (2019-nCoV) reverse transcription-quantitative polymerase chain reaction (PCR) detection kit (Bioeksen, İstanbul, Turkey) was used to conduct the SARS-CoV-2 PCR test on nasopharyngeal swabs to determine the presence of SARS-CoV-2 infection.

The Kayseri City Hospital Ethics Committee approved this study (approval no: 140121/500, date: 14.01.2021), and a signed informed consent form was obtained from all participants.

Antibody Measurement

Elecsys anti-SARS-CoV-2 electrochemiluminescence immunoassay was used in this study. This assay is intended for use on Cobase analyzers (Roche Diagnostics International Ltd, Rotkreuz, Switzerland) for the in vitro qualitative detection of antibodies (both IgA and IgG) to SARS-CoV-2 in human serum and plasma. The Elecsys anti-SARS-CoV-2 test uses the double antigen sandwich test principle and a recombinant protein representing the nucleocapsid antigen to detect SARS-CoV-2 antibodies. Regarding anti-SARS-CoV-2 antibody positivity, samples with a cut-off index (COI) (COI; signal sample/cut-off) <1.0 were considered negative, whereas samples with COI ≥1.0 were deemed positive, as per the manufacturer’s instructions. A measured antibody level magnitude above the cut-off value was not considered an indicator of the sample’s total amount of antibody present.

Statistical Analysis

Statistical analyses were performed using the IBM Statistical Package for the Social Sciences (version 26) program. In the statistical evaluation of the data obtained from the study, categorical data were summarized as frequency and percentage, continuous data with normal distribution as mean±standard deviation, and data with skewed distribution as median [25-75% interquartile range (IQR)]. Normality controls of continuous measurements were tested using the Shapiro-Wilk test. Independent samples t-tests were used to compare two groups of continuous variables with a normal distribution. The non-parametric Mann-Whitney U test was used to perform two-group comparisons for variables not consistent with the normal distribution. The chi-square test was used to compare categorical variables. The statistical significance level was considered as 0.05.

Results

A total of 269 hospital personnel, 168 women (62.5%), were included in our study. The mean age of the participants was 37.7±8.6. Of the participants, 144 (53.7%) were nurses, 42 (15.6%) were medical secretaries, and 32 (11.9%) were physicians. At least one comorbid disease was present in 77 (28.6%) of the study participants. The most common comorbid disorders were hypertension (7.8%), diabetes (3.7%), and cardiovascular diseases (2.6%).

Comorbid diseases and occupational distribution are shown in Table 1.

Table 1: Basic characteristics of healthcare workers

Antibody levels were negative in 81 (30.1%) participants, while 188 (69.9%) were positive. The median antibody value of SARS-CoV-2 antibody-positive HCWs was 9.2 COI (IQRs=3-34.7).

A comparison of the participants’ antibody levels according to mean age reavealed a statistically significant difference, and the mean age of particiapants with negative antibodies was higher (p=0.001) (Table 2).

Table 2: Comparison of basic characteristics and antibody levels of HCWs

A comparison of the participants’ antibody levels in terms of gender revealed a statistically significant difference, and the antibody positivity rate of women was higher than that of men (p<0.001) (Table 2).

There was no statistically significant association of antibody levels with the time elapsed after vaccination and the presence of comorbid diseases was found (p values: 0.147 and 0.408, respectively). Similarly, no statistically significant association was found between the development of post-vaccine side effects and antibody levels (p=0.782) (Table 2).

It was found that one or more side effects developed in 45.7% of the participants after vaccination. No serious side effects were observed. The most common side effects were local side effects such as pain at the injection site (15.2%), headache (13.8%), weakness-fatigue (13.4%), and muscle-joint pains (8.6%) (Table 3).

Table 3: Adverse events

Discussion

The first randomized controlled phase 3 trial on inactivated vaccines was conducted with 40,000 adults in China. Two inactive vaccines were examined in this study and shown to significantly reduce the risk of symptomatic COVID-19 in those vaccinated with either vaccine^[5].

According to the results of the CoronaVac^® vaccine phase 3 trial in Turkey, the vaccine’s effectiveness is 83.5%; the rate of prevention of hospitalization was reported to be 100%, and no death was observed^[6]. According to the results from Chile, where the other phase 3 study was conducted, it was shown that the vaccine reduced symptomatic infection by 67%, hospitalizations by 85%, and mortality rates by 80%^[7]. The World Health Organization gave emergency use approval for the CoronaVac^® vaccine on June 1, 2021. It has been reported that the vaccine prevents 51% of symptomatic diseases and 100% of severe illness and hospitalizations^[8].

Presenting real-life data about the CoronaVac^®vaccine is beneficial in providing information about the vaccine’s effectiveness. Zhang et al.^[9] showed that a cellular immune response with the release of Th1 cytokines and a humoral immune response with the release of Th2 cytokines were triggered by the CoronaVac^® vaccine, especially in the following days after the 2^nd dose. Evaluation of the antibodies formed after vaccination gives an idea about the humoral response triggered by the vaccine.

A study of multiple HCWs found that anti-spike or anti-nucleocapsid antibodies significantly reduced the risk of SARS-CoV-2 reinfection over the following six months. In this study, anti-spike and anti-nucleocapsid IgG negative HCWs had 1.08 per 10,000 days at risk, both baseline assays positive HCWs had 0.07 per 10,000 days at risk, and only one assay positive HCWs had 0.49 per 10,000 days at risk^[10]. The advantage of inactivated vaccines is that all their proteins can act as antigens with the complete inactivation of the virus.

In a study conducted with 264 HCWs in Turkey, antibody levels developed after the CoronaVac^® vaccine were examined by measuring anti-RBD anti-SARS-CoV-2 IgG antibodies (Quantivac-Euroimmun/Germany) by ELISA. The antibody response was found to be 91.6% in the study. Factors reducing antibody response are male gender (p=0.007), advanced age (p<0.005), no history of COVID-19 (p=0.015), and previous H1N1 vaccination (p=0.044)^[11]. Additionally, the antibody response was higher in this study because 27% of the participants had a history of COVID-19 before they were vaccinated. In our study, all volunteer HCWs did not have COVID-19 disease, and our findings are similar in terms of male gender and age.

In a retrospective study of 75 HCWs who had negative IgM and IgG results before vaccination, IgM and IgG were measured 14-21 days after two doses of the CoronaVac^® vaccine. It was observed that 100% of the HCWs developed IgG, but IgM was negative^[12].

Preliminary results of a study conducted in Turkey among volunteer HCWs who were administered CoronaVac^® vaccine to investigate neutralizing antibodies formed against the region in the SARS-CoV-2 spike protein that binds to human cells were obtained using two different tests: quantitative and qualitative. Immunity was determined as 25.3% at least 28 days after the 1^st dose and 97% at least 28 days after the 2^nd dose. In addition, similar to our study, although both sexes had adequate immunity, a significantly higher vaccine immune response was obtained in women than in men^[13].

In the CoronaVac^® vaccine phase 1 study, a maximum of 38% of adverse events were recorded^[14]. In our study, the rate of side effects was slightly higher at 45%. In the phase 3 trial conducted in Turkey, side effects after the CoronaVac vaccine were reported as fatigue, headache, myalgia, fever, chills, and pain at the injection site. Our study shows that the rates of side effects such as local side effects, muscle pain, and headache are higher than in the phase 3 study^[11].

It has been reported that immune response and vaccine response decrease with age, and stronger immune response development and increasing functional antibody development are seen in women compared to men^[15-17]. Our study’s differences in antibody response according to age and gender seem to be expected.

Different antibody response rates in studies may be related to the test being studied measuring different antibodies or using varied kits/methods. Again, the features mentioned above, such as age, gender, and previous history of COVID-19, can also be considered factors affecting the antibody response. In general, most people appear to have an antibody response with the CoronaVac® vaccine. A study of HCWs showed that neutralizing antibody levels decreased significantly within two months^[18].

Study Limitations

There are some limitations of our study. First of all, our study was conducted in a single center with a limited number of people in fifteen days. Secondly, the inability to follow-up on the presence of antibodies in disease development is a limitation of our study. Third, using a qualitative method in antibody measurement is another limitation of our study.

Conclusion

This study provides data on antibody levels and side effects obtained with the CoronaVac^® vaccine among HCWs. The antibody levels offered by the vaccine and the clinical efficacy of the vaccine; will be clearer with further study.

Ethics

Ethics Committee Approval: The Kayseri City Hospital Ethics Committee approved this study (approval no: 140121/500, date: 14.01.2021).

Informed Consent: A signed informed consent form was obtained from all participants.

Peer-review: Externally and internally peer-reviewed.

Authorship Contributions

Concept: A.K.T., İ.Ç., Design: İ.Ç., Data Collection or Processing: A.K.T., E.E., A.T.Ö., A..K., Analysis or Interpretation: A.K.T., A.T.Ö., İ.T., Literature Search: D.Ç.Ö., İ.T., Writing: A.K.T., D.Ç.Ö.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.

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Anti-nucleocapsid Antibody Response After Two Doses of CoronaVac® Among Healthcare Workers