INTRODUCTION
The Coronavirus Disease 2019 (COVID-19) outbreak was first reported in Wuhan, Hubei Province, China, in December 2019 and rapidly spread across the globe as a severe acute respiratory syndrome caused by the novel coronavirus, SARS-CoV-2. The disease resulted in substantial morbidity and mortality worldwide, posing an unprecedented public health challenge. Recognizing the alarming levels of transmission and severity, the World Health Organization (WHO) officially declared COVID-19 a global pandemic on 11 March 2020.
SARS-CoV-2 is a highly transmissible virus that threatens human health and safety. Millions of lives have been lost globally due to COVID-19, with studies indicating that males are generally more vulnerable to severe disease outcomes than females.
One of the most effective strategies for protecting individuals from COVID-19 is the development of adequate immunity through natural infection, vaccination, or a combination of both. Previous studies have demonstrated that antibody titers following a single dose of COVID-19 vaccine in previously infected individuals were higher than those observed in infection-naïve individuals even after receiving two vaccine doses.
Antibodies play a critical role in preventing SARS-CoV-2 infection by inhibiting the binding of the viral spike protein to the angiotensin-converting enzyme 2 (ACE2) receptors on host cells. In Bangladesh, the nationwide COVID-19 vaccination program commenced on 7 February 2021. The government initially prioritized frontline workers, elderly individuals, and other high-risk groups for vaccination.
Studies conducted in Bangladesh following administration of the Oxford-AstraZeneca vaccine demonstrated that previously infected individuals developed antibody titers approximately six times higher than those without prior infection. Consequently, vaccination strategies prioritized individuals at greater risk of severe illness, including healthcare workers, elderly populations, and immunocompromised persons.
Considering the importance of both natural infection and vaccination in developing immunity against SARS-CoV-2, the present study was designed to evaluate and compare antibody status between previously infected vaccinated healthcare personnel and non-infected vaccinated healthcare personnel.
MATERIALS AND METHODS
This cross-sectional observational study was conducted in the Department of Biochemistry and Molecular Biology at Bangladesh Medical University (BMU), formerly Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh, from March 2021 to February 2022.
Inclusion Criteria


Age between 25 and 65 years.


Both male and female participants.


Healthcare personnel previously infected with SARS-CoV-2 (confirmed by RT-PCR) within the previous 8–12 months and vaccinated with two doses of the Oxford-AstraZeneca COVID-19 vaccine within the last 4–6 months.


Healthcare personnel without prior SARS-CoV-2 infection who had received two doses of the Oxford-AstraZeneca COVID-19 vaccine within the last 4–6 months.


Exclusion Criteria


Subjects with acute infection.


Pregnant women.


Lactating mothers.


Patients with heart failure.


Subjects with systemic diseases such as chronic liver disease or chronic kidney disease.


Patients diagnosed with endocrine disorders, including thyroid disease.


Subjects suffering from immunosuppressive disorders such as cancer or systemic lupus erythematosus (SLE).


Based on the inclusion and exclusion criteria, a total of 70 healthcare personnel from different departments of Bangladesh Medical University Hospital were enrolled in the study. The participants were divided into two groups:
Group A
Healthcare personnel who had previously been infected with SARS-CoV-2 and subsequently received two doses of the Oxford-AstraZeneca COVID-19 vaccine.
Group B
Healthcare personnel who had not been infected with SARS-CoV-2 but had received the same two-dose Oxford-AstraZeneca vaccination schedule.
Each group consisted of 35 participants. Participants were further categorized according to age as follows:


Young age group: 25–35 years


Middle age group: 35–45 years


Elder age group: 45–55 years


Older age group: 55–65 years


A structured data collection sheet was used to record participant information. Under strict aseptic precautions, 5 mL of venous blood was collected from the antecubital vein. Of the collected sample, 2 mL was immediately transferred into a sodium fluoride tube (grey-top tube), while the remaining 3 mL was placed into a plain tube (red-top tube).
All blood samples were centrifuged to separate serum, which was then transferred into properly labeled Eppendorf tubes and stored at –65°C until analysis. Serum IgG levels were measured using a chemiluminescent microparticle immunoassay (CMIA) on the Abbott Alinity i Autoanalyzer (Abbott Inc., USA).
All immunological analyses were performed in the Department of Biochemistry and Molecular Biology, Bangladesh Medical University (BMU), Dhaka, Bangladesh.
Continuous variables were expressed as mean ± standard deviation (SD) and compared between groups using the unpaired Student’s t-test. Categorical variables were presented as frequencies and percentages and analyzed using the Chi-square test. The Mann–Whitney U test was used to compare serum IgG levels between the SARS-CoV-2 infected vaccinated group and the SARS-CoV-2 non-infected vaccinated group.
A p-value of less than 0.05 was considered statistically significant at a 95% confidence interval. Statistical analyses were performed using Statistical Package for Social Sciences (SPSS) version 20.0 for Windows.
Ethical approval for the study was obtained from the Institutional Review Board (IRB) of Bangladesh Medical University (BMU), Dhaka, Bangladesh.