# Evaluation of the Humoral Immune Response Following Two Doses of a Coronavirus Disease 2019 Vector-Based Vaccine During the Initial Rollout in Bangladesh

## TL;DR

This study evaluated the immune response to the ChAdOx1 vaccine in Bangladesh and found that most people developed strong antibodies, especially if they had a prior infection.

## Contribution

The study identifies a specific antibody threshold (359.5 BAU/mL) that may predict protection against future infections.

## Key findings

- 86.7% of participants had positive SARS-CoV-2 IgG antibody titers after two doses of ChAdOx1.
- Individuals with prior infection had significantly higher antibody levels than those without.
- A threshold of 359.5 BAU/mL was associated with preliminary immunity against reinfection.

## Abstract

Introduction: The rollout of coronavirus disease 2019 (COVID-19) vaccination was crucial in addressing the pandemic in Bangladesh, with the ChAdOx1 vaccine being the primary vaccine administered to most of the population. This study aims to assess the humoral immune response by measuring severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) IgG titers after two doses of the ChAdOx1 vaccine in the Bangladeshi population, irrespective of prior COVID-19 infection status, and to identify antibody titer levels that can predict future COVID-19 infection.

Methods: This was a cross-sectional study conducted among 256 individuals exhibiting COVID-19-like respiratory symptoms who had completed two doses of ChAdOx1 between December 2021 and March 2022. Participants were tested for COVID-19 via nasopharyngeal swabs using real-time polymerase chain reaction (PCR), and SARS-CoV-2 IgG antibody titers (≥33.8 binding antibody unit (BAU)/mL was considered positive) were measured from blood samples with chemiluminescence immunoassay (CLIA) at the COVID-19 laboratory, Department of Virology, Bangladesh Medical University (BMU). Demographic and clinical data were collected, and results were analyzed using IBM SPSS Statistics software version 22 (IBM Corp., Armonk, NY), with a P-value of <0.05 deemed statistically significant.

Results: The overall seropositivity rate for SARS-CoV-2 IgG antibodies was 86.7%, with a median antibody titer of 501.0 BAU/mL (range: 4.81-2080.0 BAU/mL). Individuals with a prior COVID-19 infection exhibited significantly higher antibody titers (P<0.01) (mean: 1262.12±864.46 BAU/mL, median: 1465 BAU/mL) compared to those without a prior infection. The receiver operating characteristics (ROC) curve analysis (area under the curve (AUC): 0.823, 95% confidence interval (CI): 0.769-0.877, P<0.001) identified a minimum antibody threshold of 359.5 BAU/mL for preliminary immunity against future COVID-19 infections (sensitivity: 81.3%, specificity: 65.9%, Youden’s Index: 0.47). Despite vaccination and a history of previous COVID-19, 1.5% of the studied population presented as reinfection with SARS-CoV-2, confirmed by real-time PCR testing. Comorbidity variables, such as diabetes mellitus, asthma, chronic obstructive pulmonary disease (COPD), and hypertension, showed no statistically significant association with the antibody response.

Conclusion: This study demonstrated that ChAdOx1 elicits robust antibody responses in the majority of individuals, with significantly stronger reactions observed among those with prior COVID-19 infection. Moreover, it provides effective protection against recurrent infection in this population, provided that a SARS-CoV-2 IgG titer is sustained above the minimum threshold of 359.5 BAU/mL. Nevertheless, additional research is warranted to characterize the SARS-CoV-2 IgG response across diverse populations receiving heterologous or multi-regimen vaccination strategies.