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Ashraful Hoque,1 Anindita Das Barshan,2 Forhad Uddin Hasan Chowdhury,2 Jannatul Fardous,3 Mohammad Jahid Hasan,3 Md Abdullah Saeed Khan,3 Ahmedul Kabir4 1Department of Blood Transfusion, Sheikh Hasina National Institute of Burn and Plastic Surgery, Dhaka, Bangladesh; 2Department of Medicine, Dhaka Medical College Hospital, Dhaka, Bangladesh; 3Tropical Medicine and Infectious Disease Division, Pi Research Consultancy Center, Dhaka, Bangladesh; 4Department of Medicine, Mugda Medical College Hospital, Dhaka, BangladeshCorrespondence: Mohammad Jahid Hasan Email dr.jahid61@gmail.com; jahid@pircc.orgPurpose: The aim of the study was to assess the antibody response to the ChAdOx1-nCoV vaccine in individuals who were not previously infected by COVID-19.Patients and Methods: All people aged 18– 65 years who received their first vaccination with ChAdOx1-nCoV from March to May 2021 were approached for inclusion. Individuals with sufficient antibody titers against SARS-CoV-2 infection before vaccination were considered previously infected and were excluded from the analysis. We observed viral spike protein RBD-S1-specific IgG antibody levels at day 28 of the first dose of vaccination and day 14 of the second dose of vaccination (74 days from index vaccination). An optical density ratio (ODR) of > 1.1 was considered to have a positive antibody response, 0.8 to 1.1 borderline and < 0.8 was denoted as negative. Informed consent was ensured before enrollment, and ethical principles conformed with the current Declaration of Helsinki.Results: This observational study comprised 769 infection-naïve individuals (mean age 40.5 years, 38.9% female). Spike-specific IgG antibody responses elicited after the first and second doses of vaccine were 99.9% and 100%, respectively. The median ODR was 5.43 (interquartile range [IQR]: 4.32– 6.98) and 10.90 (IQR 9.02– 11.90) after the first and second doses. Higher age was associated with lower antibody levels after both dosages. However, no sex-specific variation was seen. People with comorbidity had a lower antibody level after the second dose. Tenderness (51.46%) and fever (19.30%) were the most common local and systemic side effects after vaccination.Conclusion: This study was one of the earlier attempts in the country to assess the antibody response to ChAdOx1-nCoV vaccine recipients. The results imply that general people should be encouraged to take the vaccine at their earliest.Keywords: ChAdOx1-nCoV, vaccine, COVID-19, Bangladesh, antibody response