Abstract
Mass vaccination is an effective method for controlling the outbreak of coronavirus disease 2019 (COVID-19) and limiting the consequent mortality due to severe COVID-19. After the second dose, immunity can decline in certain cases over time; therefore, a third booster dose should be administered. Therefore, the present study aimed to assess the immunogenicity of the third dose of the messenger ribonucleic acid BioNTech COVID-19 vaccine and determine the effect of the third booster dose of messenger ribonucleic acid COVID-19 vaccines, specifically (Oxford/AstraZeneca COVID-19 vaccine/AZD1222), BioNTech COVID-19 vaccine, and Sinopharm among healthcare workers. This longitudinal panel design was conducted with 256 healthcare workers in Duhok Province, Iraq, from June to October 2022. Most participants had a normal body mass index (44% and 41% in the first and second phase, respectively). In the first phase, significant associations were observed between COVID-19 vaccines and positivity (P value ≤ .001), and between age groups and positivity (P value = .001). The mean severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) anti-spike receptor-binding domain immunoglobulin G antibody level in the ninth month was the highest among those who had received the Pfizer vaccine (6.7930), followed by AstraZeneca (2.8492), and Sinopharm (0.3060). In the 12th month, all 82 participants received Pfizer as a booster dose, and the highest mean SARS-CoV-2 anti-spike receptor-binding domain immunoglobulin G antibody in the 12th month belonged to those whose second dose was Pfizer (46.8835), followed by AstraZeneca (36.4635), and Sinopharm (21.7815). The Pfizer vaccine is highly effective in restoring SARS-CoV-2-specific immune responses and is well-tolerated. However, further investigation is required to determine the duration of disease protection of the third dose of the COVID-19 vaccine.