Evaluation of serum level of anti-SARS-CoV-2 IgG antibodies among vaccinated individuals in Diyala Province, Iraq

Background: Monitoring antibody titer and antibody decline as markers of humoral immune response is essential when evaluating the efficacy of COVID-19 vaccines. Therefore, the present study aimed to estimate the prevalence of IgG antibodies associated with Pfizer/Sinopharm COVID-19 vaccines among vaccinated individuals. Methods: A cross-sectional study was conducted during the period extended between December, 2023 and May, 2024. Blood samples were collected from 120 individuals who received a minimum of a two-dose COVID-19 vaccine. The concentration of anti SARS-CoV-2 IgG antibodies was investigated using ELISA technique. Results: The study found that 50.83% of participants had moderate neutralization rate (10-100 IU/ml), while only 21.67% had a high serum level (100-400 IU/ml). However, 27.50% had a concentration of antibodies less than 10 IU/ml. After 3 weeks from the last dose of vaccine, age 50-59 for the Pfizer vaccine and 40-49 for the Sinopharm vaccine were significantly associated with high antibody concentrations (P-value <0.01). Vaccinated participants who had never been infected with the virus showed the highest mean and SD for both vaccines. Furthermore, the findings showed that the mean serum IgG antibody levels were significantly higher in those who received three or more doses of the vaccine compared to those who received only two doses (P-value <0.05). Conclusion: The neutralization rate of anti-SARS-CoV-2 IgG antibodies was moderate in participants vaccinated with Pfizer and Sinopharm vaccines, which is lower than previous global studies, suggesting the need for further surveys to assess the effectiveness of vaccines and determine the need for additional doses.