Seroprevalence and persistence of humoral immunity in response to SARS-CoV-2 vaccination and mild infection in Morocco: a cross-sectional study

Abstract

Introduction: assessing the seroprevalence and persistence of humoral immunity through population-based serological surveys is crucial for predicting reinfection risk and optimizing vaccination strategies. In Morocco, a mass vaccination campaign against SARS-CoV-2 has been launched to achieve herd immunity and reduce virus transmission. This study evaluated the humoral immune response to the BBIBP-CorV and ChAdOx1 nCoV-19 vaccines in healthy donors with varying doses and infection histories. Methods: IgG levels were measured via Vircell COVID-19 ELISA, and NAb levels were assessed via the GenScript Surrogate Virus Neutralization Test (sVNT). The study included individuals with different vaccination doses and prior infection histories. Results: the third vaccine dose produced the highest IgG titers, while nonvaccinated individuals and those receiving two doses had lower levels. Prior SARS-CoV-2 infection led to increased antibody levels, suggesting that hybrid immunity offers prolonged protection. The highest NAb titers were observed in individuals receiving three doses, with 96% of previously noninfected individuals being positive for NAb. Risk factor analysis indicated that prior infection (OR = 8.64, 95% CI: 2.52-29.6, p < 0.001) and three doses (OR = 22.1, 95% CI: 2.74-178, p < 0.001) increased immunity. Conclusion: hybrid immunity, involving prior infection and full vaccination, offers enhanced and longer-lasting protection. These results support the need for targeted vaccination strategies to optimize immunity and reduce the risk of adverse outcomes, especially in high-risk populations.