Binding Antibodies Responses to SARS-COV-2 Infection in Hospitalized Patients and Vaccinated Subjects: A Longitudinal Prospective Observational Study

Background: Prevalence of seropositivity following SARS-CoV-2 infection is vital in evaluating herd immunity. However, depending on illness severity, it remains unclear whether the breadth and magnitude of immune response to SARS-CoV-2 infection is for short or long term. Objective: To test the persistence of humoral antibody responses after SARS-CoV-2 exposure in patients with different illness severity and among volunteers who had been vaccinated. Methods: This study was conducted in two Saudi Arabian tertiary hospitals. Participants were categorized as critically ill COVID-19 patients, non-critically ill COVID-19 patients, or vaccinated volunteers. We collected demographic data, COVID-19 exposure history, symptoms, vaccination details, and serum samples to analyze antibody persistence. We evaluated SARS-CoV-2 antibody concentrations in COVID-19 patients with varying disease severity and age groups, as well as in BNT162b2-vaccinated individuals, focusing on IgG levels against the S.FL and S1 domains of the spike protein. Results: The study included 172 adults: 92 unvaccinated hospitalized COVID-19 patients and 80 vaccinated volunteers. All vaccinated subjects demonstrated seropositivity to the SARS-CoV-2 spike protein, with nearly 80% having a median antibody titer of 13,500 AU/mL. Notably, vaccinated subjects exhibited significantly higher IgG levels than naturally infected patients (P < 0.001), including higher S.FL and S1 titers, regardless of severity. Age, comorbidities, and previous infections influenced S-specific antibody levels. Among hospitalized patients, 58% required intensive care, with 28- and 90-day mortality rates of 23% and 43%, respectively. Conclusion: These findings shed light on the immune response dynamics following SARS-CoV-2 infection compared to vaccinated individuals, where the latter showed significantly higher level of antibodies response, providing crucial insights for evaluating short-term herd immunity and the effectiveness of natural infection-induced immunity.