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. 2025 Jan-Mar;13(1):39-46.
doi: 10.4103/sjmms.sjmms_282_24. Epub 2025 Jan 11.

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

Mohammed Saeed Alshahrani  1   2 , Razan Aldahhan  3 , Nabela Calamata Macadato  3 , Zahra Alkhalifah  1   2 , Mohamed R El Tahan  4 , Amani Alnimr  5 , Mohammed Alabdrab Alnabi  6 , Fatima Almishkab  1 , Jawaher Almusairii  1 , Laila Perlas Asonto  1   2 , Sajjad Mohammed Almusawi  6 , Mohammed Almussalam  6 , Rawan Alsanea  6 , Mubarak Khalifa Albakheet  6 , Talal Ali Albrahim  1 , Moaz Saad Alammar  1 , Maan Albehair  7 , Suzan A Alkhater  8 , Amal Alsulaibikh  2 , Mohammed Aljumaan  2 , Saad M AlQahtani  9 , Ahmed Abdelhady  1   10 , Charlene Mapusao  1 , Ashraf Attia  1 , Iman Almansour Alzamil  3
Affiliations

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

Mohammed Saeed Alshahrani et al. Saudi J Med Med Sci. 2025 Jan-Mar.

Abstract

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.

Keywords: Antibody concentration; coronavirus disease of 2019 vaccine; critically ill; herd immunity; sero-surveillance; severe acute respiratory syndrome coronavirus 2.

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Conflict of interest statement

There are no conflicts of interest.

Figures

Figure 1
Figure 1
Comparison of full-length S specific IgG levels between critically and noncritically ill COVID-19 patients
Figure 2
Figure 2
Comparison of S1 specific IgG levels between critically and noncritically ill COVID-19 patients
Figure 3
Figure 3
Comparison of (a) full-length S specific and (b) S1 specific IgG levels between the COVID-19 patients and vaccinated subjects
See this image and copyright information in PMC

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