Retrospective neutralization analysis of SARS-CoV-2 variants in early pandemic sera

Introduction: As the COVID-19 pandemic enters its sixth year, effective vaccination strategies remain a cornerstone, particularly given the limitations in access, timing, and efficacy of currently available antiviral therapies. Methods: In this retrospective study, we analyzed neutralizing antibody responses in serum samples from 100 vaccinated and boostered individuals using standardized cell-culture and in vitro neutralization assays. Samples were tested against the original Wuhan-Hu-1 spike protein as well as major SARS-CoV-2 variants of concern (B.1.351, B.1.617, and B.1.1.529/Omicron). We also investigated the potential for antibody-dependent enhancement (ADE) in monocyte-derived macrophages. Results: Only 61% of serum samples effectively neutralized the Wuhan-Hu-1 variant; among these, 20.6% demonstrated cross-neutralization of both B.1.351 and B.1.617. Of those cross-neutralizers, 66.6% were also able to neutralize Omicron. Notably, individuals who had been both vaccinated and previously infected showed stronger neutralizing responses than those who were only vaccinated and boostered. ADE was observed in 1% of samples. Discussion: This retrospective analysis offers a valuable insight to contextualize immune responses in real-world settings, revealing how actual immunological outcomes diverged from early expectations; at least in our studied population, and underscores the importance of continuously reassessing vaccine strategies as viral evolution unfolds.