SARS-CoV-2 Neutralization Breadth Is Shaped by Variant Exposure and Immune History

Despite widespread COVID-19 vaccination, emerging SARS-CoV-2 variants continue to evade neutralizing responses. To determine how exposure history influences neutralizing breadth against SARS-CoV-2, plasma collected from 46 participants in the Clinical, Virological and Immunological COVID-19 study cohort (CVIC) was analyzed. Participants were categorized into six groups reflecting distinct SARS-CoV-2 exposure profiles: individuals infected with either Wuhan-Hu-1 or Delta variants, infection-naïve individuals who received two or four doses of mRNA vaccine (including a bivalent BA.4/5 booster), previously infected individuals subsequently vaccinated, and individuals with four vaccine doses plus breakthrough Omicron infection. Neutralizing breadth was assessed using a pseudoparticle panel representing major variants of interest/concern (Alpha, Beta, Gamma, Delta, Lambda, and Kappa) and a panel of representative Omicron subvariants (BA.1, BA.2, BA.5, BA.2.75, BA.2.86, BQ.1, XBB.1.5, EG.5, HV.1, and JN.1). Primary exposure through infection elicited potent homologous but variable heterologous neutralization, whereas vaccination alone induced lower yet more consistent responses across variants. Additional antigenic exposures, through either repeated vaccination or breakthrough infection, markedly enhanced cross-variant neutralization, with the greatest potency and breadth of neutralizing responses observed in individuals with the highest number of SARS-CoV-2 exposures. Nonetheless, neutralizing activity against more recently emerged Omicron subvariants, including EG.5, HV.1, and JN.1, remained low. Taken together, these findings demonstrate that both the nature and number of antigenic exposures can shape the potency and breadth of SARS-CoV-2 neutralizing responses. While cumulative exposures improve cross-variant immunity, ongoing viral evolution continues to erode neutralization capacity, emphasizing the need for continued variant monitoring and periodic vaccine reformulation to maintain population-level protection.