WANG, W.; Goguet, e.; Lusvarghi, S.; Paz, S.; Shrestha, L.; Vassell, R.; Pollett, S.; Mitre, E.; Weiss, C. D.

Background: Widespread immunity from vaccination and infection has reduced COVID-19 morbidity and mortality, but this immunity varies across the population. Understanding how repeated antigenic exposures influence antibody responses helps to inform future vaccination strategies. Methods: Serum samples collected one and six months after XBB.1.5 vaccination from 25 generally healthy healthcare workers with varying exposure histories were assessed for neutralizing activity against a range of variants, from pre-Omicron variants to latest Omicron JN.1 sublineage variants and divergent BA.3.2 variants, using lentiviral pseudoviruses. Participants were stratified by vaccination and infection history. Results: XBB.1.5 vaccination elicited broad neutralizing responses, with strong boosting against previously encountered antigens relative to vaccine-matched XBB.1.5 and newer variants. Geometric mean neutralization titers were generally comparable across exposure groups, indicating limited influence of prior Omicron infection or bivalent vaccination, though intra-group heterogeneity was observed. At six months, overall titers declined by 36-62%. Titers remained highest against the pre-Omicron and lowest against JN.1 sublineage variants. N-terminal glycosylation (DelS31, T22N) modestly affected neutralization. Conclusions: XBB.1.5 vaccination elicited broad neutralizing antibody responses against previously encountered and vaccine-matched antigens regardless of exposure history, but titers waned after six months. This waning, compounded by continued emergence of immune-evasive variants and heterogenous population immunity, underscores the need for continually monitoring neutralizing antibody durability and breadth to guide evidence-based COVID-19 vaccine formulation updates.