Differential functional immune recall across homologous and heterologous COVID-19 vaccination regimens.

INTRODUCTION: Understanding how different COVID-19 vaccine combinations shape long-term immunity is essential for improving durability and guiding booster strategies. Despite extensive characterization of neutralizing antibodies, long-term memory B and T cell responses after homologous and heterologous vaccination regimens remain poorly understood. METHODS: In this study, peripheral blood mononuclear cells (PBMCs) from 50 individuals were analyzed 12 months after completion of a homologous two-dose primary COVID-19 vaccination series and had received a single booster dose (12-month follow-up). Participants initially received BNT162b2, ChAdOx1 nCoV-19, or CoronaVac as the primary vaccination series, followed by either a homologous booster or a heterologous BNT162b2 booster (ChAdOx1 nCoV-19/BNT162b2 and CoronaVac/BNT162b2). Unvaccinated individuals served as controls. B cell phenotypes were assessed using flow cytometry, while B and T cell recall responses were determined using ELISpot. IgG levels and cytokine/chemokine/growth factor profiles were evaluated using ELISA and Bio-Plex multiplex before and after in vitro stimulation. RESULTS: The homologous BNT162b2 vaccination regimen exhibited significantly higher frequencies of memory B cells recognizing the ancestral (Wuhan strain-derived) SARS-CoV-2 Spike protein compared to the ChAdOx1 nCoV-19, CoronaVac, and unvaccinated controls. The CoronaVac/BNT162b2 vaccination regimen demonstrated significantly elevated RBD-specific IgG+ memory B cells and higher stimulated IgG levels, together with the highest IFN-γ-producing T cell responses. Homologous CoronaVac and CoronaVac/BNT162b2 vaccination regimens showed broader cytokine activation, including IL-6, IL-9, IL-15, TNF-α, and other cytokines. IL-6 levels were positively associated with memory B cell frequencies, suggesting a potential association with memory B cell differentiation and maintenance. DISCUSSION: Different vaccination regimens were associated with distinct long-term cellular immune profiles under real-world conditions, with both homologous BNT162b2 and heterologous CoronaVac/BNT162b2 vaccination regimens maintaining robust memory B cell signatures and functional recall responses. The results highlight how priming combinations shape the durability and quality of immune memory, supporting the potential utility of mixed-platform booster strategies for sustaining long-term immunity.