Longitudinal Evaluation of Humoral and Cellular Immunity After BNT162b2 COVID-19 Vaccination: Influence of Booster Type, Infection and Chronic Health Conditions

<b>Background/Objectives:</b> Understanding the durability of immunity induced by mRNA COVID-19 vaccines, especially in individuals with chronic health conditions, remains essential for guiding booster strategies. We conducted a longitudinal study to evaluate humoral and cellular immune responses up to 21 months after a primary two-dose BNT162b2 vaccination followed by a booster, either homologous (BNT162b2) or heterologous (mRNA-1273). <b>Methods:</b> Twenty-eight adults, mostly with chronic conditions, were assessed at approximately 9, 12 and 21 months post-primary vaccination. Serum anti-trimeric Spike IgG levels were quantified, and peripheral blood mononuclear cells were analyzed at 21 months for Spike-specific memory B-cell and T-cell responses by flow cytometry. <b>Results:</b> Participants were stratified by booster type, prior SARS-CoV-2 infection and health status. Anti-Spike IgG persisted in all participants but declined over time. The heterologous mRNA-1273 booster induced higher antibody titers at 9 months, while the homologous BNT162b2 booster led to more sustained antibody levels and higher frequencies of Spike-specific memory B cells at 21 months. Prior infection significantly enhanced antibody titers, particularly in homologous booster recipients. Surprisingly, individuals with chronic health conditions exhibited equal or higher antibody levels compared to healthy participants at all time points. At 21 months, robust Spike-specific class-switched memory B cells and polyfunctional CD4⁺ and CD8⁺ T-cell responses were detected. <b>Conclusions:</b> These findings demonstrate that BNT162b2 vaccination elicits durable, multi-layered immunity lasting nearly two years, even in individuals with chronic conditions, and support the use of both homologous and heterologous mRNA boosters to sustain protection in diverse populations.