Third exposure to COVID-19 infection or vaccination differentially impacts T cell responses

Background: In 2021, the rapid rollout of two doses of SARS-CoV-2 vaccines reduced COVID-19 severity and mortality. However, further vaccine doses as a prime-boost schedule were limited, and lifting of public health restrictions by late 2021 frequently led to infection, rather than vaccine, as a third exposure. Objective: To compare how the third exposure through mRNA booster or SARS-CoV-2 infection shapes humoral and cellular immunity following two vaccine doses. Methods: We compared immune responses after the third exposure in healthy adults enrolled in the UCLH-Crick Legacy cohort study (NCT04750356) between those receiving ancestral spike-encoded mRNA booster (vaccine immunity, n = 38) or COVID-19 infection (hybrid immunity, n = 13) following two vaccine doses. Immune profiles were evaluated using live virus neutralization assays, IFN-γ ELISpot, Luminex assay, flow cytometry and mass cytometry. Results: Both total anti-Spike IgG and variant-specific neutralising antibodies were comparable following infection or vaccine as a third exposure. Overall, T cell populations were similar but functionally different. CD8⁺ Effector Memory (TEM) cells in the vaccine group showed higher expression of CD69 and Granzyme B following stimulation with SARS-CoV-2 Spike peptides. In contrast, the hybrid group produced higher levels of innate immune associated cytokines IL-10 and IL-34, as well as the T cell homing chemokine CCL25, after stimulation. Conclusions: While both exposures generated comparable breadth of protection against SARS-CoV-2 variants, our findings suggest that the route of third exposure influences different aspects of the immune response, warranting further investigation into long-term immunity at both systemic and mucosal sites.