Discussions of immune responses to COVID-19 vaccines invariably center on virus-specific antibody responses. Although the protective role of neutralizing antibodies is well established, the role of memory cells, specifically memory T helper cells and cytotoxic T cells, in protection against symptomatic infection has not been extensively studied. To determine the relative importance of these memory cells in resolution of COVID-19 symptoms, a group of Australian researchers studied 23 people who had been previously immunized but were now suffering COVID-19 caused by the delta or omicron variants (Koutsakos M, Reynaldi A, Lee WS, et al. SARS-CoV-2 breakthrough infection induces rapid memory de novo T cell responses. Immunity. 2023 Apr 11;56(4):879-892.e4).

Researchers found that recall of virus-specific memory helper T cells was rapid, occurring as early as one day after the beginning of symptoms. However, the effector arm that most directly correlated with the resolution of symptoms and viral clearance was memory cytotoxic T cells. The authors concluded, “Our study demonstrates that a rapid and extensive recall of memory T cell populations occurs early after breakthrough infection and suggests that CD8+ T cells contribute to the control of viral replication in breakthrough SARS-CoV-2 infections” (p.879).

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