The Role of CD4 T Cell Repertoire and Immune Memory Mechanisms in Vaccination and Infection Immunity

CD4 + T cells orchestrate adaptive immunity against respiratory viruses through functionally distinct subsets, yet the qualitative differences between vaccine‐ and infection‐induced responses remain incompletely understood. This review summarizes current evidence on how CD4 + T cell subsets shape the magnitude, breadth, and durability of immunity to SARS‐CoV‐2 and influenza virus. mRNA vaccines elicit a potent Th1/Tfh1‐biased response that generates high‐affinity neutralizing antibodies and cytotoxic CD4 + T cells (CD4‐CTLs) with stem‐cell memory properties. However, this strong type I bias may come at the cost of long‐term maintenance of humoral memory. Natural infection additionally establishes tissue‐resident memory CD4 + T cells (CD4 TRMs) at respiratory mucosal surfaces, providing a frontline defense that current intramuscular vaccines fail to recapitulate efficiently. Crucially, this T cell immunity proves far more resilient than neutralizing antibodies against Variants of Concern, as many memory T cell epitopes from the ancestral strain are conserved across successive variants, a robustness further reinforced by pre‐existing cross‐reactive CD4 + T cells primed through prior seasonal coronavirus exposure or influenza infection. These observations argue for a fundamental shift in vaccine design toward mucosal delivery and conserved‐epitope antigens to achieve durable, broadly cross‐reactive protection.