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.