https://www.nature.com/articles/s41598-025-11575-w Abstract Human NK cells exhibit functional heterogeneity, yet the clinical conditions and cellular processes driving this diversity remain insufficiently understood. Here, we report that NK cell diversity emerges during peripheral development, influenced by the divergence of adaptive NK cells from various stages of canonical NK cell maturation during the resolution phase of severe COVID-19. Using scRNA-seq, we analyzed blood NK cells from patients with severe COVID-19 (acute phase and post-intensive care), individuals vaccinated with the SARS-CoV-2 mRNA vaccine (BNT162b2), and healthy donors. The frequencies of immature CD56bright and proliferating NK cells increased following BNT162b2 vaccination. In contrast, the frequency of adaptive CD56dim cells was markedly elevated in patients recovering from severe COVID-19, alongside clonal expansion and enhanced mitochondrial oxidative phosphorylation. Trajectory analysis revealed a bifu...