Post‐Pandemic Emergence H1N1 Strains With Antiviral Resistance–Associated Substitutions and H1/H3 Reassortment in Pediatric Cases: Genomic and Epidemiological Insights From Ningbo, China (2020–2024)

Influenza A (H1N1) remains a persistent threat to global public health due to its high mutation rate and immune evasion capacity. This study investigates the epidemiological trends in pediatrics, genetic characterizations, and transmission dynamics of H1N1 viruses in Ningbo, from 2020 to 2024. A total of 6632 clinical samples from children with influenza‐like illness (ILI) were analyzed. A marked resurgence of H1N1 activity was observed in 2023 and 2024, following low circulation during the COVID‐19 pandemic. Genomic analysis of 33 strains revealed dynamic lineage replacement: hemagglutinin (HA) sequences mainly clustered within 6B.1A.5a.2a, while neuraminidase (NA) lineages shifted from Clade C.5.2/C.5.3 in 2023 to D in 2024. Notably, we identified one H1N1 reassortant strain containing internal genes from H3N2 and several 2024 strains carrying the H275Y and S247N mutations previously reported to be associated with reduced susceptibility. Comparative analysis showed increasing genetic divergence between circulating strains and World Health Organization (WHO) recommended vaccine strains, raising concerns about antigenic drift or changes in vaccine match. Bayesian skyline plots indicated a stable H1N1 population in Ningbo with increasing genetic uncertainty in recent years, potentially reflecting cryptic transmission. Phylogeographic reconstruction revealed historical viral inflows from Hangzhou and Wenzhou and recent bidirectional transmissions between Ningbo and Huzhou. These findings highlight the post‐pandemic resurgence and genetic evolution of H1N1, emphasizing the need for continued genomic surveillance, antiviral resistance monitoring, and timely vaccine strain updates to mitigate future outbreaks.