ObjectiveTo analyze the evolutionary characteristics and genetic variations of the HA (hemagglutinin) and NA (neuraminidase) genes of influenza A(H1N1) viruses in Shanghai during 2024, to investigate their transmission patterns, and to evaluate their potential impact on vaccine effectiveness. MethodsFrom January to October 2024, throat swab specimens were collected from influenza like illness (ILI) patients at 4 hospitals in Shanghai. Real-time fluorescence ploymerase chain reaction (RT-PCR) was used for virus detection and isolation of H1N1 influenza viruses. Forty influenza A(H1N1) virus strains were sequenced using Illumina NovaSeq 6000 platform, followed by phylogenetic analyses, genetic distance analysis, and amino acid variation analyses of HA and NA genes. ResultsPhylogenetic tree of the HA and NA genes revealed that the 40 influenza A(H1N1) virus strains circulating in Shanghai in 2024 exhibited no significant geographic clustering, with a broad origin of strains and complex transmission chains. Genetic distance analyses demonstrated that the average intra-group genetic distances of HA and NA genes among the Shanghai strains were 0.005 1±0.000 6 and 0.004 6±0.000 6, respectively, which were comparable to or higher than those observed in global surveillance strains. Both HA and NA genes displayed frequent mutations. Compared to the 2023‒2024 and 2024‒2025 Northern Hemisphere A(H1N1) vaccine strains (WHO-recommended), the HA proteins of 40 Shanghai strains exhibited amino acid substitutions at positions 120, 137, 142, 169, 216, 223, 260, 277, 356 and 451, with critical mutations at positions 137 and 142 located within the Ca2 antigenic determinant. Furthermore, mutations in the NA protein were observed at positions 13, 50, 200, 257, 264, 339 and 382. ConclusionThe genetic background of the 2024 Shanghai influenza A(H1N1) virus strains is complex and diverse, and antigenic variation may affect vaccine effectiveness. Therefore, it is recommended to enhance genomic surveillance of influenza viruses, evaluate vaccine suitability, and implement more targeted prevention and control strategies against imported influenza viruses.

ObjectiveTo analyze the epidemiological characteristics and trends of influenza‑like illness (ILI) in Jiading District from 2013 to 2023, to explore the prevalence patterns, so as to provide a scientific basis for a better prevention and control of influenza. MethodsData of ILI cases, pathogenetic surveillance results and pathogen detection data from influenza surveillance network laboratories in Jiading District from 2013 to 2023 were collected for statistical analysis. ResultsFrom 2013 to 2023, the overall proportion of medical visits for ILI cases in Jiading District was 2.70%. ILI cases were mainly distributed in the age group of ≥25 years, with a seasonal prevalence characteristic, mainly concentrated in the winter and spring, with an occasional small peak in the summer. A total of 12 423 specimens were tested for pathogenetic surveillance and monitoring, 3 651 of which were tested positive for nucleic acid detection, with a positive rate of 29.39%. The dominant strain was influenza A (H3N2) virus, accounting for 55.05%. There were statistical significant differences in the positive detection rates by different streets and townships(χ²=24.73，P<0.05）. The influenza network laboratory isolated and cultured influenza viruses from 3 154 nucleic acid‑positive samples sent by national influenza sentinel surveillance hospitals using 2 methodsMadin‑Darby canine kidney (MDCK) cells and specific pathogen free (SPF) chicken embryos, respectively, with a total isolation rate of 39.51%, and the isolation rate of MDCK cells (35.45%) was higher than that of SPF chicken embryos (5.36%). From 2013 to 2023, a total of 14 ILI outbreaks were reported in Jiading District, mainly in primary schools, kindergartens and secondary schools. ConclusionInfluenza in Jiading District, Shanghai, is mainly prevalent in the winter and spring, with different subtypes of influenza viruses alternating or co⁃circulating in different monitoring year. It is recommended to promote influenza vaccination, further improve influenza prevention and control measures, enhance health promotion to the age group of ≥25 years in winter and spring, and strengthen surveillance and monitoring on ILI outbreaks in collective units such as primary schools and childcare institutions, so as to reduce the disease burden.