To clarify the epidemiological characteristics, viral subtype distribution and population infection patterns of influenza in Ya′an City during surveillance years 2019-2024, so as to provide scientific evidence for targeted prevention and control strategies.

Influenza surveillance data from March 2019 to February 2025 were collected from two national sentinel hospitals in Ya′an City, with zero-positive data generated during epidemic prevention and control in 2020 excluded. Reverse transcription-polymerase chain reaction (RT-PCR) was performed for viral subtyping identification. The chi-square (χ²) test was adopted to compare differences in positive detection rates across different years, seasons and populations.

A total of 13, 454 samples were included, with 2, 535 positive cases (18.84%). The annual differences were significant (P<0.001), and 803 positive cases (29.82%) were detected in 2023. Virus typing was dominated by seasonal influenza A(H3N2) subtype (seasonal H3) with 1, 411 cases (55.66%), followed by influenza B virus Victoria lineage (BV lineage) with 1, 725 cases (28.60%) and 2009 pandemic influenza A(H1N1) virus (novel H1N1) with 399 cases (15.74%); influenza B virus Yamagata lineage (BY lineage) was not detected. The seasonal distribution showed a bimodal pattern, with 1, 020 cases (27.54%) in autumn and 792 cases (24.00%) in winter, and the lowest number of 255 cases (7.93%) in summer; the difference was statistically significant (P<0.001). The 5-14 years age group had 1, 303 positive cases (30.23%); the positive rates among 1, 207students(33.07%) and15 teachers (37.50%) were significantly higher than those in other occupations (P<0.001). The positive rate in the school setting 1, 711 cases( 25.76%) was significantly higher than that outside school 824 cases(12.09%) (P<0.001).

Influenza epidemics in Ya′an City prevail in autumn, winter and spring, with the most prominent peak occurring in autumn. Alternating circulation of seasonal H3, BV and pdm09 A/H1N1 strains is identified, and students and teachers constitute high-risk populations. It is recommended to prioritize influenza vaccination for school-aged children aged 5-14 years, teachers and preschool children, strengthen morning and afternoon temperature checks as well as absence tracking due to illness in key settings such as schools, and establish a two-dimensional early warning system covering viral subtype and seasonal dynamics.