Epidemiological and genomic surveillance of influenza A virus (pdm09 H1N1 and H3N2) strains from 2017 to 2025 in Tianjin, China

Influenza A virus (IAV) remains a global public health concern, causing influenza-like illness and severe respiratory tract infections. Two major subtypes, A/pdm09 H1N1 and A/H3N2, circulate globally, and their epidemics are influenced by multiple factors, especially during the COVID-19 pandemic. Based on data from the National Influenza Surveillance Program in China, we analyzed the epidemiological and genomic data in Tianjin collected from 2017 to 2025. A total of 77,473 throat swabs were collected, of which 9144 were IAV-positive. The A/pdm09 H1N1 and A/H3N2 lineages exhibited distinct epidemics across different influenza seasons, with a decline in cases observed during the COVID-19 pandemic. We sequenced the genomes of 128 A/pdm09 H1N1 and 113 A/H3N2 clinical isolates and characterized their temporal evolution and genetic diversity using time-scaled phylogenetic analysis. Additionally, we conducted a genetic risk evaluation of the hemagglutinin and neuraminidase segments, identifying key amino acid residues associated with viral adaptation, transmissibility, virulence, and drug resistance. Moreover, no antigenic variants were found in clinical isolates during the recent influenza seasons, though reduced sensitivity to oseltamivir and zanamivir was observed in individual strains. Our surveillance highlights the epidemiology and evolution of IAV before and after the COVID-19 pandemic in Tianjin.