Surveillance and analysis of influenza-like illness cases in a sentinel hospital in Nantong from 2014 to 2021

Objective To analyze the surveillance results of influenza-like illness （ILI） cases in a sentinel hospital in Nantong from 2014 to 2021 and to understand the seasonal and etiological characteristics of influenza.

Methods The routine ILI case information in a sentinel hospital in Nantong from 2014 to 2021 was collected from China Influenza Surveillance Information system， and the surveillance results were described and analyzed.

Results A total of 64 110 ILI cases were reported in the hospital from 2014 to 2021， and accounted for 3.93% of outpatient and emergency visits. ILI is mainly concentrated in children under 15 years old， accounting for 55.39% of the total ILI cases. ILI% roughly showed two epidemic peaks in spring and winter. A total of 8 639 nasopharyngeal swab samples of ILI cases were collected during 2014‒2021， of which 838 were influenza positive， with an average positive rate of 10.01% （3.27%‒16.75%）. The epidemic strains varied from year to year and in different age groups. In 2014 and 2015， H3N2 influenza was the epidemic strain； in 2016 and 2017， H3N2 influenza and B-Victoria influenza were the co-epidemic strains； in 2018， H1N1 influenza was the epidemic strain； and in 2019‒2021， B Victoria influenza was the epidemic strain. The epidemic strain in 0‒4 years old， 15‒24 years old， 25‒59 years old and >60 years old was H3N2， and the epidemic strain in 5‒14 years old was B Victoria. There was a significant difference in the positive rate between different gender groups （χ²=7.364， P=0.006）.

Conclusion Influenza often occurs in winter and spring in a sentinel hospital in Nantong. Influenza subtypes are prevalent alternately in different age groups and different years. We need to closely monitor the etiology of influenza virus in Nantong to provide a basis for influenza prevention and control.