Comparative analysis of influenza epidemiological characteristics in Quzhou, China from 2016 to 2024

This study analyzed influenza-like illness (ILI) and influenza-positive rate (IPR) in Quzhou from 2016 to 2024 to assess the impact of the COVID-19 pandemic.

The weekly surveillance data of ILI were collected from sentinel hospitals and network laboratories. Real-time reverse transcription-polymerase chain reaction (RT-PCR) was used to detect and identify the specific types of influenza viruses.

A total of 77,912 ILI cases were reported (ILI% = 3.9%). Both ILI% and IPR showed seasonal winter-spring peaks. A marked decline in ILI% (4.7% to 2.7%) and IPR (19.2% to 10.7%) was observed during the COVID-19 pandemic period (2020-2022), followed by a post-pandemic rebound. Children under 15 years old accounted for the highest proportion of ILI cases (65.9%). The 5-14 age group had the highest IPR (33.6%). Influenza A(H3N2) (39.2%) and B/Victoria (36.0%) were the predominant strains. The correlation between ILI% and IPR varied significantly across COVID-19 pandemic phases: weak positive pre-pandemic (r_s = 0.325), moderate negative during the pandemic (r_s = -0.597), and moderate positive post-pandemic (r_s = 0.584). Age-stratified analysis revealed distinct correlation patterns in different COVID-19 pandemic phases, with the strongest correlation in the 15-59 age group (r_s = 0.56, p = 0.004).

The COVID-19 pandemic has significantly influenced influenza activity and altered the relationship between ILI% and IPR. The inconsistent correlation between ILI and IPR, especially during the COVID-19 pandemic, highlights the necessity of integrated virological surveillance and appropriate definition of ILI for effective influenza monitoring and public health response.