Impact of Multiple Environmental Factors on Influenza–Like Illness in Fujian Province, China, 2015–2023: A Multicity Study

Yao Xiaoyan^1,2*Yanhua Zhang²Yuemei Hu^1,2Enjun Cui³Fen Lin^1,2Guangmin Chen²Jianfeng Xie²Yuwei Weng²Kuicheng Zheng^1,2*

• ¹Fujian Medical University, Fuzhou, China
• ²Fujian Provincial Center for Disease Control and Prevention, Fuzhou, China
• ³Weihai Municipal Hospital, Weihai, China

Introduction: Influenza-like illness (ILI) represents a significant global public health challenge influenced by environmental factors. While previous studies have demonstrated associations, most have been limited to single-city analyses with inconsistent findings. This multi-city study systematically examines the effects of meteorological and air pollution factors on ILI across diverse urban environments. Study design: We analyzed daily ILI surveillance data (2015-2023) from 18 sentinel hospitals across nine Fujian Province cities, combined with concurrent air quality and meteorological data. Using LASSO regression for variable selection, we employed distributed lag non-linear models (DLNMs) to characterize exposure-response relationships in each city, followed by random-effects multivariate meta-analysis to pool estimates and assess heterogeneity. Results: A total of 2,995,909 ILI cases were collected. ILI cases in nine cities of Fujian Province exhibited significant seasonal fluctuations, peaking in winter or early summer. LASSO regression identified temperature, NO₂, and SO₂ as key environmental factors. our results indicated that the combined cumulative effect of average temperature on ILI across nine cities decreased with rising temperatures, with a risk peak at –0.1°C. The NO₂–ILI association was non–linear, resembling an inverted "U" shape, with a risk peak at 40.5 µg/m³. SO₂ exposure had a large degree of heterogeneity in its effect on ILI. Conclusions: This study provides robust evidence that ambient temperature and NO₂ levels significantly influence ILI risk in Fujian Province, with distinct exposure-response patterns. Public health strategies should prioritize cold-weather preparedness and air quality management, particularly during high-risk seasons. Our two-stage analytical approach addresses previous limitations in multi-city environmental health studies.