Effects and interaction of humidex and air pollution on influenza: A national analysis of 319 cities in mainland China

Abstract

Influenza imposes a significant global health burden. This study investigates the effects of humidex and air pollution on influenza and their interactions, using multi-city surveillance data in China. Daily data on reported influenza cases, meteorological factors and air pollution from 319 cities in mainland China over the study period of 2014-2019 were collected. A two-stage analytical framework, comprising distributed lag non-linear model and multivariate meta-analysis, was employed to assess the associations between humidex, air pollution and influenza. Hierarchical and joint effect models were employed to examine their interaction. Nationally, an approximately L-shaped relationship between humidex and influenza was observed, with the highest relative risk (RR) of 2.603 (95% confidence interval [CI]: 2.195–3.086). Per interquartile range increases in PM_2.5, PM₁₀, NO₂, SO₂, CO and O₃ were associated with influenza risk increments of 0.035 (95% CI: 0.010–0.061), 0.029 (95% CI: 0.003–0.055), 0.191 (95% CI: 0.152–0.231), 0.239 (95% CI: 0.166–0.317), 0.038 (95% CI: 0.001–0.076) and -0.171 (95% CI: -0.238–-0.099), respectively. A synergistic interaction effect was identified between low humidex and high air pollution as well as different air pollutants. Subgroup analyses indicated females and individuals aged 7-18 years old exhibited higher risks. Stronger effects were observed during winter season and in large cities. This study underscores the urgent need for tailored interventions to mitigate the health impacts in regions with concurrent low humidex and high air pollution.