Heat effects on influenza-like illness and heterogeneity factors across regions: based on 333 cities in China

While cold weather's impact on influenza-like illness (ILI) is well-studied, heat effects remain underexplored. This study examines the relationship between heat and influenza risk across China, examining regional variations and quantifying heat-attributable disease burden.

We analyzed weekly ILI surveillance data and temperature records from 333 Chinese cities (2010-2018), focusing on warm seasons (May-October). Using distributed lag non-linear models combined with the generalized additive quasi-Poisson regression model, we assessed non-linear heat-ILI associations in each city. China was divided into seven geographical regions to evaluate how economic development, healthcare resources, and population composition modify the adaptation of ILI to heat effects.

The merged results of heat effects on ILI vary in seven regions. Nationally, influenza risk increases when temperatures exceed the region-specific minimum risk temperature. In the northeast, northern, northwest, and southern regions, risk rises significantly at temperatures of 17 °C, 16.4 °C, 14.7 °C, and 21.9 °C respectively. Conversely, in the central and southwest regions, risk declined with rising temperatures before showing minimal subsequent growth. In the eastern region, risk increased steadily above 11.8℃, with only a slight decline at extreme high temperatures. These patterns collectively highlight the pronounced effect of high temperatures on ILI epidemics. Elderly population proportion significantly modified heat effects in most regions. Population size modified ILI's heat adaptability in Northwest/Southwest, while the proportion of low-income populations across cities influenced Northern/Southern/Northwest/central regions. Quantitative analysis revealed that in 139 cities, temperature spikes contributed to an estimated 3,128,629 excess ILI cases in total.

This nationwide study demonstrates that heat effects were associated with influenza-like illness risk across China, with variations across regions. These regional differences in heat-ILI associations varied with local healthcare conditions, economic status, social environment, and population age structure. The findings support the need for region-specific public health interventions that consider these modifying factors.