Urban-rural disparities in heatwave effects on under-5 mortality in China, 2009-2019: a nationwide case-crossover study.

Background: Children under five years are physiologically susceptible to heat stress and significantly more affected by their surrounding environmental conditions, yet urban-rural differences in their vulnerability remain poorly understood, particularly regarding different heatwave timing patterns. We aimed to investigate whether and how the impacts of daytime, nighttime, and compound heatwaves on under-5 mortality differ between urban and rural areas of China. Methods: Using a nationwide case-crossover design, we analyzed 61,464 under-5 mortality cases across China from 2009 to 2019. High-resolution (1-km) temperature data were matched to each case location. We defined daytime, nighttime, and compound heatwaves as ≥2 consecutive days with maximum, minimum, or both temperatures exceeding the local 90th percentile, respectively. We employed a time-stratified case-crossover study design with conditional logistic regression and distributed lag models, adjusting for daily mean temperature, to estimate the cumulative effects of different heatwave events over 0-7 days. We examined eight principal causes of under-5 mortality, including preterm birth/low birth weight, birth asphyxia, pneumonia, diarrhea, leukemia, meningitis/encephalitis, and congenital cardiac defects. Stratified analyses were conducted by urban-rural residence, cause of death, age (neonatal: <28 days; post-neonatal: 28-364 days; early childhood: 1-4 years), sex, warm-season mean temperature, and temperature variability (standard deviation of daily mean temperatures from May to October), to identify vulnerable subgroups and potential effect modifiers. Findings: We analyzed 61,464 under-5 deaths (13,727 urban; 47,737 rural) across China from 2009 to 2019, with neonatal deaths (<28 days) constituting the largest age group in both urban (57.0%) and rural areas (53.8%). Nighttime and compound heatwaves were significantly statistically associated with increased under-5 mortality risk in rural areas (OR = 1.10, 95% CI: 1.03-1.17 and OR = 1.09, 95% CI: 1.03-1.16, respectively) but not in urban areas (OR = 0.96, 95% CI: 0.86-1.06 and OR = 0.95, 95% CI: 0.89-1.02, respectively), with significant urban-rural differences (p < 0.05). Rural vulnerability was particularly pronounced for child deaths related to preterm birth or low birth weight during both nighttime (OR = 1.18, 95% CI: 1.02-1.44) and compound heatwaves (OR = 1.21, 95% CI: 1.01-1.40). Post-neonatal infants and children in regions with lower mean temperature or temperature variability showed greater urban-rural disparities. Interpretation: Rural children in China are exposed to higher risks of heat-related mortality than their urban peers, especially during nighttime and compound heatwaves. Effective heat adaptation strategies should prioritize rural vulnerabilities and the threats posed by elevated nighttime temperatures in the context of a warming climate. Funding: National Natural Science Foundation of China (82430105) and Shanghai Municipal Science and Technology Major Project (2023SHZDZX02).