Effects of ambient temperature on under-five mortality: a nationwide space-time-stratified case-crossover study in...

Although climate change poses increasing risks to child health, particularly through temperature extremes, epidemiological evidence on its impact on child mortality is still limited. This study investigated the short-term effects of ambient temperatures (heat and cold) on under-five mortality in Brazil.

We conducted a space-time-stratified case-crossover study across 5570 Brazilian municipalities from 2000 to 2019, using data from the Brazilian Mortality Information System and from the Brazilian Daily Weather Gridded Data (BR-DWGD). We estimated relative risks (RRs) and 95 % confidence intervals (CIs) using conditional quasi-Poisson regression models for extreme cold (1st percentile) and extreme heat (99th percentile) relative to the minimum mortality temperature (MMT), accounting for cumulative lag effects with DLNM stratified by region, age (neonatal, post-neonatal, and childhood), and specific causes of deaths.

1,061,229 under-five deaths were included, and the temperature-mortality relationship followed a U-shaped pattern. Compared to MMT, mortality risk was 1.95 (95 %CI: 1.66-2.30) times higher after extreme cold and 1.29 (95 %CI: 1.19-1.40) after extreme heat. Cold-related mortality was highest in the South (RR: 2.17, 95 %CI: 1.60-2.94), and heat-related mortality in the Northeast (RR: 2.02, 95 %CI: 1.60-2.94). Age-stratified analyses showed the highest vulnerability to cold in the post-neonatal period (RR: 4.64, 95 %CI: 3.47-6.22), while heat-related mortality increased with age, peaking in children aged 1-4 years (RR:1.85, 95 %CI: 1.49-2.29). Cause-specific analyses showed that both extreme cold and extreme heat were associated with higher mortality from diarrhoea and all infectious diseases, while respiratory mortality was associated only with heat. The associations were particularly strong for diarrhoeal causes.

Extreme temperatures increase under-five mortality in Brazil, with risks varying by region, age, and cause of death. There is an urgent need for targeted interventions to adapt to and mitigate the impact of extreme temperatures on child health.