Characterizing the relationship between temperature and mortality in tropical and subtropical cities: a distributed lag non-linear model analysis in Hue, Viet Nam, 2009-2013

Background: The relationship between temperature and mortality has been found to be U-, V-, or J-shaped in developed temperate countries; however, in developing tropical/subtropical cities, it remains unclear. Objectives: Our goal was to investigate the relationship between temperature and mortality in Hue, a subtropical city in Viet Nam. Design: We collected daily mortality data from the Vietnamese A6 mortality reporting system for 6,214 deceased persons between 2009 and 2013. A distributed lag non-linear model was used to examine the temperature effects on all-cause and cause-specific mortality by assuming negative binomial distribution for count data. We developed an objective-oriented model selection with four steps following the Akaike information criterion (AIC) rule (i. e. a smaller AIC value indicates a better model). Results: High temperature-related mortality was more strongly associated with short lags, whereas low temperature-related mortality was more strongly associated with long lags. The low temperatures increased risk in all-category mortality compared to high temperatures. We observed elevated temperature-mortality risk in vulnerable groups: elderly people (high temperature effect, relative risk [RR] = 1.42, 95% confidence interval [CI] = 1.11-1.83; low temperature effect, RR = 2.0, 95% CI = 1.13-3.52), females (low temperature effect, RR = 2.19, 95% CI = 1.14-4.21), people with respiratory disease (high temperature effect, RR = 2.45, 95% CI = 0.91-6.63), and those with cardiovascular disease (high temperature effect, RR = 1.6, 95% CI = 1.15-2.22; low temperature effect, RR = 1.99, 95% CI = 0.92-4.28). Conclusions: In Hue, the temperature significantly increased the risk of mortality, especially in vulnerable groups (i. e. elderly, female, people with respiratory and cardiovascular diseases). These findings may provide a foundation for developing adequate policies to address the effects of temperature on health in Hue City.