Heat exposure specification determines estimates of pandemic period heatstroke mortality across 47 Japanese prefectures from 2020 to 2022
Year: 2026
Type: article
Abstract: Pandemic period excess mortality for specific causes is widely reported, but such estimates depend on a baseline model whose exposure definition is rarely validated. We examined whether heatstroke mortality in Japan departed from pre-pandemic expectation during 2020 to 2022, and how far that conclusion depends on how heat exposure is measured. We analysed annual heatstroke deaths (ICD-10 T67) in all 47 Japanese prefectures from 2013 to 2022 (470 prefecture-years). Daily maximum and minimum temperatures for June to September yielded counts of extremely hot days (at least 35 °C), hot days (at least 30 °C) and tropical nights (minimum at least 25 °C), alongside monthly mean temperature. Expected deaths for 2020 to 2022 were estimated from quasi-Poisson fixed-effects models trained on 2013 to 2019 and selected by out-of-sample forecast error. Negative-binomial and Gaussian linear rate models were fitted as distributional sensitivity analyses. We report both the national aggregate departure and a non-cancelling regional excess measure that sums only positive region-level threshold exceedances. Heat-day specifications predicted substantially better than monthly mean temperature. Under rolling-origin validation, mean absolute percentage error fell from 37.0% with the monthly mean to 22.1% with the selected specification, and root mean squared error fell from 616 to 419 deaths. With the selected quasi-Poisson specification, the observed 1,522 deaths in 2020 lay within the simulated interval (expected 1,312; 95% interval 1,099 to 1,552), as did 2021 and 2022. Negative-binomial and Gaussian linear rate models gave the same three classifications. A non-cancelling regional analysis nevertheless identified 202 thresholded excess deaths in Kanto in 2020. In the full-panel fixed-effects model, mortality was associated with extremely hot days (incidence rate ratio 1.034 per day; 95% confidence interval 1.020 to 1.048) and tropical nights (1.007; 1.001 to 1.014), but not with hot days at or above 30 °C. With an exposure specification selected for out-of-sample prediction, no pandemic year showed a statistically supported national departure from expectation, and this finding was unchanged by negative-binomial or linear modelling. Because a total within its prediction interval does not establish that excess mortality was absent, this result is reported as a non-detection rather than as evidence of no excess. Localised excess can coexist with a national aggregate within expectation, so national and non-cancelling regional estimands should be reported separately. Cause-specific excess-mortality studies should validate baseline specification, test distributional sensitivity and distinguish aggregate from local excess.
Source: Discover Public Health
Authors: Akira Kimura, Taishi Goto
Institution: Gumma Paz College
Language: English
Subfield: Health, Toxicology and Mutagenesis
Field: Environmental Science
Domain: Physical Sciences
Open Access status: diamond
OpenAlex ID: W7219773325