Temporal Changes in the Association between Wet-Bulb Globe Temperature and Heat-Related Emergency Ambulance Dispatches across Japan: A Multi-Prefecture Time-Series Analysis Spanning the COVID-19 Pandemic and National Heatstroke Alert Implementation (2015–2023)

We examined temporal changes in the association between wet-bulb globe temperature (WBGT) and heat-related emergency ambulance dispatches (EAD) across Japan’s 47 prefectures during June–September 2015–2023, spanning the COVID-19 pandemic and national Heatstroke Alert implementation. Distributed lag nonlinear models (lag 0–5 days) and two-stage random-effects meta-analysis compared prepandemic (2015–2019), pandemic (2020–2021), and postpandemic (2022–2023) periods. We recorded 577,027 EAD across 51,606 calendar prefecture-days, of which 51,602 had observed WBGT. At the 95th WBGT percentile (31.9 °C), pooled cumulative relative risk (RR) declined from 10.45 (95% CI: 9.69–11.26) to 8.78 (8.17–9.45) and 5.90 (5.44–6.39), respectively: a 43.6% reduction (post/pre-RR ratio: 0.564, 95% CI: 0.506–0.630; P < 0.0001), with the larger step after the pandemic period. Severe-case RRs were higher but also declined (21.38 [17.66–25.88], 16.22 [11.20–23.48], and 12.20 [8.77–16.97]); the period × severity interaction against nonsevere cases was null (Wald χ2 = 3.18, df = 6, P = 0.79). Annualized heat-attributable counts nevertheless increased by 18.1%, versus a 26.1% increase in total EAD, indicating that attenuation offset but did not reverse growth in absolute burden. Latitude modified associations in the first two periods (both P < 0.001) but not postpandemically (P = 0.113), when the proportion aged ≥65 years was the strongest (P = 0.0002). Thus, exposure–response associations attenuated across severity levels while absolute burden increased; exploratory meta-regressions suggested possible changes in geographic patterning.