Extreme weather events and pandemics pose significant global health challenges. While the individual impacts of these crises are well-studied, their combined effects remain largely unexplored. This study addresses this gap by investigating the compound mortality impacts of extreme temperatures and the COVID-19 pandemic in England and Wales between January 2020 and December 2022. The UK provides a suitable case study due to its well-documented healthcare system and readily available data on temperature variability and COVID-19 mortality at a sub-national level. The study focuses on excess mortality associated with non-optimal temperatures (both heat and cold), as this represents the primary short-term health impact of weather hazards in the UK. The research aims to quantify the independent and combined effects of extreme temperatures and COVID-19 on mortality, providing crucial insights for improving climate adaptation strategies and pandemic preparedness. The unprecedented confluence of a record-breaking heatwave and a major pandemic within this period offers a unique opportunity to examine the compound health impacts of such events. Understanding these interactions is vital for developing effective public health solutions that address both climate change and pandemic threats simultaneously. The study’s importance lies in its potential to inform evidence-based policy decisions concerning resource allocation, public health messaging, and the development of interventions to mitigate the risks associated with these compound crises.

Existing literature extensively documents the individual health burdens of extreme weather events and pandemics like COVID-19. Studies highlight the significant mortality associated with heatwaves and cold snaps, particularly among vulnerable populations. Similarly, the COVID-19 pandemic led to a substantial global excess mortality burden. However, research quantifying the compound health effects of these crises co-occurring is limited. While the climatic-conflict nexus has been explored, investigations into the interplay between climate change and other parallel crises, such as pandemics, are relatively scarce. This study contributes to this nascent research area by providing empirical evidence of the combined mortality impacts of extreme temperatures and COVID-19.

The study employed distributed lag nonlinear models (DLNMs) to analyze daily mortality data in England and Wales from 1981 to 2022. Data on daily all-cause mortality, excluding deaths explicitly attributed to COVID-19, were obtained from the Office for National Statistics. Daily average temperatures were derived from the HadUK-Grid Climate Observations dataset. The DLNMs modeled the non-linear association between daily average temperature and mortality, accounting for time lags to capture the delayed effects of temperature exposure on mortality. Natural cubic splines with three internal knots were used to represent the non-linear temperature-mortality relationship. The model incorporated a 21-day lag period to capture both short-term and delayed effects. Heatwaves and cold snaps were identified using data from the UK Health Security Agency (UKHSA) Heatwave Mortality Reports and Level 3 Cold Health Alerts. For the periods before 2016 when these reports were unavailable, the events were identified using temperature thresholds. Excess mortality attributable to heat and cold was calculated for each day, and 95% confidence intervals were estimated using Monte Carlo simulations. The study compared cumulative temperature-related deaths with cumulative COVID-19 deaths for the entire study period and during specific heatwave and cold snap events to assess their respective and combined mortality impacts. Regional population data were used to standardize mortality rates to deaths per 100,000 population. The methodology involved careful removal of COVID-19-related deaths from the all-cause mortality data to isolate the temperature-related mortality component.

The study revealed that temperature-related mortality significantly exceeded COVID-19 mortality in several regions of England and Wales during parts of the study period (June to October 2020, March to August 2021, and September 2021 to the end of 2022). During the July 2022 heatwave, daily heat-related mortality peaked at 580 deaths (95% CI: 634–670), exceeding the mean daily level of the previous ten years. Over the entire study period, 8481 excess deaths were attributable to extreme temperatures (1283 from cold and 6298 from heat). During the ten heatwave episodes, temperature-related deaths outnumbered COVID-19 deaths in nine of ten regions. The ratios of temperature-related deaths to COVID-19 deaths ranged from 1.7 to 2.7 in southern regions during heatwaves. During eight cold snaps, temperature-related deaths were lower than COVID-19 deaths across all regions, although this is likely due to large COVID-19 mortality surges early in the pandemic. Compound mortality (heat-related and COVID-19) during heatwaves ranged from 19 to 24 deaths per 100,000 population, at least twice as high as the previous decade. The study period showed higher additional mortality pressures than the previous ten years. An analysis of average temperature-related deaths per day versus the average temperature during heatwaves and cold snaps indicated potential confounding effects of COVID-19 on temperature-related mortality, although the sample size was limited.

The findings highlight the substantial and often overlooked contribution of temperature-related mortality to overall excess deaths, even during a major pandemic like COVID-19. The observed compound mortality emphasizes the need for public health interventions that consider both extreme weather and infectious disease risks. The higher excess deaths compared to the previous decade demonstrate the escalating risk from these compound crises. The results underscore the urgent need for enhanced preparedness and integrated strategies that address both extreme temperatures and pandemic threats simultaneously. Improved health system capacity, targeted interventions for vulnerable populations, and effective public health messaging are crucial for mitigating the health impacts of these crises. The potential confounding effect of COVID-19 on temperature-related mortality warrants further investigation using longer time series data.

This study demonstrates the significant compound mortality impacts of extreme temperatures and the COVID-19 pandemic in England and Wales. The results highlight the need for integrated public health strategies and resource allocation that address both climate change and pandemic preparedness. Future research should focus on disentangling the confounding effects of COVID-19 on temperature-related mortality, using longer datasets and exploring finer spatial scales. Further investigation into the effectiveness of various interventions to mitigate heat and cold-related mortality is also needed.

The study's reliance on death certificate data might underestimate the true burden of COVID-19-related deaths. The limited data for the COVID-19 period (2020-2022) constrains the analysis of long-term trends. The study focused on England and Wales, and the findings may not be generalizable to other regions with different climatic conditions, healthcare systems, or population characteristics. The analysis of confounding effects is limited by the relatively short time period.