The rise and fall of excess winter mortality in New Zealand from 1876 to 2020

Abstract

Excess winter mortality (EWM) has been used as a measure of how well populations and policy moderate the health effects of cold weather. We aimed to investigate long-term changes in the EWM of Aotearoa New Zealand (NZ), and potential drivers of change, and to test for structural breaks in trends. We calculated NZ EWM indices from 1876 (4,698 deaths) to 2020 (33,310 deaths), total and by age-group and sex, comparing deaths from June to September (the coldest months) to deaths from February to May and October to January. The mean age and sex-standardised EWM Index (EWMI) for the full study period, excluding 1918, was 1.22. However, mean EWMI increased from 1.20 for 1886 to 1917, to 1.34 for the 1920s, then reduced over time to 1.14 in the 2010s, with excess winter deaths averaging 4.5% of annual deaths (1,450 deaths per year) in the 2010s, compared to 7.9% in the 1920s. Children under 5 years transitioned from a summer to winter excess between 1886 and 1911. Otherwise, the EWMI age-distribution was J-shaped in all time periods. Structural break testing showed the 1918 influenza pandemic strain had a significant impact on trends in winter and non-winter mortality and winter excess for subsequent decades. It was not possible to attribute the post-1918 reduction in EWM to any single factor among improved living standards, reduced severe respiratory infections, or climate change.

Exploring the paradoxical nature of cold temperature mortality in Europe

“…While the overall pattern of annual mortality cycles seems to have largely stabilized in the last half-century, suggesting a period of relative consistency, earlier dynamics clearly hinted that moderately high temperatures used to be a major environmental hazard before the widespread adoption of modern sanitary standards 9,52,53 . Nevertheless, even in recent decades, our resilience to environmental hazards is steadily increasing, as there has been both an observed decrease in EWM 35,54 and a reduction in vulnerability to heatwaves [54][55][56] . Thus, the future relationship is likely in flux and depends not only on our climate but also on the relative success in mitigating specific threats.…”

“…The onset of the recent COVID-19 pandemic served as a natural experiment to elucidate the modulation mechanism by providing interruption of a non-climatic nature, followed by an ultimate significant rebound in infections. During the initial phase of the pandemic, due to restrictions and voluntary behavior changes, respiratory infection visits 33 , 34 and mortality reduced 7 , 35 . This was accompanied by a decreasing detectable antibody level for common viruses 36 .…”