# Seasonal Variation in Myocardial Infarction Hospitalisations and Ischaemic Heart Disease Deaths in New Zealand

## TL;DR

This study found that heart attacks and heart disease deaths in New Zealand are more common in winter than in summer, especially among older people.

## Contribution

The study provides new evidence of significant seasonal variation in heart disease outcomes in New Zealand.

## Key findings

- Winter had 1,278 more heart attack hospitalisations and 1,100 more heart disease deaths than summer.
- The winter-to-summer event ratio for heart attacks was 1.29 and for heart disease deaths was 1.35.
- The seasonal effect was strongest in people aged 80 and older.

## Abstract

Background

Seasonal variation in the incidence of myocardial infarction (MI) has been reported, but the literature is conflicting regarding its magnitude and geographical differences. This study examined the seasonal variation in MI incidence and ischaemic heart disease (IHD) mortality across three New Zealand cities: Auckland, Wellington, and Christchurch.

Methods

This multi-centre retrospective cohort study used New Zealand Ministry of Health data from 2005 to 2014. Poisson regression analysis was conducted, controlling for age, city, and secular trends. Event counts were grouped by season and multivariate models calculated seasonal event ratios with summer as the reference season.

Results

From 2005 to 2014, there were 42,846 MI hospitalisations and 15,466 deaths from IHD. Compared to summer, winter had 1,278 additional MI hospitalisations and 1,100 more IHD-related deaths. Winter-to-summer event ratios were 1.29 (95% confidence interval (CI): 1.15-1.45, p<0.001, χ²=900.84) for MI hospitalisations and 1.35 (95% CI: 1.28-1.43, p<0.001, χ²=61.10) for IHD mortality. The seasonal effect was most pronounced among those aged 80 years and older. No statistically significant differences were observed between cities, genders, ethnicities, or socioeconomic groups.

Conclusion

MI incidence varies by season, peaking during the winter and declining in summer. Given the substantial seasonal burden, targeted interventions such as public awareness campaigns, influenza vaccinations, improved indoor heating, and energy subsidies for vulnerable populations may help reduce winter cardiovascular disease (CVD) risk.