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Seasonal patterns of mortality in relation to social factors
  1. Simon Hales,
  2. Tony Blakely,
  3. Rachel H Foster,
  4. Michael G Baker,
  5. Philippa Howden-Chapman
  1. Department of Public Health, University of Otago, Wellington, Wellington, New Zealand
  1. Correspondence to Dr Simon Hales, Department of Public Health, University of Otago, Wellington, 23a Mein Street, Newtown, Wellington 6242, New Zealand; simon.hales{at}


Background New Zealand is a temperate country with substantial excess winter mortality. We investigated whether this excess winter mortality varies with social factors.

Methods Records from New Zealand censuses in 1981, 1986, 1991, 1996 and 2001 were each anonymously and probabilistically linked to 3 years of subsequent mortality data creating five cohort studies of the New Zealand adult population (age 30–74 years at census) each with 3 years' follow-up. Logistic regression analysis was used to model the risk of dying in winter compared to summer with winter deaths classified ‘1’ and summer deaths ‘0’. There were 75 138 eligible mortality records with complete data on social variables recorded for 58 683 (78%).

Results Adjusting for age, sex, census year, ethnicity and tenure, those in the lowest tertile of income were at increased risk of winter death compared to those in the highest tertile: OR 1.13 (95% CI 1.08 to 1.19). Compared to home owners, people living in rented accommodation were at greater risk of winter death: OR 1.05 (95% CI 1.01 to 1.10). Urban dwellers were also at significantly increased risk. The strongest associations were seen for infectious diseases.

Conclusions There was an increased risk of dying in winter for most New Zealanders, but more so among low-income people, those living in rented accommodation and those living in cities. Exact causal mechanisms are not known but possibly include correlated poorer health status, low indoor temperatures and household crowding.

  • Social inequality
  • seasonal variation
  • mortality
  • environmental epidemiology

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  • Summary Statistics New Zealand Security Statement The New Zealand Census-Mortality Study is a study of the relation between social factors and mortality in New Zealand based on the integration of anonymised population census data from Statistics New Zealand and mortality data from the New Zealand Health Information Service. This project was approved by Statistics New Zealand as a Data Laboratory project under the Microdata Access Protocols in 1997. The datasets created by the integration process are covered by the Statistics Act and can be used for statistical purposes only. Only approved researchers who have signed Statistics New Zealand's declaration of secrecy can access the integrated data in the Data Laboratory.

  • Funding The study was funded by the New Zealand Ministry of Health and the National Heart Foundation of New Zealand. The New Zealand Census-Mortality Study is conducted in collaboration with Statistics New Zealand and within the confines of the Statistics Act 1975. The authors take full responsibility for the paper and Statistics New Zealand will not be held accountable for any error or inaccurate findings. The New Zealand Census-Mortality Study was funded by the Health Research Council of New Zealand and receives continuing funding from the Ministry of Health.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.