Seasonal Mortality in Denmark. The Role of Sex and Age

Roland Rau, Max Planck Institute for Demographic Research
Gabriele Doblhammer, Max Planck Institute for Demographic Research

The detrimental effects of the environment, namely cold temperatures, are reflected in the seasonal fluctuations in mortality. Although mortality peaks in the cold season, one can not necessarily equate colder climate with larger monthly oscillations as previous studies for the UK, Canada and Mediterranean countries showed. We analyzed the influence of the factors sex and age on these seasonal fluctuations in Denmark between 1968 and 1998, a country which sets itself apart from its neighbors in health and mortality trends. Our presentation is the first longitudinal analysis of seasonal mortality that is based on a sample of the general population using not estimated but correct exposure times (person-months lived). For our main analysis, we used discrete-time survival models (logistic regression) to estimate the influence of the time-varying covariate “current month” on mortality. Our data consist of all people in Denmark who were 50 years or older on 1 April 1968 (~1.4 Mio individuals). To be able to process the huge amount of data (in our reshaped time-varying data-set, one record resembled one person-month lived and not one individual), we drew a random sample from that base-population, which was made up of 46.619 persons. They were divided into four cohorts and followed for 30 years until March 1998. We first calculated a descriptive measurement of winter excess mortality comparable to previous studies. The result of 6.69 percent winter excess mortality places Denmark in a mid-table position among European countries. However, a comparison with Denmark’s neighboring countries lets us conclude that further progress can be made in reducing the annual cold-related death toll. The highest odds-ratios for dying are found in winter for all cohorts. Mortality is also elevated during spring and fall for the whole population. Further analyzes showed that previous findings of neglectable sex differences in seasonal mortality have to be re-considered. We find a relatively similar seasonal pattern in mortality only in the youngest age-group (born between April 1908 – March 1918; Ages 50 – 90). At older ages (Individuals born before April 1908; Ages 60+), women and men showed remarkable diverse seasonal mortality. Generally, men’s excess mortality during winter is higher than that of women. Seasonal fluctuations for men increase from younger ages onwards. We found a secondary summer peak only for the oldest male cohort. Our results suggest that this phenomenon that has been described before in the literature was not induced by age but was an outcome of a period effect. Women show almost no increase in seasonal mortality fluctuations up to age 90 and a sharp increase after this age. Seasonal mortality among the elderly will gain relevance in public health because the chances to survive into these ages are increasing and – as argued in the literature – the annual amount of excess mortality may be reducible by preventive health and safety measures.