A large decrease in the magnitude of seasonal fluctuations in mortality explains part of the increase in longevity in Sweden during the 20th century

Abstract

AbstractBACKGROUNDMortality rates of are known to depend on the seasons and in temperate climates rates are highest during winter. The magnitude of these seasonal fluctuations in mortality have decreased substantially during the 20th century in many countries, but the extent to which this decrease has contributed to the concurrent increase in life expectancy, is not known. Here I describe how the seasonality of all-cause mortality among persons aged 60 years or more in Sweden has changed in the time period from 1860 to 1995, and furthermore investigate how this change has contributed to the increase in life expectancy observed during the same period.METHODSYearly sex-specific birth cohorts consisting of all persons born in Sweden between 1800 and 1901, and who reached at least 59 years of age, were obtained from a genealogical database. The mortality rates of each cohort were modeled by an exponential function of age modulated by a sinusoidal function of time of year. The contribution of seasonal fluctuations to life expectancy was investigated by a novel decomposition of the total mortality rate into a seasonal part and a part independent of the seasons. Cohort life expectancy at 60 was used to quantify changes in lifespan during the time period.RESULTSThe magnitude of seasonal variations of mortality rates decreased substantially over the time period. For cohorts born in 1800, the risk of dying during the winter season was almost twice that of dying during summer. For cohorts born in 1900, the relative increase in winter mortality was 10%. Cohort life expectancy at 60 increased with 4.3 years for men and 6.8 years for women and the decrease of seasonal variability accounted for approximately 40% of this increase in average lifespan.CONTRIBUTIONBy following a large number of extinct cohorts it was possible to show how the decrease in seasonal mortality fluctuations has contributed to an increase in life expectancy. The decomposition of total mortality introduced here might be useful to better understand the processes and mechanisms underlying the marked improvements in life expectancy seen over the last 150 years.