Department of Public Health Sciences, Stockholm University, Stockholm, SE-106 91, Sweden.
BMC Public Health. 2020 Nov 9;20(1):1674. doi: 10.1186/s12889-020-09749-4.
Mortality rates are known to depend on the seasons and, in temperate climates, rates are highest during winter. The magnitude of these seasonal fluctuations in mortality has decreased substantially in many countries during the 20th century, 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 people ages 60 years or more has changed in Sweden between 1860 and 1995, and investigate how this change has contributed to the increase in life expectancy observed during the same time period.
Yearly sex-specific birth cohorts consisting of all people born in Sweden between 1800 and 1901 who reached at least 59 years of age were obtained from a genealogical database. The mortality rates for each cohort were modeled by an exponential function of age modulated by a sinusoidal function of time of year. The potential impact of seasonal fluctuations on 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 age 60 was used to quantify changes in lifespan during the time period.
The magnitude of seasonal fluctuations in mortality rates decreased substantially between 1860 and 1995. 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 age 60 increased by 4.3 years for men and 6.8 years for women, and the decrease in seasonal mortality fluctuations accounted for approximately 40% of this increase in average lifespan.
By following a large number of extinct cohorts, it was possible to show how the decrease in seasonal fluctuations in mortality 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.
众所周知，死亡率取决于季节，在温带气候中，死亡率在冬季最高。在 20 世纪，许多国家的死亡率季节性波动幅度都大大降低，但这种降低在多大程度上促成了同期预期寿命的延长尚不清楚。在这里，我描述了 1860 年至 1995 年间瑞典 60 岁及以上人群全因死亡率的季节性变化，并研究了这种变化对同期观察到的预期寿命增长的贡献。
从一个家谱数据库中获得了由 1800 年至 1901 年期间在瑞典出生且至少达到 59 岁的所有人群组成的每年特定性别出生队列。通过年龄的指数函数和年度时间的正弦函数对每个队列的死亡率进行建模。通过将总死亡率分解为季节性部分和与季节无关的部分，来研究季节性波动对预期寿命的潜在影响。使用 60 岁时的队列预期寿命来量化该时间段内寿命的变化。
1860 年至 1995 年间，死亡率季节性波动幅度大幅降低。对于 1800 年出生的队列，冬季死亡的风险几乎是夏季的两倍。对于 1900 年出生的队列，冬季死亡率的相对增加为 10%。男性 60 岁时的队列预期寿命增加了 4.3 年，女性增加了 6.8 年，季节性死亡率波动的减少约占平均寿命延长的 40%。
通过跟踪大量已灭绝的队列，我们可以展示死亡率季节性波动的减少如何促成预期寿命的增加。这里介绍的总死亡率分解方法可能有助于更好地理解过去 150 年预期寿命显著提高的过程和机制。
