Seasonal variation in cause-specific mortality: Are there high-risk groups? 25-year follow-up of civil servants from the first Whitehall study

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Abstract
Objectives To determine the seasonal effect on all-cause and cause-specific mortality and to identify high-risk groups. Methods A 25-year follow-up of 19 019 male civil servants aged 40–69 years. Results All-cause mortality was seasonal (ratio of highest mortality rate during winter versus lowest rate during summer 1.22, 95% CI : 1.1–1.3), largely due to the seasonal nature of ischaemic heart disease. Participants at high risk based on age, employment grade, blood pressure, cholesterol, forced expiratory volume, smoking and diabetes did not have higher seasonal mortality, although participants with ischaemic heart disease at baseline did have a higher seasonality effect (1.38, 95% CI : 1.2–1.6) than those without (1.18, 95% CI : 1.1–1.3) (P = 0.03). Conclusions Seasonal mortality differences were greater among those with prevalent ischaemic heart disease and at older ages, but were not greater in individuals of lower socioeconomic status or with a high multivariate risk score. Since seasonal differences showed no evidence of declining over time, elucidating their causes and preventive strategies remains a public health challenge.