Estimation of seasonal variations in risk factor profiles and mortality from coronary heart disease

Objective: Seasonal variations in coronary heart disease (CHD) and related risk factors have been reported previously. However, no studies to date quantify the contribution of seasonal variations in risk factors to actual mortality in both men and women using a single database of sufficient size and follow-up. Methods: We investigated the database from the Western Austrian Vorarlberg Health Monitoring and Promotion Programme (VHM&PP) including over 450.000 repeated measurements of 149.650 individuals between 1985 and 1999. Results: Of a total of 1266 deaths from CHD (ICD-9 410-414), 353 deaths occurred between December and February (27.9%), in contrast to 275 (21.7%) between June and August. While the frequency of deaths through acute myocardial infarction (ICD-9 410) was similar over the seasons, chronic forms of CHD (ICD-9 414) occurred significantly (p<0.001) more frequently in winter. Total cholesterol, blood pressure and body mass index showed pronounced seasonal variations with average levels significantly higher during the winter months in all age groups and both sexes, giving an estimated increase in SCORE risk of 6.8% in men and 3.6% in women. However by contrast, use of single time point risk factor data tended to over-estimate subsequent 10 year mortality if measured in winter and the converse in summer. Conclusion: For the first time, this study quantifies the contribution of seasonal risk factor variation to CHD mortality. The consistent effect across demographic groups suggests that this is a real physiological phenomenon and not an artefact of living conditions. Interpretation of standard risk scores should take account of this seasonal fluctuation in subsequent investigation and follow-up.