Introduction Seasonality of disease has been long recognised in epidemiology. More recently, researchers have established standard ways of measuring seasonality. In temperate countries there has been a particular interest in excess winter mortality (EWM), and, to a lesser degree, excess winter hospitalisations (EWH). Understanding the aetiology of seasonality is important for identifying interventions and potential future climate change effects.
Method We measured EWM and EWH in 60–95 year olds between 2000 and 2008, by ICD-10 chapter, and the contribution of each chapter to all-cause winter excess. We then compared indices and percentage contributions for mortality to those for hospitalisation.
Results Indices do not lie in the same direction for all ICD-10 chapters. Excesses lay within the same range only for respiratory deaths and hospitalisations. Circulatory illness showed an excess for both hospitalisation and death, but the excess was higher for mortality than for morbidity. However, neoplasms, digestive diseases, and genitourinary diseases all showed a winter mortality excess but a non-winter hospitalisation index. Other chapters also lacked a relationship between indices for hospitalisation and death. Similarly, there was a difference in contribution to winter excess by chapter.
Conclusion The comparison of EWM and EWH indicates that caution may be needed in extrapolating causal results from one to the other. As their disease and excess distribution is different, factors which contribute to EWH may play a different role in EWM, and vice versa. Research that can identify the reasons for these differences would improve our understanding of the mechanisms causing disease seasonality.
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