The occurrence of myocardial infarction (MI), ischemic stroke, and hemorrhagic stroke has decreased in recent years, but trends in seasonal occurrence remain unclear.
Using Danish healthcare databases, we identified all patients with a first-time MI, ischemic stroke, or hemorrhagic stroke during the study period (1977–2016). We summarized monthly cases for each disease separately and computed the peak-to-trough ratio as a measure of seasonal occurrence of one cycle. To examine trends over time in seasonal occurrence, we computed the peak-to-trough ratio for each of the 40 years. We also quantified the amount of bias arising from random error in peak-to-trough ratios.
Before consideration of bias, the peak-to-trough ratio of summarized monthly cases was 1.11 (95% confidence interval [CI] = 1.10, 1.12) for MI, 1.08 (95% CI = 1.07, 1.09) for ischemic stroke, and 1.12 (95% CI = 1.10, 1.14) for hemorrhagic stroke. The peak-to-trough ratio of MI occurrence increased from 1.09 (95% CI = 1.04, 1.15) in 1977 to 1.16 (95% CI = 1.09, 1.23) in 1999. The trend then remained stable. The peak-to-trough ratio of ischemic stroke occurrence declined continuously during the study period, dropping from 1.12 (95% CI = 1.02, 1.24) in 1977 to 1.06 (95% CI = 1.00, 1.12) in 2016. The peak-to-trough ratio of hemorrhagic stroke occurrence remained stable over time. However, after adjusting for potential bias, time trends in peak-to-trough ratios were almost flat.
We found no substantial seasonality for MI, ischemic stroke, or hemorrhagic stroke occurrence during 1977−2016. Modest peak-to-trough ratios should be interpreted after considering bias induced by random variation.
From the aDepartment of Clinical Epidemiology, Aarhus University Hospital, Aarhus N, Denmark
bRTI Health Solutions, Research Triangle Institute, Research Triangle Park, NC.
Submitted September 29, 2017; accepted July 10, 2018.
Data sharing: No additional data are available. Danish legislation does not allow data to be shared.
Transparency declaration: The lead author affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned have been explained.
This study was supported by the Program for Clinical Research Infrastructure (PROCRIN) established by the Lundbeck Foundation and the Novo Nordisk Foundation.
The authors report no conflicts of interest.
Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com).
N.S., J.S., K.A., and H.T.S conceived the study idea and designed the study. N.S. directed the analyses, which were conducted by N.S. and E.H.-P. All authors participated in the discussion and interpretation of the results. N.S. reviewed the literature, organized the writing, and wrote initial drafts. All authors critically revised the manuscript for intellectual content and approved the final version. H.T.S is the guarantor.
Correspondence: Nils Skajaa, Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, Denmark. E-mail: email@example.com.