BACKGROUND: The occurrence of myocardial infarction (MI), ischemic stroke, and hemorrhagic stroke has decreased in recent years, but trends in seasonal occurrence remain unclear.
METHODS: 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.
RESULTS: 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.
CONCLUSIONS: 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.