Seasonality of Influenza‐Like‐Illness and Acute Cardiovascular Events Are Related Regardless of Vaccine Effectiveness

BACKGROUND: Influenza has been identified as a trigger for stroke and myocardial infarction (MI) with prior studies demonstrating that influenza vaccination may decrease risk of stroke and MI. METHODS AND RESULTS: We used data from the New York Department of Health Statewide Planning and Research Cooperative System to evaluate whether annual variability in influenza vaccination effectiveness (VE) would be associated with cardiovascular events. Daily and monthly counts of outpatient and inpatient visits for influenza‐like illness (ILI), stroke, and MI were identified using International Classification of Diseases, Ninth Revision (ICD‐9) codes; VE data for each year are publicly available. We identified pertinent lags between ILI, stroke, and MI using prewhitening cross‐correlation functions and applied them to autoregressive integrated moving average time series regression models. Time series forecasting systems assessed correlations among ILI, stroke, and MI, and the effect of VE on these relationships. Cross‐correlation functions indicated stroke events increased 1 month after increases in ILI rates; MIs increased immediately. Accounting for seasonality and lag, peaks in ILI rates were significantly related to peaks in stroke (P=0.04) and MI (P=0.01). Time forecasting analyses indicated no relationship between VE and cardiovascular events. CONCLUSIONS: We identified that seasonality of cardiovascular events may be associated with seasonality in ILI, though VE did not modify this relationship.