Background and Purpose: Influenza has been identified as a trigger for stroke, and prior work demonstrated a decreased risk of stroke in people vaccinated for influenza. We aimed to identify the relationship between seasonal influenza rates, stroke rates, and the year-to-year variability in influenza vaccination effectiveness (VE).
Methods: Using data from 2004-2015 from the New York Department of Health Statewide Planning and Research Cooperative System (SPARCS), we removed any common temporal patterns using cross correlation functions to assess potential correlations between influenza rates and total stroke (ischemic and hemorrhagic) rates, with a sub-analysis investigating ischemic stroke and hemorrhagic stroke rates separately. Time series regression models (autoregressive integrated moving average: ARIMA) and time series forecasting systems were used to assess the interrelationship of influenza rates, stroke rates, and VE. Validated ICD-9 codes were used to identify emergency department and hospital visits for influenza and stroke, and influenza VE data for each year were available publically from the CDC.
Results: In the raw data, stroke hospitalizations appeared to increase shortly after increases in influenza. The cross correlation function for all stroke yielded a significant lag of 1 month, suggesting that stroke hospitalizations increased one month after an increase in influenza presentations. Accounting for seasonality and the lag, peaks in influenza rates were significantly related to peaks in stroke hospitalizations (P=0.04). When investigating ischemic and hemorrhagic stroke separately, there appeared to be a similar but non-statistically significant relationship with a 1 month lag of both stroke subtype peaks after influenza peaks (p=0.1195 and p=0.1921, respectively). Time forecasting analysis did not support a relationship between VE and stroke hospitalization rates; however, VE estimates were based on national data presented by the CDC and may differ for New York State specifically, in addition to varying vaccination rates.
Conclusion: The seasonality of stroke may be attributed to the seasonality in influenza, though influenza vaccine effectiveness was not associated with the seasonality of stroke.