title: Influenza epidemics, seasonality, and the effects of cold weather on cardiac mortality.
abstract: BACKGROUND
  More people die in the winter from cardiac disease, and there are competing hypotheses to explain this. The authors conducted a study in 48 US cities to determine how much of the seasonal pattern in cardiac deaths could be explained by influenza epidemics, whether that allowed a more parsimonious control for season than traditional spline models, and whether such control changed the short term association with temperature.
  
  
  METHODS
  The authors obtained counts of daily cardiac deaths and of emergency hospital admissions of the elderly for influenza during 1992-2000. Quasi-Poisson regression models were conducted estimating the association between daily cardiac mortality, and temperature.
  
  
  RESULTS
  Controlling for influenza admissions provided a more parsimonious model with better Generalized Cross-Validation, lower residual serial correlation, and better captured Winter peaks. The temperature-response function was not greatly affected by adjusting for influenza. The pooled estimated increase in risk for a temperature decrease from 0 to -5°C was 1.6% (95% confidence interval (CI) 1.1-2.1%). Influenza accounted for 2.3% of cardiac deaths over this period.
  
  
  CONCLUSIONS
  The results suggest that including epidemic data explained most of the irregular seasonal pattern (about 18% of the total seasonal variation), allowing more parsimonious models than when adjusting for seasonality only with smooth functions of time. The effect of cold temperature is not confounded by epidemics.
authors.affiliation: Department of Environmental Health, Exposure Epidemiology and Risk Program, Harvard School of Public Health, Boston, MA, USA.
authors.forename: Stephanie
authors.initials: S
authors.lastname: von Klot
authors.forename: Antonella
authors.initials: A
authors.lastname: Zanobetti
authors.forename: Joel
authors.initials: J
authors.lastname: Schwartz
mesh_terms: D000368:Aged; D002318:Cardiovascular Diseases / Q000401:mortality*; D002423:Cause of Death; D002947:Cities; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007251:Influenza, Human / Q000453:epidemiology*; D012621:Seasons; D013696:Temperature; D014481:United States / Q000453:epidemiology