Characterizing Repeated Outbreak Patterns in Influenza Time Series

In some influenza seasons, morbidity and mortality closely follow the expected seasonal variation. In these years, approaches such as Serflingís model and seasonal-based syndromic outbreak detectors, in use in EARS, work well. In other years, though, short but intense variations occur in addition to the longer term seasonal variation. These intense outbreaks, which are often multimodal, have important implications for both syndromic surveillance and influenza epidemiology. Unfortunately, they are both difficult to characterize and poorly understood. In this paper, we apply techniques from time-frequency distribution theory to identify the temporal location, duration, and amplitude of intense outbreaks occurring in the presence of longer time scale variations.