Systematic Assessment of Multiple Routine and Near-Real Time Indicators to Classify the Severity of Influenza Seasons and Pandemics in the United States, 2003–04 Through 2015–2016

Abstract

Assessments of influenza season severity can guide public health action. We used the Moving Epidemic Method to develop intensity thresholds (IT) for 3 US surveillance indicators from the 2003–04—2014–15 influenza season (excluding the 2009 pandemic): 1) outpatient visits for influenza-like illness, 2) influenza-related hospitalizations, and 3) influenza- and pneumonia-related deaths. ITs were developed for the overall population and children, adults, and older adults separately and were the upper limit of the 50% (IT₅₀), 90% (IT₉₀), and 98% (IT₉₈) one-sided CIs of the geometric mean of each season's 3 highest values. Severity was classified as low if ≥2 systems peaked below IT₅₀, moderate if ≥2 peaked between IT₅₀ and IT₉₀, high if ≥2 peaked between IT₉₀ and IT₉₈, and very high if ≥2 peaked above IT₉₈. We piloted this method with the 2015–16 season and the 2009 pandemic. Overall, 4 seasons were classified as low severity, 7 as moderate, 2 as high, and none as very high. While older adults had the most seasons (n = 3) classified as high, children were the only group to have seasons (n = 2) classified as very high. We will apply this method to classify the severity of future seasons and inform pandemic response.