Seasonal Incidence of Symptomatic Influenza in the United States

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Abstract

<div class="section"> <a class="named-anchor" id="S1">  </a> <h5 class="section-title" id="d3319414e102">Background</h5> <p id="P1">The seasonal incidence of influenza is often approximated as 5%–20%.</p> </div><div class="section"> <a class="named-anchor" id="S2">  </a> <h5 class="section-title" id="d3319414e107">Methods</h5> <p id="P2">We used 2 methods to estimate the seasonal incidence of symptomatic influenza in the United States. First, we made a statistical estimate extrapolated from influenza-associated hospitalization rates for 2010–2011 to 2015–2016, collected as part of national surveillance, covering approximately 9% of the United States, and including the existing mix of vaccinated and unvaccinated persons. Second, we performed a literature search and meta-analysis of published manuscripts that followed cohorts of subjects during 1996–2016 to detect laboratory-confirmed symptomatic influenza among unvaccinated persons; we adjusted this result to the US median vaccination coverage and effectiveness during 2010–2016. </p> </div><div class="section"> <a class="named-anchor" id="S3">  </a> <h5 class="section-title" id="d3319414e112">Results</h5> <p id="P3">The statistical estimate of influenza incidence among all ages ranged from 3.0%–11.3% among seasons, with median values of 8.3% (95% confidence interval [CI], 7.3%–9.7%) for all ages, 9.3% (95% CI, 8.2%–11.1%) for children <18 years, and 8.9% (95% CI, 8.2%–9.9%) for adults 18–64 years. Corresponding values for the meta-analysis were 7.1% (95% CI, 6.1%–8.1%) for all ages, 8.7% (95% CI, 6.6%–10.5%) for children, and 5.1% (95% CI, 3.6%–6.6%) for adults. </p> </div><div class="section"> <a class="named-anchor" id="S4">  </a> <h5 class="section-title" id="d3319414e117">Conclusions</h5> <p id="P4">The 2 approaches produced comparable results for children and persons of all ages. The statistical estimates are more versatile and permit estimation of season-to-season variation. During 2010–2016, the incidence of symptomatic influenza among vaccinated and unvaccinated US residents, including both medically attended and nonattended infections, was approximately 8% and varied from 3% to 11% among seasons. </p> </div>

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Systematic Assessment of Multiple Routine and Near Real-Time Indicators to Classify the Severity of Influenza Seasons and Pandemics in the United States, 2003–2004 Through 2015–2016

Daniel B. Jernigan, Shikha Garg, Erin Burns … (2018)

Assessments of influenza season severity can guide public health action. We used the Moving Epidemic Method to develop intensity thresholds (IT) for 3 U.S. 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% (IT 50 ), 90% (IT 90 ), and 98% (IT 98 ) one-sided CIs of the geometric mean of each season’s 3 highest values. Severity was classified as low if ≥2 systems peaked below IT 50 , moderate if ≥2 peaked between IT 50 and IT 90 , high if ≥2 peaked between IT 90 and IT 98 , and very high if ≥2 peaked above IT 98 . 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.