Seasonal Incidence of Symptomatic Influenza in the United States

Abstract

Background: The seasonal incidence of influenza is often approximated as 5%-20%.

Methods: 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.

Results: 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.

Conclusions: 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.

Figure 1

Numbers of manuscripts screened and included in the study. *Did not follow for a full influenza season (n = 4), data from the 2009–2010 pandemic (n = 3), clinical trial without a placebo group (n = 3), subjects not from the general population (n = 3). Abbreviation: CINAHL, Cumulative Index to Nursing and Allied Health Literature.

Figure 2

Influenza incidence in unvaccinated children aged <18 years (A), adults (B), and all ages (C), by season and study. Data is shown for all seasons, but forest plots and summary incidence include only seasons of moderate severity. “Year” denotes first year in the influenza season (eg, “1996” denotes the 1996–1997 influenza season). Thirteen of the 14 study seasons include only adults aged 18–64 years; 1 [30] may include some adults ≥65 years of age. Abbreviations: CI, confidence interval; IV, inverse variance; SE, standard error.