The seasonal incidence of symptomatic influenza in the United States

Abstract

Background

The seasonal incidence of influenza is often approximated as "5% to 20%".

Methods

We used two 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 2010–11 to 2015–16, 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 United States 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% (CI 8.2%, 11.1%) for children <18 years and 8.9% (CI 8.2%, 9.9%) for adults 18–64 years. Corresponding values for the meta-analysis were 7.1% (CI 6.1, 8.1) for all ages, 8.7% (6.6, 10.5) for children, and 5.1% (3.6, 6.6) for adults.

Conclusions

The two 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 United States residents, including both medically attended and non-attended infections, was approximately 8% and varied from 3% to 11% among seasons.