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

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.

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.

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.

Seasonal influenza virus infection is so common that its incidence can only be estimated. The Centers for Disease Control and Prevention (CDC) maintains surveillance for a number of measures, such as the percentage of respiratory specimens submitted to clinical laboratories that are positive for influenza and the percentage of outpatient visits to sentinel physicians that are for influenza-like illness [

A common approximation is that “5%–20% of people get influenza each season.” This figure is based on a serologic study performed in Tecumseh, Michigan, during the 1976–1977 through 1980–1981 influenza seasons [

The methods used to make this estimate have been summarized previously and are outlined in

We sought articles with the following characteristics: English language; published during a 20-year period (influenza seasons 1996–1997 to 2015–2016 but excluding the 2009–2010 pandemic year); performed in the United States or Canada; and included follow-up of a defined group of subjects to detect symptomatic, laboratory-confirmed (by culture or reverse-transcription polymerase chain reaction [RT-PCR]) seasonal influenza during at least 1 influenza season (approximately October of 1 year to May of the following year). Studies with appropriate data included the placebo arm of controlled trials and cohort studies of respiratory virus incidence. Studies were excluded if a case-control, case-cohort, or design other than cohort was used; only medically attended subjects were included; or only members of a specific group were included (eg, subjects with a specific disease, healthcare workers, children or staff in childcare facilities, residents of long-term care facilities, military personnel, and religious group members).

We made a hand-search of articles that were known to the authors, written by authors who frequently publish on this subject, or included in recent review articles [

We abstracted the following variables: influenza season, type of study (clinical trial vs cohort study), site (eg, country, state, city), ages included, active (subjects contacted routinely to ascertain symptoms) vs passive (request that patient contact researcher if has symptoms) follow-up, swab type (throat, nose, nasopharyngeal, oropharyngeal), laboratory testing method (culture or RT-PCR), percentage of subjects with current-season vaccination, total number of subjects followed, and number or percentage with symptomatic laboratory-confirmed influenza. If data were presented separately for influenza types or subtypes, the numbers were added to estimate the total number with influenza (eg, if 10 subjects were reported with influenza A and 5 with influenza B, a total of 15 subjects with influenza was calculated). In clinical trials, per-protocol rather than intention-to-treat results were preferentially included.

To assess study quality, we adapted criteria from the Newcastle-Ottawa Quality Assessment Scale for Cohort Studies [

We first performed descriptive analyses. Many studies included only unvaccinated subjects, and, where data were reported for a mix of vaccinated and unvaccinated subjects, the median percentage vaccinated was >60%, higher than in the United States general population. Therefore, to facilitate comparability with the statistical estimates, we limited meta-analyses to unvaccinated subjects and adjusted the results to approximate infection rates given median vaccination coverage and effectiveness during 2010–2016 (

The 6 influenza seasons covered by CDC estimates of influenza activity included 3 that were A(H3N2) predominant, 2 that were A(H1N1)pdm09 predominant, and 1 with mixed A(H3N2) and A(H1N1)pdm09 predominance (

The database searches identified 5347 manuscripts, most from Ovid Medline (n = 4671) (

The 16 studies spanned the 1996–1997 to 2013–2014 influenza seasons (

Only 1 study of persons ≥65 years was identified [

Of 15 manuscripts included in meta-analyses, study quality measures were high or intermediate for 9 for geographic representativeness, 13 for age representativeness, 15 for general representativeness, 12 for adequacy of follow-up, 13 for sensitivity of symptoms prompting laboratory testing, and 15 for laboratory method (

The meta-regression model included 23 study seasons with data on children or adults (

We show data for all seasons, but show forest plots and random-effects incidence rates only for seasons of moderate severity (

We used 2 methods to make an updated estimate of the seasonal incidence of symptomatic influenza, both medically attended and non–medically attended, among United States residents. The first (statistical estimation) method was based on CDC-measured rates of hospitalization with influenza that were adjusted to produce an estimate of total numbers of influenza infections. The second was a literature review and meta-analysis of published studies. The 2 methods produced similar incidence results for all ages (7%–8%) and for children (both 9%), but the statistical estimate was higher than the meta-analytic result for adults 18–64 years of age (9% vs 5%;

Because of its greater clinical relevance, we studied symptomatic influenza infection. Estimates of the percentage of influenza infections that are asymptomatic include a common approximation of 50% [

It is widely believed that influenza incidence is higher in children than in adults [

The statistical estimation method that we present has become the primary way that CDC estimates the seasonal numbers of influenza infections, medical visits, hospitalizations and deaths due to influenza, and the numbers of these events that are prevented by vaccination. The strengths of this method include the careful yearly collection of hospitalization data from geographically representative regions that include approximately 9% of the United States population. This large sample size allows robust estimates, and yearly collection allows estimates of year-to-year variability. The 6 seasons we studied included 1 season of low and 1 season of high severity; therefore, the range that we report should be a good estimate of seasonal variability. These advantages make the statistical estimation approach the preferred method to make updated and yearly estimates of seasonal influenza incidence.

Our literature review produced an important independent estimate of influenza incidence. We used a comprehensive search strategy (

Other limitations of the statistical estimate method have been discussed previously [

The most important way to prevent influenza is yearly vaccination, which is recommended for everyone 6 months and older [

The authors gratefully acknowledge the FluSurv-NET personnel for collecting data used in the statistical estimate; Julia Taliano, Centers for Disease Control and Prevention (CDC), for creating the literature search strategy; and Vanessa Boshuizen for assistance in manuscript review. We also acknowledge Melissa S. Stockwell, Columbia University, and Ryan Malosh and Emily Martin, University of Michigan School of Public Health, for providing updated and stratified data.

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.

Influenza incidence in unvaccinated children aged <18 years (

Estimates of the Incidence of Symptomatic Influenza by Season and Age-Group, United States, 2010–2016

Season | Predominant Virus(es) | Season Severity [ | Incidence | |||||
---|---|---|---|---|---|---|---|---|

0–4 y | 5–17 y | 18–49 y | 50–64 y | ≥65 y | All Ages | |||

2010–2011 | A/H3N2, A/H1N1pdm09 | Moderate | 14.1 | 8.4 | 5.3 | 8.1 | 4.3 | 6.8 |

2011–2012 | A/H3N2 | Low | 4.8 | 3.6 | 2.5 | 3.1 | 2.3 | 3.0 |

2012–2013 | A/H3N2 | Moderate | 18.6 | 12.7 | 8.9 | 14.3 | 9.9 | 11.3 |

2013–2014 | A/H1N1pdm09 | Moderate | 12.4 | 7.2 | 9.2 | 13.0 | 3.4 | 9.0 |

2014–2015 | A/H3N2 | High | 15.0 | 12.7 | 7.8 | 12.9 | 12.4 | 10.8 |

2015–2016 | A/H1N1pdm09 | Moderate | 11.1 | 7.4 | 7.1 | 11.0 | 3.5 | 7.6 |

Median | 13.2 | 7.9 | 7.4 | 12.0 | 3.9 | 8.3 |

Median influenza incidence 9.3% for children <18 years of age, 8.9% for adults 18–64 years of age. Confidence intervals for these incidence estimates are in

Values represent percentage of residents with influenza during the designated season estimated from hospitalization rates determined in the Influenza Hospitalization Surveillance Network (FluSurv-NET) [

Summary of Results, Incidence of Symptomatic Influenza in US Residents in Seasons of Moderate Severity, by Method of Estimation and Vaccination Category

Age Group, y | Statistical Estimation Method, |Incidence, % | Meta-analysis, Incidence, % | |
---|---|---|---|

Vaccinated and Unvaccinated | Unvaccinated | Vaccinated and Unvaccinated^{,} | |

Children <18 y | 9.3 (8.2–11.1) | 12.0 (9.2–14.7) | 8.7 (6.6–10.5) |

Adults 18–64 y | 8.9 (8.2–9.9) | 6.1 (4.3–7.9) | 5.1 (3.6–6.6) |

Adults ≥65 y | 3.9 (3.4–4.2) | No estimate | No estimate |

All ages | 8.3 (7.3–9.7) | 8.9 (7.7–10.2) | 7.1 (6.1–8.1) |

Abbreviation: CI, confidence interval.

Estimates from

Median 44% of US residents were vaccinated during 2010–2016 (

Estimates from

Calculated by reducing the incidence in unvaccinated by 28.6% for children, 16.4% for adults, and 20.4% for all ages (

Studies of Influenza Incidence Identified by Literature Review, United States and Canada, 1996–2016

Author, Publication Year | Influenza | Recruitment | Purpose | Age Group (Ages Included) | Subjects | Site(s) | Swab Type | Laboratory Method | Follow-up |
---|---|---|---|---|---|---|---|---|---|

Barrett 2011 [ | 2008–2009 | CT | Vaccine | Adult (18–49 y) | Healthy, exclude high risk | 36 sites | NP | C | Passive |

Belshe 2000 [ | 1996–1997, 1997–1998 | CT | Vaccine | Child (15–85 mo) | Healthy, exclude high risk | 10 sites | Not reported | C | Active (weekly) |

Falsey 2005 [ | 1999–2000, 2000–2001, 2001–2002, 2002–2003 | Cohort | Respiratory viruses | Senior (≥65 y) | Healthy, exclude if disabling disease | Rochester, NY | NP | C, P, S | Passive |

Hoberman 2003 [ | 1999–2000, 2000–2001 | CT | Vaccine | Child (6–24 mo) | Healthy, multiple exclusion criteria | Pittsburgh, PA | T | C | Active (biweekly) |

Jackson 2010 [ | 2005–2006, 2006–2007 | CT | Vaccine | Adult (18–49 y) | Healthy, multiple exclusion criteria | 37–44 sites | NP/OP | C | Active (weekly) |

Langley 2011 [ | 2003–2004 | CT | Vaccine | Adult (18–64 y) | Healthy, multiple exclusion criteria | 28 sites, Canada | N, T | C | Active (biweekly) |

Monto 2009 [ | 2007–2008 | CT | Vaccine | Adult (18–49 y) | Healthy, exclude high risk | 4 college campuses, MI | T | C, P | Passive |

Monto 2014 [ | 2011–2012 | Cohort | Respiratory viruses | Child (< 18 y), Adult (18–64 y), | Households with ≥4 members, ≥2 children <18 y | Ann Arbor, MI | N (age <3 y), N, T (age ≥3 y) | P | Active (weekly) |

Ohmit 2006 [ | 2004–2005 | CT | Vaccine | Adult (18–46 y) | Healthy, exclude high risk | 2 university, 2 community sites in MI | T | C, P | Active (biweekly) |

Ohmit 2008 [ | 2005–2006 | CT | Vaccine | Adult (18–48 y) | Healthy, exclude high risk | 4 university, 2 community sites in MI | T | C, P | Passive |

Ohmit 2013 [ | 2010–2011 | Cohort | Vaccine | Child (< 18 y) | Households with ≥4 members, ≥2 children <18 y | Ann Arbor, MI | N (age <7 y), T (age ≥7 y) | P | Active (weekly) |

Ohmit 2015 [ | 2012–2013 | Cohort | Vaccine | Child (<9 to 17 y), adult (18 to ≥50 y), | Households with ≥4 members, ≥2 children <18 y | Ann Arbor, MI | N (age <3 y), N, T (age ≥3 y) | P | Active (weekly) |

Ohmit 2016 [ | 2013–2014 | Cohort | Vaccine | Child (<9 to 17 y), adult (18 to ≥50 y), | Households with ≥4 members, ≥2 children <18 y | Ann Arbor, MI | N (age <3 y), N, T (age ≥3 y) | P | Active (weekly) |

Smithgall 2016 [ | 2013–2014 | Cohort | Vaccine | Child (0–17 y), adult (18–64 y), all (0 to >64 y) | Households in low-income neighborhood | New York City, NY | N | P | Active (semi-weekly) |

Treanor 2007 [ | 2004–2005 | CT | Vaccine | Adult (18–49 y) | Healthy, exclude high risk | 3 sites | NP | C | Active (weekly) |

Treanor 2011 [ | 2007–2008 | CT | Vaccine | Adult (18–49 y) | Healthy, exclude high risk | 24 sites | N, T | C | Active (weekly) |

Abbreviations: C, culture; CT, clinical trial; MI, Michigan; N, nose; NP, nasopharyngeal; NY, New York; OP, oropharyngeal; P, reverse-transcription polymerase chain reaction; PA, Pennsylvania; S, serology; T, throat.

Purposes include to study the efficacy or effectiveness of influenza vaccine on influenza incidence, or to study the frequency of respiratory viruses.

Unpublished data supplied by authors.

Meta-regression of Factors Influencing the Seasonal Incidence of Influenza Among Unvaccinated Persons in Published Studies, 1996–2013

Category | Variable | Study Seasons | Estimate | (95% CI) | |
---|---|---|---|---|---|

Age | Intercept | 14 | 6.4 | (5.2–7.6) | <.0001 |

Children | 9 | 4.5 | (2.7–6.3) | <.0001 | |

Season severity | Moderate | 15 | Reference | <.0001 | |

Low | 7 | −5.2 | (−6.8 to −3.6) | ||

High | 1 | 5.1 | (−1.8 to 11.9) |

Includes 23 study seasons with data on children (<18 years) or adults. Proportion of total between-study variance explained by the model = 0.72. Other factors assessed that were nonsignificant included recruitment method (controlled trial vs cohort study), laboratory method (culture vs reverse-transcription polymerase chain reaction), follow-up (active vs passive), and secular trend.

Abbreviation: CI, confidence interval.

The estimate for the intercept represents influenza incidence in adults; 13 of the 14 study seasons included only adults 18–64 years, but 1 study [

The estimate of incidence in children is 4.5% higher than in adults, or 10.9%.