Document text
Severity, Disease Burden, and Prevented Burden for
the 2024 -2025 Influenza Season
Influenza Division, CDC
2024 -2025 Influenza Season News
Influenza Severity Assessment by Season and by Age Group
Severity Classification by Age Group
Influenza Season* 0-17 years 18-64 years ≥65 years All Ages
2009- 2010 Very High Moderate Low Moderate
2010- 2011 Moderate Moderate Moderate Moderate
2011- 2012 Low Low Low Low
2012- 2013 Moderate Moderate High Moderate
2013- 2014 Moderate Moderate Moderate Moderate
2014-2015 Moderate Moderate High High
2015- 2016 Low Moderate Low Moderate
2016- 2017 Moderate Moderate Moderate Moderate
2017- 2018 High High High High
2018- 2019 Moderate Moderate Moderate Moderate
2019- 2020 High High Moderate Moderate
2021- 2022 Low Low Low Low
2022- 2023 High Moderate Moderate Moderate
2023- 2024 Moderate Moderate Moderate Moderate
2024-2025 High High High High
*Severity assessment was not completed for 2020 -2021 season because of minimal influenza activity
Influenza Disease Burden Estimation
•Influenza is not a notifiable , reportable disease in the U.S.
-Except for pediatric deaths
•Sentinel surveillance systems in healthcare settings cannot capture all
influenza virus infections
•Not everyone who is ill seeks medical care
•Not everyone who seek medical care will be tested for influenza
•Even if someone with influenza seeks care and is tested, depending on where
in the course of an illness and type of specimen collected, they may test negative
•Influenza symptoms are non -specific and individuals could be misdiagnosed
with having a different respiratory illness especially when not testedWhy it is necessary to estimate influenza disease burden
https://www.cdc.gov/flu -burden/php/about/why -cdc-estimates.html
•Disease burden is estimated by
clinical disease severity
•Calculated using five age groups:
-0-4 years
-5-17 years
-18-49 years
-50-64 years
-65+ yearsU.S. Annual Influenza Burden Estimates
Deaths
Hospitalizations
Medically attended
illnesses
Community symptomatic
illnesses
Hospitalizations
(Crude hospitalization rate) x (adjustment for
testing practices) x population1
Symptomatic community illness
(Hospitalizations) x ( case:hospitalization ratio)3Outpatient medical visits:
(Symptomatic community illness) x
(probability of seeking medical care when ill)4Deaths
(Hospitalizations) x
(death:hospitalization ratio)2Data Source
Cause of death from death
certificate, risk of death when hospitalized, and location of death (in vs. out of hospital (ref 1)
Routine sentinel surveillance for patients hospitalized with
laboratory -confirmed
influenza and data from a
survey of influenza testing
practices at sentinel hospitals
(ref 1)
Telephone survey of healthcare -seeking behavior
during influenza -like illness
(ref 3)
Field investigations of influenza (ref 2)Burden level
MethodologyStep
#Legend
1: R eed, et al., PLoS One. 2015
2: R eed , et al., EID. 2009
3: Biggerstaff, et al., AJPH . 2012
Preliminary Influenza Disease Burden, 2024- 2025 by Age Group
Age GroupSymptomatic Illnesses
(95% Uncertainty
Interval)Medically Attended Illnesses
(95% Uncertainty Interval)Hospitalizations
(95% Uncertainty
Interval)
0-4 years3,600,000
(2,900,000, 5,800,000)2,400,000
(1,800,000, 4,000,000)25,000
(20,000, 40,000)
5-17 years11,000,000
(8,600,000, 24,000,000)5,800,000
(4,100,000, 12,000,000)31,000
(24,000, 65,000)
18-49 years22,000,000
(16,000,000, 38,000,000)8,100,000
(5,300,000, 14,000,000)120,000
(91,000, 210,000)
50-64 years14,000,000
(10,000,000, 31,000,000)6,000,000
(3,700,000, 13,000,000)150,000
(110,000, 320,000)
≥65 years4,900,000
(3,400,000, 11,000,000)2,700,000
(1,700,000, 6,200,000)440,000
(310,000, 990,000)
Total56,000,000
(47,000,000, 83,000,000)25,000,000
(20,000,000, 38,000,000)770,000
(620,000, 1,400,000)
Influenza Disease Burden Varied by Season
23,000,000
9,400,00034,000,00030,000,000 30,000,000
24,000,00029,000,00041,000,000
29,000,00034,000,000
11,000,00031,000,00040,000,00056,000,000 11,000,000
4,400,00016,000,000
13,000,000 14,000,000
11,000,00014,000,00019,000,000
14,000,00015,000,000
5,100,00014,000,00018,000,00025,000,000
2010-11 2011-12 2012-13 2013-14 2014-15 2015-16 2016-17 2017-18 2018-19 2019-20 2021-22 2022-23 2023-24 2024-25Number of Symptomatic Illnesses, Medical Visits, and
Hospitalizations
SeasonSymptomatic Illnesses Medical Visits Hospitalizations
140,000580,000
350,000590,000
280,000500,000710,000
400,000380,000
120,000370,000470,000770,000
310,000
* * *
*Preliminary Estimates
Influenza Vaccine Prevented Influenza
Disease Burden Estimation
Influenza Vaccine Prevented Disease Burden Components
56 million
illnesses
770,000 hospitalizations25 million medical visits
35–71%
(Varies by age group)38–56%
(Varies by age
group
and flu type)Disease Burden Vaccine Coverage Vaccine Effectiveness
Outpatient
Inpatient
39–62%
(Varies by age
group
and flu type)
Data Sources: Vaccine Coverage: NIS-FLU, BRFSS ; Outpatient VE estimates: VE Network , NVSN , VISION Network ;
Inpatient VE estimates: NVSN, VISION Network, IVY
https://www.cdc.gov/flu -burden/php/about -burden- prevented/index.html
Influenza Prevented Disease Burden Model
Estimated
Medical VisitsEstimated
HospitalizationsEstimated
Deaths
Observed burden
outcomes with
vaccinationExpected burden
outcomes without
vaccinationBurden outcomes
prevented by
vaccination
Use vaccine coverage
(VC) and vaccine
effectiveness (VE)
estimate risk to
susceptible personsApply risk of burden
outcomes among
susceptible persons to
hypothetical susceptible
population without
vaccinationMonte Carlo
simulation
methodology to
estimate uncertaintyDisease
burden model
Prevented
disease burden
model
Calculated monthly
by age groupEstimated
InfectionsAdjusted
Hospitalization
Rates
∝
1-VE, 1 -VC
Preliminary Influenza Disease Burden Prevented by Vaccination, 2024-
2025 by Age Group
Age Group Symptomatic Illnesses Medically Attended Illnesses Hospitalizations
0-4 years1,200,000
(910,000, 2,000,000)810,000
(600,000, 1,300,000)9,700
(7,300, 16,000)
5-17 years2,200,000
(1,600,000, 4,800,000)1,200,000
(840,000, 2,500,000)7,700
(5,600, 17,000)
18-49 years3,800,000
(2,700,000, 6,400,000)1,400,000
(1,000,000, 3,000,000)18,000
(13,000, 31,000)
50-64 years3,300,000
(2,300,000, 7,300,000)1,400,000
(1,000,000, 3,000,000)32,000
(22,000, 63,000)
≥65 years1,600,000
(1,100,000, 3,500,000)900,000
(600,000, 2,000,000)170,000
(120,000, 380,000)
Total12,000,000
(10,000,000, 18,000,000)5,700,000
(4,800,000, 8,400,000)240,000
(180,000, 460,000)
Influenza Disease and Prevented Burden 2011 -2024†Number of Illnesses, Hospitalizations, and Deaths140,000580,000
350,000590,000
280,000500,000710,000
400,000 380,000
120,000370,000470,000770,000
45,000119,000
120,000112,000
73,00065,00083,000
36,000 98,000
26,00071,000120,000240,000Hospitalizations Prevented Hospitalizations
9,400,00034,000,00030,000,000 30,000,00024,000,00029,000,00041,000,000
29,000,00034,000,000
11,000,00031,000,00040,000,00056,000,000
2,300,0008,100,0008,000,0003,200,000
5,600,0004,900,0006,000,000
2,800,0006,700,000
1,900,0005,500,0009,800,00012,500,000
2011-12 2012-13 2013-14 2014-15 2015-16 2016-17 2017-18 2018-19 2019-20 2021-22 2022-23 2023-24 2024-25Symptomatic Illnesses Prevented Sympotmatic Illnesses11,000,000
4,400,00016,000,00013,000,000 14,000,00011,000,00014,000,00019,000,00014,000,000 15,000,000
5,100,00014,000,00018,000,00025,000,000 3,100,000
1,100,0004,000,000
3,700,000 1,700,0002,700,0002,500,0003,100,000
1,500,0003,300,000
990,0002,700,0004,800,0005,700,000Medical Visits Prevented Medical Visits
*
*Preliminary Estimates; †https://www.cdc.gov/flu -burden/php/data -vis-vac/past -burden -prevented -est.html
* *
•Influenza causes significant impacts on healthcare settings and substantial
disease burden on the population
•2024- 2025 season was classified as high severity for all age ages and by age
group and is the first high severity season since 2017- 2018
•Estimated influenza disease burden was the highest the U.S. has seen in the
last decade
•Influenza vaccine prevented an estimated 240,000 hospitalizations, most in
adults aged 65 years and older, and likely prevented the season from being even more severeConclusions
For more information, contact CDC
1-800- CDC- INFO (232 -4636)
TTY: 1 -888- 232- 6348 cdc.gov
The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the U.S. Centers for Disease Control and Prevention.