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Interim Estimates of 2024 –25 Seasonal Influenza
Vaccine Effectiveness
Aaron M. Frutos, PhD, MPH
On behalf of CDC Influenza Vaccine Effectiveness CollaboratorsNational Center for Immunization & Respiratory Diseases
CDC Influenza Vaccine Effectiveness
Networks
Four networks to evaluate vaccine effectiveness (VE) against laboratory -
confirmed influenza for children and adolescents and adults in the
outpatient and inpatient settings
•Investigating Respiratory Viruses in the Acutely Ill (IVY)
•New Vaccine Surveillance Network (NVSN)
•U.S. Flu Vaccine Effectiveness Network (US Flu VE)
•Virtual SARS -CoV-2, Influenza, and Other respiratory viruses Network
(VISION)CDC Influenza Vaccine Effectiveness Networks
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VISION
IVYNVSNThese networks include all ages across settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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NVSNNVSN: all settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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NVSNUS Flu VE: Outpatient clinic and ED/UC
US Flu VE
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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NVSNVISION: ED/UC & hospitalization
US Flu VE
VISION
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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IVYIVY: hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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IVYUS Flu VE: Outpatient clinic and ED/UC
US Flu VE
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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IVYVISION: ED/UC & hospitalization
US Flu VE
VISION
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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These networks include all ages across settings
Outpatient
VISION
IVYNVSN
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
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CDC influenza VE networks include patients from 23
states
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•Enrollees: Received medical care for an acute respiratory illness
•Dates of enrollment: Fall 2024 – Early 2025
•Design: Test-negative design
–Comparing vaccination odds among case patients with influenza confirmed by
molecular assay versus control patients testing negative for influenza and SARS -CoV-2
–Vaccination status: receipt of any 2024 –25 seasonal flu vaccine according to medical
records, immunization registries, claims data, and/or self -report2024 –2025 Influenza VE Methods
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•Analysis : VE = (1 – adjusted OR) x 100%
–Adjusted for geographic region, age, calendar time of illness
»US Flu VE, and VISION also adjusted for sex and race and ethnicity
–VE estimates were calculated for influenza A subtypes A(H1N1)pdm09 and A(H3N2)
when possible
»Influenza A subtype estimates were not calculated for VISION because of
limited subtype data
–VE was not estimated for some age groups and settings when sample size was small or
when models did not converge2024 –2025 Influenza VE Methods
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Influenza A subtype by network
Influenza A subtype by network
Influenza A subtype by network, total (%)
Influenza A(H1N1)pdm09 Influenza A(H3N2)
IVY 50 (31) 110 (69)
NVSN 284 (52) 262 (48)
US Flu VE 168 (33) 337 (67)
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Pediatric VE
(aged 6 months –17 years)
Pediatric VE against any influenza
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
NVSN (Outpatient) 100/482 (21) 855/2,487 (34) 59 (47 –68)
US Flu VE (Outpatient) 54/217 (25) 256/917 (28) 32 (1 –54)
VISION (Outpatient) 1,322/9,563 (14) 5,943/27,356 (22) 60 (56 –63)
NVSN (Inpatient) 28/119 (24) 613/1,523 (40) 63 (41 –76)
VISION (Inpatient) 16/157 (10) 406/1,481 (27) 78 (60 –89)
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Pediatric VE against any influenza
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Pediatric VE against influenza A(H1N1)pdm09
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Pediatric VE against influenza A(H3N2)
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Adult VE
(aged ≥18 years)
Adult VE against any influenza
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
US Flu VE (Outpatient) 112/475 (24) 592/1,735 (34) 34 (16 –51)
VISION (Outpatient) 5,631/26,011 (22) 25,842/76,628 (34) 54 (52 –56)
IVY (Inpatient) 211/675 (31) 873/2,500 (35) 41 (28 –52)
VISION (Inpatient) 905/2,959 (31) 10,869/28,074 (39) 55 (51 –59)
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Adult VE against any influenza
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Adult VE against influenza A(H1N1)pdm09
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Adult VE against influenza A(H3N2)
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Adult (aged ≥65) VE
Adult (aged ≥65) VE against any influenza
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
US Flu VE (Outpatient) 28/56 (50) 195/332 (59) 18 (-69–60)
VISION (Outpatient) 2,575/5,731 (45) 14,978/27,525 (54) 51 (47 –54)
IVY (Inpatient) 150/341 (44) 591/1,313 (45) 38 (19 –52)
VISION (Inpatient) 693/1,897 (37) 8,903/19,271 (46) 57 (52 –61)
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Adult (aged ≥65) VE against any influenza
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Discussion
•Vaccination with a 2024 –25 influenza vaccine reduced the risk for
medically attended influenza outpatient visits and hospitalizations among
children and adolescents and adults across 23 US states
•Influenza v accination was effective against influenza A viruses with
variation by subtype across networks
•Interim Estimates of 2024 –2025 Seasonal Influenza Vaccine Effectiveness —
Four Vaccine Effectiveness Networks, United States, October 2024 –
February 2025 | MMWRSummary of four CDC influenza VE networks
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For more information, contact CDC
1-800-CDC-INFO (232 -4636)
TTY: 1 -888-232-6348 www.cdc.gov
The findings and conclusions in this report are those of the authors and do not necessarily represent the official
position of the Centers for Disease Control and Prevention.
Thank you
Aaron M. Frutos, PhD, MPH
Influenza Division
National Center for Immunization and Respiratory Diseases
[email protected]’d like to thank our many collaborators
from CDC, IVY , NVSN, US Flu VE, and
VISION
Adult (aged 18 –64) VE
Adult (aged 18 –64) VE against any influenza
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
US Flu VE (Outpatient) 84/419 (20) 397/1,403 (28) 37 (15 –53)
VISION (Outpatient) 3,056/20,280 (15) 10,864/49,103 (22) 56 (53 –58)
IVY (Inpatient) 61/334 (18) 282/1187 (24) 48 (28 –63)
VISION (Inpatient) 212/1,062 (20) 1,966/8,803 (22) 51 (41 –59)
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Adult (aged 18 –64) VE against any influenza
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Pediatric VE Tables
Pediatric VE against Influenza A(H1N1)pdm09
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
NVSN (Outpatient) 32/224 (14) 855/2,487 (34) 72 (59 –81)
US Flu VE (Outpatient) 9/50 (18) 256/917 (28) 53 (3 –79)
NVSN (Inpatient) 13/60 (22) 613/1,523 (40) 63 (30 –81)
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Pediatric VE against Influenza A(H3N2)
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
NVSN (Outpatient) 62/218 (28) 855/2,487 (34) 42 (19 –58)
US Flu VE (Outpatient) 29/107 (27) 256/917 (28) 16 (-34–49)
NVSN (Inpatient) 12/44 (27) 613/1,523 (40) 55 (14 –77)
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Adult VE Tables
(aged ≥18 years)
Adult VE against Influenza A(H1N1)pdm09
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
US Flu VE (Outpatient) 36/118 (31) 592/1,735 (34) 42 (8 –64)
IVY (Inpatient) 12/50 (24) 873/2,500 (35) 39 (-14–67)
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Adult VE against Influenza A(H3N2)
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
US Flu VE (Outpatient) 56/230 (24) 592/1,735 (34) 25 (-6–48)
IVY (Inpatient) 8/110 (26) 873/2,500 (35) 51 (22 –69)
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