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National Center for Immunization & Respiratory Diseases
Interim Estimates of 2023–24 Seasonal Influenza Vaccine
Effectiveness
Aaron M. Frutos, PhD, MPH
On behalf of CDC Influenza Vaccine Effectiveness Collaborators
Advisory Committee on Immunization Practices
February 28, 2024
CDC Influenza Vaccine Effectiveness Networks
Four networks to evaluate vaccine effectiveness (VE) against laboratory -
confirmed influenza for children, adolescents, and adults in the
outpatient and inpatient settings
3CDC Influenza Vaccine Effectiveness Networks
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)
VISION
IVYNVSN
4These networks include all ages across settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
NVSN
5NVSN: all settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
NVSN
6US Flu VE: Outpatient clinic and ED/UC
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
NVSN
7VISION: ED/UC & hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)VISION
IVY
8IVY: hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)
IVY
9US Flu VE: Outpatient clinic and ED/UC
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)US Flu VE
IVY
10VISION: ED/UC & hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)US Flu VE
VISION
VISION
IVYNVSN
11These networks include all ages across settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)Outpatient
12CDC influenza VE networks include patients from 22
states
NVSN
IVY
US Flu VE
VISION
132023- 2024 Influenza VE Methods
Enrollees: Have acute respiratory illness
Dates of enrollment: Fall 2023- Early 2024
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 2023 –24 seasonal flu vaccine according to medical records,
immunization registries, claims data, and/or self -report
142023- 2024 Influenza VE Methods
Analysis : VE = (1 – adjusted OR) x 100%
•Adjusted for geographic region, age, calendar time of illness
•IVY , US Flu VE, and VISION also adjusted for sex and race and ethnicity
•US Flu VE also adjusted for days between illness onset and enrollment and self -reported
general health status.
•VE estimates were calculated for influenza A subtypes A(H1N1)pdm09 and A(H3N2) when possible
•Subtype not available for VISION
•VE was not estimated for some age groups and settings when sample size was small or when
models did not converge
Pediatric VE
(aged 6 months –17 years)
16Pediatric VE against any influenza
Influenza test result by influenza vaccination
status, no. vaccinated/Total (%)
Influenza -positive Influenza -negative VE (95% CI)
NVSN (Outpatient) 123/622 (20) 793/2,577 (31) 59 (48, 67)
US Flu VE (Outpatient) 29/283 (10) 182/736 (25) 67 (48, 80)
VISION (Outpatient) 961/6,068 (16) 4,579/15,274 (30) 60 (57, 64)
NVSN (Inpatient) 29/128 (23) 543/1,321 (41) 61 (40, 75)
VISION (Inpatient) 21/113 (19) 299/921 (32) 52 (16, 72)
17Pediatric VE against any influenza
18Pediatric VE against influenza A
19Pediatric VE against influenza A(H1N1)pdm09
20Pediatric VE against influenza A(H3N2)
21Pediatric VE against influenza B
Adult VE
(aged ≥18 years)
23Adult 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) 177/568 (31) 803/1,807 (44) 33 (16, 47)
VISION (Outpatient) 4,501/18,385 (24) 21,356/52,657 (41) 49 (47, 51)
IVY (Inpatient) 200/632 (32) 1,517/3,872 (39) 44 (32, 54)
VISION (Inpatient) 728/1,839 (40) 7,425/14,168 (52) 41 (34, 47)
24Adult VE against any influenza
25Adult VE against influenza A
26Adult VE against influenza A(H1N1)pdm09
27Adult VE against influenza A(H3N2)
28Adult VE against influenza B
Adult (aged ≥65) VE
30Adult (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) 41/79 (52) 300/439 (68) 51 (14, 72)
VISION (Outpatient) 1,944/3,687 (53) 12,162/19,571 (62) 41 (36, 45)
IVY (Inpatient) 113/249 (45) 938/1,945 (48) 42 (23, 56)
VISION (Inpatient) 531/1,066 (50) 6,058/10,118 (60) 42 (34, 50)
31Adult (aged ≥65) VE against any influenza
32Adult (aged ≥65) VE against influenza A
33Adult (aged ≥65) VE against influenza B
Discussion
35Summary of four CDC influenza VE networks
Vaccination with a 2023 -24 influenza vaccine reduced the risk for medically
attended influenza outpatient visits and hospitalizations among children,
adolescents, and adults across 22 US States
Vaccination was effective against both influenza A (mostly subtype
A(H1N1)pdm09) and B (lineage Victoria) viruses that have circulated this season
Results were consistent across networks
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.
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 VISIONThank you
37Overall VE (pediatric and adults) by season*
*Estimates are from US Flu VE except for 2022 -23 which are from a study from WI