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National Center for Immunization & Respiratory Diseases
2023 –24 End of Season Influenza Vaccine Effectiveness – United
States
Sascha Ellington, PhD, MSPH
Influenza Division
U.S. Centers for Disease Control and Prevention
2023 –2024 Influenza Season
•A(H1N1)pdm09 predominant
•Lower levels of A(H3N2) and
B/Victoria circulation
•Peak activity 2023 week 52
•Vaccines were quadrivalent and
included A(H1N1)pdm09,
A(H3N2), and B components
H1N1
H3N2B
3CDC Influenza Vaccine Effectiveness Networks
Investigating Respiratory Viruses in the Acutely Ill (IVY)
New Vaccine Surveillance Network (NVSN)
US 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)Outpatient
NVSN
5NVSN: all settings
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)Outpatient
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)Outpatient
NVSN
7VISION: ED/UC & hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent careUS Flu VE
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)VISIONOutpatient
IVY
8IVY: hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)Outpatient
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 Outpatient
IVY
10VISION: ED/UC & hospitalization
AgeSettingOutpatient
clinic
HospitalizationEmergency
department/
urgent care
Adults
(≥18 years)Children and adolescents
(6 months – 17 years)US Flu VE
VISIONOutpatient
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 23 states
13
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142023 –2024 Influenza VE Methods
Enrollees: Have acute respiratory illness and present for medical care
Dates of enrollment: Fall 2023 –April/May 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
152023 –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 among children and adolescents aged 6 months – 17 years
17Pediatric VE against any influenza
February 2024
18Pediatric VE against any influenza
February 2024
19Pediatric VE against influenza A
February 2024
20Pediatric VE against influenza A
February 2024
21Pediatric VE against influenza B
February 2024
22Pediatric VE against influenza B
February 2024
VE among adults ages 18 –49 years
24Adult (aged 18 –49 years) VE against any influenza
25Adult (aged 18 –49 years) against VE influenza A
26Adult (aged 18 –49 years) VE against influenza B
VE among adults aged 50 –64 years
28Adult (aged 50 –64 years) VE against any influenza
February 2024, 18–64 years combined
29Adult (aged 50 –64 years) VE against any influenza
February 2024, 18–64 years combined
30Adult (aged 50 –64 years) VE against influenza A
February 2024, 18–64 years combined
31Adult (aged 50 –64 years) VE against influenza A
February 2024, 18–64 years combined
32Adult (aged 50 –64 years) VE against influenza B
February 2024, 18–64 years combined
33Adult (aged 50 –64 years) VE against influenza B
February 2024, 18–64 years combined
VE among adults aged ≥65 years
35Adult (aged ≥65) VE against any influenza
February 2024
36Adult (aged ≥65) VE against any influenza
February 2024
37Adult (aged ≥65) VE against influenza A
February 2024
38Adult (aged ≥65) VE against influenza A
February 2024
39Adult (aged ≥65) VE against influenza B
February 2024
40Adult (aged ≥65) VE against influenza B
February 2024
VE by A subtype
42VE against influenza A(H1N1)pdm09
VE against influenza A(H3N2)
43
Discussion
45Summary of four CDC influenza VE networks
Vaccination with a 2023 –2024 influenza vaccine reduced the risk of medically
attended influenza outpatient visits and hospitalizations among children,
adolescents, adults, and the elderly .
Results were consistent across 4 networks in 23 states.
Preliminary end-of-season estimates are similar to interim estimates from
February.
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.
We’d like to thank our many collaborators from CDC, IVY , NVSN, US Flu VE, and VISIONThank you