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 laboratoryconfirmed influenza for children, adolescents, and adults in the 

outpatient and inpatient settings 
3

CDC 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

IVY

NVSN

4

These networks include all ages across settings

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

US Flu VE

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)
NVSN

5

NVSN: all settings

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)
NVSN

6

US Flu VE: Outpatient clinic and ED/UC

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

US Flu VE

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)
NVSN

7

VISION: ED/UC & hospitalization

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

US Flu VE

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)

VISION
IVY

8

IVY: hospitalization

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)
IVY

9

US Flu VE: Outpatient clinic and ED/UC

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)

US Flu VE
IVY

10

VISION: ED/UC & hospitalization

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)

US Flu VE

VISION
VISION

IVY

NVSN

11

These networks include all ages across settings

Age

Setting

Outpatient 

clinic

Hospitalization

Emergency 

department/ 

urgent care

US Flu VE

Adults 

(≥18 years)

Children and adolescents

(6 months– 17 years)

Outpatient
12

CDC influenza VE networks include patients from 22 

states

NVSN

IVY

US Flu VE

VISION
13

2023-2024 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
14

2023-2024 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)
16

Pediatric 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)
17

Pediatric VE against any influenza
18

Pediatric VE against influenza A
19

Pediatric VE against influenza A(H1N1)pdm09
20

Pediatric VE against influenza A(H3N2)
21

Pediatric VE against influenza B
Adult VE

(aged ≥18 years)
23

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) 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)
24

Adult VE against any influenza
25

Adult VE against influenza A
26

Adult VE against influenza A(H1N1)pdm09
27

Adult VE against influenza A(H3N2)
28

Adult VE against influenza B
Adult (aged ≥65) VE
30

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) 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)
31

Adult (aged ≥65) VE against any influenza
32

Adult (aged ≥65) VE against influenza A
33

Adult (aged ≥65) VE against influenza B
Discussion
35

Summary 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

AFrutos@cdc.gov

We’d like to thank our many 

collaborators from CDC, IVY, NVSN, 

US Flu VE, and VISION

Thank you
37

Overall VE (pediatric and adults) by season*

*Estimates are from US Flu VE except for 2022-23 which are from a study from WI