CDC Seasonal Flu Vaccine Effectiveness Studies

What to know

CDC conducts studies each flu season to help determine how well flu vaccines are working. These vaccine effectiveness (VE) studies help regularly assess the value of flu vaccination as a public health intervention. The results of vaccine effectiveness studies can vary based on the study design, the outcome(s) measured, the population studied, and the season studied.

U.S. flu vaccine effectiveness networks

CDC has been working with researchers at universities and hospitals since the 2003-2004 flu season to estimate how well flu vaccines work through observational studies using laboratory-confirmed flu as the outcome. Over the past few years, CDC has conducted VE studies using multiple vaccine effectiveness networks. More information on CDC's vaccine effectiveness networks and studies is available at CDC's Influenza Vaccine Effectiveness Networks.

Results from prior flu seasons

The overall, adjusted vaccine effectiveness estimates for flu seasons from 2004-2024 are noted in the chart below. (Estimates are typically adjusted for study site, age, sex, underlying medical conditions, and days from illness onset to enrollment.)

Effectiveness of Seasonal Flu Vaccines from the 2009-2024 Flu Seasons

The vaccine effectiveness estimates included in the chart and tables below are vaccine effectiveness estimates from the U.S. Flu VE Network.

Chart showing the level of seasonal flu vaccine effectiveness 2009 through 2024
Chart showing the level of seasonal flu vaccine effectiveness 2009 through 2024
2009-10 2010-11 2011-12 2012-13 2013-14 2014-15 2015-16 2016-17 2017-18 2018-19 2019-20 2020-21* 2021-22 2022-23** 2023-24††
Adj. Overall VE (%) 56 60 47 49 52 19 48 40 38 29 39 Not enough data to compute 36 30 42

*2020-2021 flu vaccine effectiveness was not estimated due to low influenza virus circulation during the 2020-2021 flu season.

A point estimate is a single value that is used to estimate an unknown population parameter. Point estimates are used in statistics to make inferences about population parameters based on sample data. More information is available at How Flu Vaccine Effectiveness and Efficacy are Measured.

Overall VE

Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2024

Influenza Season

Reference

Study Site(s)

No. of Patients

Adjusted Overall VE (%)

95% CI

2004-05*

Belongia 2009

WI

762

10

-36, 40

2005-06*

Belongia 2009

WI

346

21

-52, 59

2006-07*

Belongia 2009

WI

871

52

22, 70

2007-08*

Belongia 2011

WI

1,914

37

22, 49

2008-09*

Unpublished

WI, MI, NY, TN

6,713

41

30, 50

2009-10*

Griffin 2011

WI, MI, NY, TN

6,757

56

23, 75

2010-11*

Treanor 2011

WI, MI, NY, TN

4,757

60

53, 66

2011-12

Ohmit 2014

WI, MI, PA, TX, WA

4,771

47

36, 56

2012-13

McLean 2014

WI, MI, PA, TX, WA

6,452

49

43, 55

2013-14

Gaglani 2016

WI, MI, PA, TX, WA

5,999

52

44, 59

2014-15

Zimmerman 2016

WI, MI, PA, TX, WA

9,311

19

10, 27

2015-16

Jackson 2017

WI, MI, PA, TX, WA

6,879

48

41, 55

2016-17

Flannery 2018 WI, MI, PA, TX, WA 7,410

40

32, 46

2017-18

Rolfes 2019 WI, MI, PA, TX, WA 8,436

38

31, 43

2018-19

Flannery 2019 WI, MI, PA, TX, WA 10,041

29

21, 35

2019-20

Tenforde 2021 WI, MI, PA, TX, WA 8,845

39

32, 44

2020-21**

n/a n/a n/a

n/a

2021-22

Price 2022 CA, MI, PA, TN, TX, WA, WI 4,312

36

21, 48

2022-23

Chung 2024 MI, PA, TX, WA, AZ, MO, OH 2,561 30 -9, 54

2023-2024††****

ACIP Presentation on February 28, 2024 AZ, MI, MO, PA, OH, TX, WA 3,394 42 29, 53

*From 2004-2005 through 2010-2011, the Flu VE Network also enrolled inpatients.

**2020-2021 flu vaccine effectiveness was not estimated due to low influenza virus circulation during the 2020-2021 flu season.

Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not been published.

Number of patients used in VE calculation.

****Overall VE was calculated for this table to be consistent with what has been reported historically and is available for comparison with historical estimates. All other 2023-2024 VE estimates are reported by age group.

Supporting research