What to know
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 2004-2025 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 2009-2025 Flu Seasons
The vaccine effectiveness estimates included in the chart and tables below are vaccine effectiveness estimates from multiple VE networks.
| 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 | 2024-25†† | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Adj. Overall VE (%) | 56 | 60 | 47 | 49 | 52 | 19 | 48 | 40 | 38 | 29 | 39 | Not enough data to compute | 36 | 30 | 44 | 56 |
*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.
Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2025
|
Influenza Season† |
Reference |
Study Site(s) |
No. of Patients‡ |
Adjusted Overall VE (%) |
95% CI |
|---|---|---|---|---|---|
|
2004-05* |
WI |
762 |
10 |
-36, 40 |
|
|
2005-06* |
WI |
346 |
21 |
-52, 59 |
|
|
2006-07* |
WI |
871 |
52 |
22, 70 |
|
|
2007-08* |
WI |
1,914 |
37 |
22, 49 |
|
|
2008-09*† |
Unpublished |
WI, MI, NY, TN |
6,713 |
41 |
30, 50 |
|
2009-10* |
WI, MI, NY, TN |
6,757 |
56 |
23, 75 |
|
|
2010-11* |
WI, MI, NY, TN |
4,757 |
60 |
53, 66 |
|
|
WI, MI, PA, TX, WA |
4,771 |
47 |
36, 56 |
||
|
WI, MI, PA, TX, WA |
6,452 |
49 |
43, 55 |
||
|
WI, MI, PA, TX, WA |
5,999 |
52 |
44, 59 |
||
|
WI, MI, PA, TX, WA |
9,311 |
19 |
10, 27 |
||
|
WI, MI, PA, TX, WA |
6,879 |
48 |
41, 55 |
||
| Flannery 2018 | WI, MI, PA, TX, WA | 7,410 |
40 |
32, 46 | |
| Rolfes 2019 | WI, MI, PA, TX, WA | 8,436 |
38 |
31, 43 | |
| Flannery 2019 | WI, MI, PA, TX, WA | 10,041 |
29 |
21, 35 | |
| Tenforde 2021 | WI, MI, PA, TX, WA | 8,845 |
39 |
32, 44 | |
| n/a | n/a | n/a |
n/a |
||
| Price 2022 | CA, MI, PA, TN, TX, WA, WI | 4,312 | 36 | 21, 48 | |
| Chung 2024 | MI, PA, TX, WA, AZ, MO, OH | 2,561 | 30 | -9, 54 | |
| Chung 2025 | AZ, MI, MO, PA, OH, TX, WA | 9,061 | 44 | 36, 51 | |
| Frutos 2025 | MN, WI, UT, CA, OR, WA, IN, CO | 139,558 | 56 | 54, 58 |
*From 2004-2005 through 2010-2011, the Flu VE Network also enrolled inpatients.
†Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not been published.
**2020-2021 flu vaccine effectiveness was not estimated due to low influenza virus circulation during the 2020-2021 flu season.
‡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.
†† VE estimates for 2024-2025 flu season are preliminary.