CDC Seasonal Flu Vaccine Effectiveness Studies
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 2004-2023 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.)
The vaccine effectiveness estimates included in the chart and tables below are vaccine effectiveness estimates from the U.S. Flu VE Network.
| 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** | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Adj. Overall VE (%) | 56 | 60 | 47 | 49 | 52 | 19 | 48 | 40 | 38 | 29 | 39 | Not enough data
to compute |
36 | n/a |
*2020-2021 flu vaccine effectiveness was not estimated due to low flu virus circulation during the 2020-2021 flu season.
**Preliminary 2022-2023 flu vaccine effectiveness against severe outcomes was estimated by three CDC networks. Flu vaccine effectiveness against medically attended flu illness was not estimated.
|
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Unpublished |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 | |
| n/a | n/a | n/a |
n/a |
*From 2004-2005 through 2010-2011, the Flu VE Network also enrolled inpatients.
**2020-2021 flu vaccine effectiveness was not estimated due to low flu virus circulation during the 2020-2021 flu season.
***Preliminary 2022-2023 flu vaccine effectiveness against severe outcomes was estimated by three CDC networks. Flu vaccine effectiveness against medically attended flu illness was not estimated.
† Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not been published.
‡ Number of patients used in VE calculation.
