CDC Seasonal Flu Vaccine Effectiveness Studies
CDC conducts studies to measure the benefits of seasonal flu vaccination each flu season to help determine how well flu vaccines are working. These vaccine effectiveness (VE) studies regularly assess the value of flu vaccination as a public health intervention. Study results of vaccine effectiveness can vary based on the study design, the outcome(s) measured, the population studied and the season in which the flu vaccine was 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 vaccine works through observational studies using laboratory-confirmed flu as the outcome. CDC uses four networks to estimate vaccine effectiveness: the U.S. Flu VE Network, the Influenza Vaccine Effectiveness in the Critically Ill (IVY), the New Vaccine Surveillance Network (NVSN), and the VISION VE Network. The Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) was previously used to estimate vaccine effectiveness but ended enrollment on July 31, 2021. CDC will continue to collect information on adults hospitalized with flu through its other VE networks. The U.S. Flu VE Network currently consists of seven study sites across the United States that measure the flu vaccine’s effectiveness at preventing outpatient medical visits due to laboratory-confirmed influenza. CDC’s observational studies at U.S. Flu VE Network sites measure outpatient visits for laboratory-confirmed influenza virus infections using a highly accurate lab test called PCR to test for the flu virus in respiratory specimens. NVSN consists of seven study sites in the U.S. that measure the flu vaccine’s effectiveness at preventing hospitalizations due to laboratory-confirmed influenza. NVSN captures influenza hospitalizations among children 6 months to 17 years of age, and persons hospitalized with respiratory illness are tested for influenza using molecular tests (including PCR). These studies compare the frequency of influenza vaccination among patients who test positive for influenza to vaccination in a comparable group of patients without influenza. IVY consists of 21 large, adult hospitals in 20 U.S. cities and was created in 2019 to estimate how well the flu vaccine works at preventing severe flu illness among intensive care unit (ICU) patients. As of April 1, 2021, IVY has expanded to enroll all adults hospitalized with COVID-19 and flu. The VISION VE Network collects data on emergency department visits, hospitalizations, and intensive care unit (ICU) admissions. The network was established in 2019 and includes the following eight U.S. sites:
- Baylor Scott and White Health (BSHW; Texas)
- Columbia University Irving Medical Center (CUIMC; New York)
- HealthPartners (HP; Minnesota and Wisconsin)
- Intermountain Healthcare (IH; Utah)
- Kaiser Permanente Northern California (KPNC; California)
- Kaiser Permanente Northwest (KPNW; Oregon and Washington)
- Regenstrief Institute (RG; Indiana)
- University of Colorado (UCO; Colorado).
The overall, adjusted vaccine effectiveness estimates for influenza seasons from 2004-2021 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 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Adj. Overall VE (%) | 56 | 60 | 47 | 49 | 52 | 19 | 48 | 40 | 38 | 29 | 39 | Not enough data
to compute |
35 |
*2020-2021 flu vaccine effectiveness was not estimated due to low flu virus circulation during the 2020-2021 flu season.
Table. Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2022
CDC calculates vaccine effectiveness estimates through the U.S. Flu VE Network
|
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 | |
|
2020-21** |
n/a | n/a | n/a |
n/a |
|
| Chung 2022 | CA, MI, PA, TN, TX, WA, WI | 4,705 |
35 |
19, 47 |
*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.
***Interim VE estimates
† Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not been published.
‡ Number of patients used in VE calculation.
