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 and confirm the value of flu vaccination as a public health intervention. Study results of vaccine effectiveness can vary based on study design, outcome(s) measured, population studied and the season in which the flu vaccine was studied.
CDC conducts studies to measure the benefits of seasonal flu vaccination each flu season to help determine how well flu vaccines are working.
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 three networks to estimate vaccine effectiveness: the U.S. Flu VE Network, the Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN), and the New Vaccine Surveillance Network (NVSN). The U.S. Flu VE Network currently consists of five 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 infections using a highly accurate lab test called PCR to test for the flu virus in respiratory specimens. HAIVEN and NVSN currently consists of four and seven study sites, respectively, in the U.S. that measure the flu vaccine’s effectiveness at preventing hospitalizations due to laboratory-confirmed influenza. HAIVEN captures adults >17 years of age and NVSN captures influenza hospitalizations among children 6 months to 17 years of age. At both networks, 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.
The overall, adjusted vaccine effectiveness estimates for influenza seasons from 2004-2018 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.)
Figure. Effectiveness of Seasonal Flu Vaccines from the 2009-2019 Flu Seasons
The vaccine effectiveness estimated included in the chart and table below are vaccine effectiveness estimates from the U.S. VE Network. These estimates do not include vaccine effectiveness estimates from HAIVEN at this time.
Table. Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2019
CDC calculates vaccine effectiveness estimates through the U.S. 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 |
1914 |
37 |
22, 49 |
|
|
2008-09**† |
Unpublished |
WI, MI, NY, TN |
6713 |
41 |
30, 50 |
|
2009-10** |
WI, MI, NY, TN |
6757 |
56 |
23, 75 |
|
|
2010-11** |
WI, MI, NY, TN |
4757 |
60 |
53, 66 |
|
|
WI, MI, PA, TX, WA |
4771 |
47 |
36, 56 |
||
|
WI, MI, PA, TX, WA |
6452 |
49 |
43, 55 |
||
|
WI, MI, PA, TX, WA |
5999 |
52 |
44, 59 |
||
|
WI, MI, PA, TX, WA |
9311 |
19 |
10, 27 |
||
|
WI, MI, PA, TX, WA |
6879 |
48 |
41, 55 |
||
| Flannery 2018external icon | WI, MI, PA, TX, WA | 7410 |
40 |
32, 46 | |
| Rolfes 2019external icon | WI, MI, PA, TX, WA | 8,436 |
38 |
31, 43 | |
| Flannery 2019external icon | WI, MI, PA, TX, WA | 10,041 |
29 |
21, 35 | |
| Flannery, 2020 | WI, MI, PA, TX, WA | 8,845 |
39 |
32, 44 |
** From 2004-05 through 2010-11, the Flu VE Network also enrolled inpatients.
† Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not yet been published.
‡ Number of patients used in VE calculation.
