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.

Info on Flu Vaccine Effectiveness

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 2009-2018 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.

*Vaccine effectiveness estimates for 2018-2019 were presented to ACIP on June 27, 2019pdf icon

Table. Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2018

CDC calculates vaccine effectiveness estimates through the U.S. VE Network

Table. Adjusted vaccine effectiveness estimates for influenza seasons from 2004-2018CDC 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**

Belongia 2009external icon

WI

762

10

-36, 40

2005-06**

Belongia 2009external icon

WI

346

21

-52, 59

2006-07**

Belongia 2009external icon

WI

871

52

22, 70

2007-08**

Belongia 2011external icon

WI

1914

37

22, 49

2008-09**†

Unpublished

WI, MI, NY, TN

6713

41

30, 50

2009-10**

Griffin 2011external icon

WI, MI, NY, TN

6757

56

23, 75

2010-11**

Treanor 2011external icon

WI, MI, NY, TN

4757

60

53, 66

2011-12

Ohmit 2014external icon

WI, MI, PA, TX, WA

4771

47

36, 56

2012-13

McLean 2014external icon

WI, MI, PA, TX, WA

6452

49

43, 55

2013-14

Gaglani 2016external icon

WI, MI, PA, TX, WA

5999

52

44, 59

2014-15

Zimmerman 2016external icon

WI, MI, PA, TX, WA

9311

19

10, 27

2015-16

Jackson 2017external icon

WI, MI, PA, TX, WA

6879

48

41, 55

2016-17

Flannery 2018external icon WI, MI, PA, TX, WA 7410

40

32, 46

2017-18

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

38

31, 43

2018-19

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

29*

21, 35

** 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.

Supporting Research

Belongia EA, Kieke BA, Donahue JG, et al. Effectiveness of inactivated influenza vaccines varied substantially with antigenic match from the 2004-2005 season to the 2006-2007 season. J Infect Dis. 2009 Jan 15;199(2):159-67. doi:10.1086/595861. PubMed PMID: 19086915.external icon

Belongia EA, Kieke BA, Donahue JG,et al. Influenza vaccine effectiveness in Wisconsin during the 2007-08 season: comparison of interim and final results. Vaccine. 2011 Sep 2;29(38):6558-63. doi: 10.1016/j.vaccine.2011.07.002. Epub 2011 Jul 19. PubMed PMID: 21767593.external icon

Ferdinands JM, Gaglani M, Martin ET, Middleton D, Monto AS, Murthy K, Silveira FP, Talbot HK, Zimmerman R, Alyanak E, Strickland C, Spencer S, Fry AM; HAIVEN Study Investigators. Prevention of influenza hospitalization among adults in the US, 2015-16: Results from the US Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN). J Infect Dis. 2018 Dec 14. doi:10.1093/infdis/jiy723. [Epub ahead of print] PubMed PMID: 30561689.external iconexternal icon

Flannery B, Clippard J, Zimmerman RK, Norwalk MP, Jackson ML, Jackson LA, Monto AS, Petrie JG, McLean HQ, Belongia EA, Gaglani M, Berman L, Foust A, Sessions W, Thaker SN, Spencer S, Fry AM. Early Estimates of Seasonal Influenza Vaccine Effectiveness – United States, January 2015. MMWR Morb Mortal Wkly Rep. 2015 Jan 13;64(1);10-15.(https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6401a4.htm)

Griffin MR, Monto AS, Belongia EA, et al. Effectiveness of non-adjuvanted pandemic influenza A vaccines for preventing pandemic influenza acute respiratory illness visits in 4 U.S. communities. PLoS One. 2011;6(8):e23085. doi: 10.1371/journal.pone.0023085. Epub 2011 Aug 12. PubMed PMID: 21857999.external icon

McLean HQ, Thompson MG, Sundaram ME, Kieke BA, Gaglani M, Murthy K, Piedra PA, Zimmerman RK, Nowalk MP, Raviotta JM, Jackson ML, Jackson L, Ohmit SE, Petrie JG, Monto AS, Meece JK, Thaker SN, Clippard JR, Spencer SM, Fry AM, Belongia EA. Influenza Vaccine Effectiveness in the United States During 2012-2013: Variable Protection by Age and Virus Type. J Infect Dis. 2014 Nov 18. pii: jiu647. [Epub ahead of print] PubMed PMID: 25406334.external icon

Ohmit SE, Thompson MG, Petrie JG, et al. Influenza vaccine effectiveness in the 2011-2012 season: protection against each circulating virus and the effect of prior vaccination on estimates. Clin Infect Dis. 2014 Feb;58(3):319-27. doi: 10.1093/cid/cit736. Epub 2013 Nov 13.external icon

Treanor JJ, Talbot HK, Ohmit SE, et al. Effectiveness of seasonal influenza vaccines in the United States during a season with circulation of all three vaccine strains. CID 2012; 55(7):951-959. Epub 2012 Jul 25. PubMed PMID: 22843783.external icon

Jackson ML, Chung JR, Jackson LA, Phillips CH, Benoit J, Monto AS, Martin ET, Belongia EA, McLean HQ, Gaglani M, Murthy K, Zimmerman R, Nowalk MP, Fry AM, Flannery B. N Engl J Med. 2017 Aug 10;377(6):534-543. doi: 10.1056/NEJMoa1700153. PMID: 2879286external icon