How Effective is Influenza Vaccine?
Like all vaccines, the influenza vaccine only gives a temporary artificial immunity and, in the case of flu shots, that temporary artificial immunity is confined to the influenza virus strains contained in the vaccine. The only way to get more complete and longer lasting immunity to a strain of type A or B influenza is to recover from the illness. Natural immunity to a particular strain of influenza can be protective against severe illness symptoms if that strain or a closely related strain circulates in the future.
The flu vaccine, however, only provides temporary immunity to selected type A and B strains and those strains may or may not be prevalent each year. Since 2010, U.S. public health officials have directed doctors to give every American over six months old a flu shot every year, whether or not they are healthy or at high risk for influenza complications.1 2
Every year, public health officials at the World Health Organization (WHO) and U.S. Centers for Disease Control and Prevention (CDC) try to guess which three or four influenza strains are most likely to be circulating and causing illness in the U.S. the following year to determine which strains will be included in vaccine manufactured for the upcoming flu season. Despite elaborate influenza monitoring systems by the WHO and U.S.3 and scientists worldwide to select the strains to be targeted in the following year’s flu vaccine, studies by the U.S. Flu Vaccine Effectiveness Network have shown that, since the 2004/2005 flu season, the vaccine has never been more than 60 percent effective.4
A 2005 study on the impact of flu vaccination on mortality in adults 65 years of age and older determined that an increase in flu vaccine coverage after 1980 in senior adults had no impact on reducing mortality rates in any age group.5
Between the 2004/2005 and 2017/2018 flu seasons, the influenza vaccine was reported to be less than 50 percent effective in 10 out of 14 flu seasons. In the 2014-2015 flu season, the influenza vaccine was only 19 percent effective.6 A 2011 review of existing research determined that the inactivated influenza vaccine had a pooled efficacy of 59 percent for adults 18 to 65 years of age for 8 out of 12 seasons. Similar data for inactivated influenza vaccine for adults over 65 years of age and children between 2 and 17 years of age was reported to be lacking. This same review found that live-attenuated influenza vaccine (LAIV), had a pooled efficacy of 83 percent in 9 of the 12 seasons analyzed for children aged six months to seven years. Similar data for LAIV efficacy for children aged 8 to 17 years was lacking and requires additional study.7
A Canadian study published in 2015 found that the effectiveness of the flu vaccine decreased when a person received the shot two years in a row. Because of these negative findings which implied that the vaccine increased a person’s susceptibility to influenza, study authors recommended further research be completed. 8 9
In 2016, CDC officials advised against use of the live attenuated nasal influenza vaccine based on effectiveness data collected between 2013 and 2016 which showed LAIV vaccine to be only 3 percent effective against any influenza virus among children 2 to 17 years of age.10 However, in February 2018, CDC officials voted to approve that a new formulation of LAIV be added as an option, when appropriate, for the 2018-2019 flu season. Vaccine efficacy studies on whether this new formulation of FluMist have not been completed and this approval was based on data analysis provided by the manufacturer.11 FluMist will remain available for use during the 2019/2020 flu season.12
In the fall of 2017, scientists reported that the H3N2 influenza virus strain mutated during the 2014-2015 season and, although the 2016-2017 seasonal flu vaccine was updated to include the mutated strain, another mutation occurred that season in manufacturing labs when the H3N2 strain was grown in chicken eggs to produce the vaccine. There is emerging evidence that H3N2 influenza viruses cannot be grown in chicken eggs without adaptive mutations occurring.13 The 2017/2018 influenza vaccine reportedly was only 10 percent effective in the southern hemisphere because the mutated H3N2 strain was the one making most people sick, but it was not included the vaccine.14
In 2018, the Cochrane Collaboration published a review of medical literature on the effects of the influenza vaccination in the elderly and concluded that:
“The available evidence relating to complications is of poor quality, insufficient, or old and provides no clear guidance for public health regarding the safety, efficacy, or effectiveness of influenza vaccines for people aged 65 years or older.”15
The Cochrane Collaboration’s 2014 review of the medical literature on influenza vaccine had previously noted bias in the publication of influenza vaccine research on effectiveness and safety and reported that:
“An earlier review of 274 influenza vaccine studies in all age groups (including most of the studies in this review) showed an inverse relationship between risk of bias and the direction of study conclusions. Conclusions favourable to the use of influenza vaccines were associated with a higher risk of bias. In these studies, the authors made claims and drew conclusions that were unsupported by the data they presented. In addition, industry-funded studies are more likely to have favourable conclusions, to be published in significantly higher-impact factor journals and to have higher citation rates than non-industry-funded studies. This difference is not explained by either their size or methodological quality (Jefferson 2009a). Any interpretation of the body of evidence in this review should be made with these findings in mind.” 16
This 2014 Cochrane review also concluded that recommendations for routine use of influenza vaccine as a routine public health measure was not supported by the published evidence base and stated:
“The results of this review provide no evidence for the utilisation of vaccination against influenza in healthy adults as a routine public health measure. As healthy adults have a low risk of complications due to respiratory disease, the use of the vaccine may only be advised as an individual protective measure.” 17
The 2018 Cochrane review of influenza vaccination in healthy adults found that the flu vaccine may only have a modest impact on reducing the number of cases of influenza and influenza-like illness, but the data was insufficient to determine whether vaccination had any impact on working days lost or on reducing serious complications of the flu during influenza season.18
In June 2019, the CDC reported that the 2018-2019 seasonal flu vaccine offered no protection against the circulating H3N2 flu strain which emerged in late February and overall flu vaccine effectiveness was reported at only 29 percent. Among adults hospitalized for the flu, the vaccine’s effectiveness against the H3N2 strain was reported at -43 percent. A negative percentage indicates that persons who were vaccinated with the 2018-2019 seasonal flu vaccine were more likely to be hospitalized for flu, than those who were not. 19
IMPORTANT NOTE: NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.
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1 CDC Prevention and Control of Influenza with Vaccines – Recommendations of the Immunization Practices Advisory Committee (ACIP), 2010 MMWR Aug. 6, 2010. 59(RR08);1-62
2 Mayo Clinic Flu shot: Your best bet for avoiding influenza. Sept. 12, 2019
3 World Health Organization(WHO) Global Influenza Surveillance and Response System (GISRS). 2019.
4 CDC CDC Seasonal Flu Vaccine Effectiveness Studies Sept. 4, 2019
5 Simonsen L, Reichert TA, Viboud C, et al. Impact of Influenza Vaccination on Seasonal Mortality in the US Elderly Population Arch Intern Med 2005;165(3):265-272.
6 CDC CDC Seasonal Flu Vaccine Effectiveness Studies Sept. 4, 2019
7 Osterholm MT, Kelley NS et al. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis 2012;12:36-44.
8 Skowronski DM, Chambers C et al. Interim estimates of 2014/15 vaccine effectiveness against influenza A(H3N2) from Canada's Sentinel Physician Surveillance Network, January 2015. Euro Surveill 2015 Jan 29;20(4).
9 Yourex-West H, McIntosh S, Canadian study finds flu shot could increase risk of getting sick. Global News. Jan. 30, 2015.
10 CDC. ACIP votes down use of LAIV for 2016-2017 flu season. June 22, 2016
11 Schnirring, L. CDC vaccine panel brings back FluMist for 2018-19 season. CIDRAP Feb. 21, 2018
12 CDC Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2019–20 Influenza Season MMWR August 23, 2019; 68(3);1–21
13 Perelman School of Medicine. H3N2 mutation in last year’s flu vaccine responsible for lowered efficacy. Medical Xpress Nov. 6, 2017.
14 Dwyer D. Q&A: Why the flu vaccine isn’t always effective. Boston Globe Dec. 7, 2017.
15 Demicheli V, Jefferson T, et al. Vaccines for preventing influenza in the elderly. Cochrane Database of Systematic Reviews. Feb. 1, 2018
16 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews Mar. 13, 2014.
18 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews. Feb 1, 2018
19 Flannery B Preliminary Estimates of 2018–19 Seasonal Influenza Vaccine Effectiveness against Medically Attended Influenza from three U.S. Networks Advisory Committee on Immunization Practices meeting June 27, 2019