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How effective is Influenza vaccine?

Updated October 12, 2023

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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.   

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.  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. 

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. 

Between the 2004/2005 and 2022/2023 flu seasons, the influenza vaccine was reported to be less than 50 percent effective in 13 out of 19 flu seasons. In the 2014-2015 flu season, the influenza vaccine was only 19 percent effective.  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.  

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.    

Again in 2019, researchers found that vaccine effectiveness was lower against certain strains of flu (B and H3N2) for those vaccinated during two consecutive flu seasons when compared with persons vaccinated only with the current season’s shot. 

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.  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 effectiveness studies on the new formulation of FluMist have not been completed and this approval was based on data analysis provided by the manufacturer.  FluMist will remain available for use during the 2023/2024 flu season. 

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.  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. 

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.” 

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.”  

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.”  

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. 

Another 2018 published study found that the influenza virus may be spread simply by breathing (i.e., no coughing or sneezing required) and that repeated vaccination increases the amount of influenza virus released into the air. Study authors reported that “Little is known about the amount and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission,” and “We show that sneezing is rare and not important for—and that coughing is not required for—influenza virus aerosolization.”  

This study also found that a recently vaccinated individual who received the live attenuated influenza vaccine (LAIV) may potentially shed and transmit the virus. According to the study, people who were vaccinated for influenza shed more than six times more virus into the air than those who were not: 

“Self-reported vaccination for the current season was associated with a trend toward higher viral shedding in fine-aerosol samples; vaccination with both the current and previous year’s seasonal vaccines, however, was significantly associated with greater fine-aerosol shedding in unadjusted and adjusted models.

In adjusted models, we observed 6.3 times more aerosol shedding among cases with vaccination in the current and previous season compared with having no vaccination in those two seasons.”

More investigation is needed given that annual vaccination may increase aerosol viral shedding and result in reduce vaccine effectiveness. Study authors reported: 

“The association of current and prior year vaccination with increased shedding of influenza A might lead one to speculate that certain types of prior immunity promote lung inflammation, airway closure, and aerosol generation. This first observation of the phenomenon needs confirmation. If confirmed, this observation, together with recent literature suggesting reduced protection with annual vaccination, would have implications for influenza vaccination recommendations and policies.”

It is likely possible that after vaccination, a person may become a contagious silent carrier of influenza. A person with symptoms of influenza (i.e. body aches, fever, cough) would likely stay at home. However, a vaccinated individual who is silently contagious would continue to go to public places such as work, school, and stores, and be unaware that they are spreading the virus, even with regular breathing.

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.  

A 2020 study conducted by the U.S. Department of Defense found that receipt of the influenza vaccine increases a person’s risk to coronavirus by 36 percent. 

The effectiveness of the 2020-2021 flu vaccine was not estimated because public health officials reported that circulation of flu viruses was low during this flu season. The preliminary effectiveness estimate of the flu vaccine during the 2021/2022 flu season was reported to be 36 percent. 

In 2022, the CDC preferentially recommended the high dose quadrivalent influenza vaccine (HD-IIV4), the quadrivalent recombinant influenza vaccine (RIV4), and the quadrivalent adjuvanted inactivated flu vaccine (aIIV4) for persons 65 years of age and older. The decision to preferentially recommend these vaccines in seniors was primarily based on studies of the trivalent high-dose flu vaccine, a vaccine no longer available for use in the U.S. According to the CDC, certain studies reported a benefit for these vaccines when compared to the standard inactivated flu shot, but the studies were few in number and based on diagnosis codes and not laboratory confirmed flu samples. Public health officials, however, acknowledged that not all studies showed a benefit to using these vaccines in individuals 65 years and older. Additionally, data on the use of the newer quadrivalent flu vaccines in comparison to the standard inactivated flu shot was not available. 

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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