Despite the ever-growing number of influenza vaccines available in the U.S. market, the effectiveness of influenza vaccines has not improved very much over the years. In 2003-2004, the CDC increased research efforts to determine just how well seasonal flu vaccine works in preventing cases of influenza in vaccinated persons.
Since 2004, the seasonal influenza vaccine has failed to prevent influenza in vaccinated persons more than half the time, demonstrating a low of 10 percent effectiveness in 2004/2005 to a high of 60 percent effectiveness in 2010/2011. The average effectiveness of influenza vaccines over the past 20 flu seasons has been less than 40 percent.
The Cochrane Collaboration’s 2014 review of the medical literature on influenza vaccine noted bias in the publication of influenza vaccine research on effectiveness and safety:
“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.”
The 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.
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.
Only trivalent influenza vaccines containing two influenza A viruses and one influenza B virus will be available in the U.S. for the 2024/2025 flu season. Despite a lack of safety or efficacy studies, the CDC states that the influenza vaccine may be administered at the same time as all authorized COVID-19 vaccines.
