Influenza Vaccines Only 42 Percent Effective in Adults This Year

The influenza vaccines currently being administered in the United States are estimated by the U.S. Centers for Disease Control and Prevention (CDC) to be 42 percent effective in adults. This is an interim estimate for the 2023-2024 “flu season” published in the agency’s Morbidity and Mortality Weekly Report (MMWR) on Feb. 29, 2024.^{1 2 3}

By “effective,” the CDC is referring to reducing the risk of influenza-related hospitalizations or “medically attended influenza virus infection,” not actually preventing influenza infections—notably influenza A (H1N1) and B (Victoria lineage).¹ In its report, the agency stated:

These findings indicate that the 2023–24 seasonal influenza vaccine is effective at reducing the risk of influenza-associated outpatient visits and hospitalization.¹

Less Than 25 Percent of Suspected ILI Cases are Influenza

Most influenza-like-illness (ILI) which occurs during the “flu season” is not actually caused by  type A or type B influenza. Studies show that less than 25 percent of suspected ILI cases turn out to be influenza after lab testing. There are many other respiratory infections that look like influenza but are caused by other types of viruses and bacteria.⁴

According to Sascha Ellington, PhD, leader of the CDC’s influenza prevention and control team, the 42 percent influenza vaccine effectiveness rate falls within the “range” that the CDC “typically” sees when the vaccine is a “good match with the [influenza] viruses that are circulating.” This season’s rate thus far lags behind the estimated 54 percent effectiveness rate for the flu shot in 2022-2023, but it is better than the 36 percent rate for 2021-2022.^{1 2 3 5 6}

Influenza Vaccine Effectiveness Rate Under 50 Percent for Most Years

The 42 percent influenza vaccine effectiveness rate this year is roughly comparable to most annual rates during the past two decades. Since the 2004-2005 flu season, the CDC has estimated the flu shot to be more than 50 percent effective only five times—52 percent in 2006-2007, 56 percent in 2009-2010, 60 percent in 2010-2011, 52 percent in 2013-2014 and 54 percent in 2022-2023. In other words, the influenza vaccine the CDC recommends children and adults get every year has been more than 50 percent ineffective in preventing type A or type B influenza infections nearly three out of every four years.^{6 7}

Influenza activity in the U.S. tends to peak in February and can last as late as May, although the CDC has noted that since the start of the COVID pandemic, the “timing and duration of influenza infection activity has been less predictable.” The CDC usually provides an estimate in June for the effectiveness of the influenza vaccine for the second half of the “flu season,” as well as an effectiveness estimate for the entire season.^{3 8}

Nine Licensed Influenza Vaccines and All Contain Allergens

There are nine influenza vaccines that are licensed by the U.S. Food and Drug Administration (FDA) for distribution during the 2023-2024 flu season, including the Afluria Quadrivalent (produced by Sequris), Fluad Quadrivalent (Sequris), Fluarix Quadrivalent (GSK), Flublok Quadrivalent (Sanofi Pasteur), Flucelvax Quadrivalent (Sanofi Pasteur), FluLaval Quadrivalent (Sanofi Pasteur), FluMist Quadravalent (MedImmune), Fluzone Quadrivalent (Sanofi Pasteur) and Fluzone High-Dose Quadrivalent (Sanofi Pasteur).⁹

Each of these influenza vaccines contains at least one ingredient that is a potential allergen.

• Alfuria contains the mercury-based preservative thimerosal (multiple-dose vials), egg protein, neomycin and polymyxin.
• Fluad contains egg protein, kanamycin, neomycin and polysorbate 80.
• Fluarix contains egg protein, gentamicin and polysorbate 80.
• Flublok contains polysorbate 20.
• Flucelvax contains thimerosal (multiple-dose vials) and polysorbate 80.
• FluLaval contains egg protein and polysorbate 80.
• FluMist contains egg protein, gentamicin, pork gelatin and monosodium glutamate (MSG).
• Fluzone contains egg protein.
• Fluzone High-Dose contains thimerosal (multiple-dose vials) and egg protein.⁹

If you would like to receive an e-mail notice of the most recent articles published in The Vaccine Reaction each week, click here.