Flu Vaccine Effectiveness Averaged 39% Across 18 Seasons
The study found 9 of 18 seasons below 40% effectiveness, but it does not estimate how well the 2026–27 vaccine is working.
By shahzad
6 min read2 readers
In this story
Key takeaways
A peer-reviewed JAMA Network Open analysis found that flu vaccine effectiveness averaged 39.3% across 18 U.S. seasons from 2004–05 through 2023–24.
Half of those seasons had estimates below 40%, but the study does not measure the 2026–27 vaccine.
CDC still recommends an annual flu vaccine for everyone 6 months and older who does not have a contraindication; the latest season-specific effectiveness is not yet known.
A meta-analysis published November 10, 2026, found that the flu vaccine's average season-specific effectiveness was about 39% across 18 U.S. seasons. That result describes historical performance, not how well the 2026–27 vaccine is working, and it does not change CDC's current vaccination recommendation.
What did the 18-season analysis find?
The study, published in JAMA Network Open, examined end-of-season vaccine-effectiveness estimates from the U.S. Influenza Vaccine Effectiveness Network. The researchers covered the 2004–05 through 2023–24 seasons and pooled 18 season-level estimates drawn from 96,335 participants.
The unweighted mean effectiveness was 39.3%, and the median was 39.5%. The researchers also calculated a pooled estimate of 41.3%, with a 95% confidence interval of 35.6% to 46.5%. Those figures are different statistical summaries of the same historical evidence, not competing estimates for the current season.
The year-to-year range was wide. Season-specific estimates ran from 10% in 2004–05 to 60% in 2010–11. Nine of the 18 seasons fell below 40%; the other nine were between 40% and 60%. None exceeded 60%.
The study also calculated a 95% prediction interval of 15.6% to 59.3% for a single future season. That interval describes the uncertainty and variation suggested by the historical data. It is not a forecast for the 2026–27 season.
One factor associated with lower effectiveness was antigenic mismatch: a difference between a vaccine strain and the influenza virus circulating in the community. The researchers reported a mismatch in at least one vaccine component in 11 of the 18 seasons. Pooled effectiveness was 36.2% in mismatch seasons, compared with 50.4% in seasons without a reported mismatch.
That comparison is an association, not proof that mismatch alone caused the difference. The authors noted that seasons can also differ in prior vaccination, population immunity and other factors.
Why does the 39% figure matter?
Analysis: The familiar 40% to 60% range is often used to describe flu vaccine performance when vaccine strains are well matched to circulating viruses. The new analysis suggests that this range does not describe every ordinary season: half of the seasons studied came in below 40%.
That context can help readers understand why flu vaccination does not guarantee that someone will avoid infection. Vaccine effectiveness is a relative reduction in the risk of the measured outcome—in this case, medically attended, laboratory-confirmed influenza—not the percentage of vaccinated people who are completely protected.
A result below 40% does not mean a vaccine has no value. The CDC's flu-prevention guidance says vaccination reduces the risk of flu and can also reduce the severity of illness in people who get vaccinated but still become sick. Preventing some infections and reducing severe outcomes can matter even when protection against infection varies.
The study also has limits. It combines season-level estimates from one U.S. network, does not include individual participant characteristics, and does not establish how well a vaccine will work in a particular person or season.
What should people do for the 2026–27 flu season?
Do not use the 39.3% historical average as a personal prediction or as this season's effectiveness estimate. The study's data stop at 2023–24.
For the current season, CDC recommends an annual flu vaccine for everyone 6 months and older who does not have a contraindication. Its 2026–27 clinical considerations say vaccination should continue after October and throughout the season while influenza viruses are circulating and unexpired vaccine is available. If you are unsure which vaccine is appropriate for your age or health circumstances, ask a healthcare professional or pharmacist.
A practical checklist:
Not vaccinated for 2026–27? Check with a pharmacy or healthcare provider about an age-appropriate vaccine.
At higher risk of serious flu complications? Ask your healthcare professional about vaccination and what to do if flu symptoms develop.
Still get sick after vaccination? Follow public-health advice and contact a clinician if you are at higher risk or your symptoms are severe.
Big Zero has also reported on another vaccine-preventable illness in its coverage of the U.S. measles outbreak. Influenza and measles are different diseases, but both show why prevention remains a public-health priority.
What is still unknown and what should readers watch next?
The JAMA analysis did not include the 2024–25 or 2025–26 seasons. CDC's broader effectiveness table now lists preliminary estimates of 56% for 2024–25 and 36% for 2025–26, drawing on multiple vaccine-effectiveness networks. Those preliminary figures are not part of the JAMA study's 18-season analysis, so they should not be treated as direct additions to its pooled result. CDC explains that estimates can vary with study design, population, outcome and season in its seasonal vaccine-effectiveness data.
The latest CDC FluView report, published November 6, 2026, covered the week ending October 31, 2026. It said flu activity was increasing in many parts of the country and was higher than typically observed at that point in the season. CDC also said it was too early to know which viruses would predominate or what implications they might have for vaccine effectiveness.
The next useful signal will be updated CDC surveillance and, later, season-specific effectiveness estimates for 2026–27. Until those data are available, the historical average should be treated as context—not a score for this year's vaccine.
FAQ
Does 39.3% effectiveness mean 60.7% of vaccinated people get the flu?
No. Vaccine effectiveness is a relative comparison of risk between vaccinated and unvaccinated groups for a defined outcome. It does not divide vaccinated people into a protected group and an unprotected group.
Is the 2026–27 flu vaccine 39% effective?
The study does not answer that question. Its analysis ends with the 2023–24 season, and CDC says it is too early to assess this season's effectiveness.
Should people still get a flu vaccine?
CDC recommends annual flu vaccination for everyone 6 months and older who does not have a contraindication. The historical study does not replace that guidance; people with questions about their circumstances should speak with a healthcare professional.
Why does flu vaccine effectiveness change from year to year?
Influenza viruses change, and the match between vaccine strains and circulating viruses can vary. Population immunity, previous vaccination and other seasonal differences can also affect measured effectiveness.
Bottom line
The study shows that flu vaccine effectiveness varies and that the 40% to 60% shorthand does not describe every season. It does not measure the 2026–27 vaccine or overturn CDC guidance, so use it to set expectations—not as a reason to skip vaccination.
