#17,932
Uptake of the flu seasonal flu vaccine, which had been rising over the past decade, appears to have peaked in 2020-2021 (see chart above) and has since experienced a slow decline. Some of this likely due to pandemic fatigue, rampant anti-vaccine rhetoric on social media, and a significant amount of `breakthrough' infection among the vaccinated.
Up until about a dozen years ago, the oft repeated mantra from the CDC was: ". . . for healthy adults under the age of 65, in years when the vaccine is a good match to circulating strains, effectiveness ranges from 70%-90%."
In 2011 the CDC lowered expectations somewhat, stating in a FAQ: `. . . recent RCTs of inactivated influenza vaccine among adults under 65 years of age have estimated 50-70% vaccine efficacy during seasons in which the vaccines' influenza A components were well matched to circulating influenza A viruses.'Based on more recent studies, the CDC's revised assessment of flu vaccine VE now reads:
While vaccine effectiveness (VE) can vary, recent studies show that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well-matched to those used to make flu vaccines. In general, current flu vaccines tend to work better against influenza B and influenza A(H1N1) viruses and offer less protection against influenza A(H3N2) viruses.
As the following CDC chart shows, flu vaccine effectiveness can vary greatly from one year to the next.
The measure of a vaccine's `effectiveness' can vary between studies, with some looking at outpatient visits or other medically attended illnesses, and others looking at more serious outcomes like hospitalizations or deaths.
In other words, a vaccine doesn't necessarily have to prevent symptomatic infection in order to be considered effective.
Last September, in a bid to change the public's perception of the benefits of the flu vaccine (see CDC Hopes to `Reset' Flu Vaccine Expectations With New Campaign), the CDC began emphasizing the vaccines ability to reduce the severity of flu infection, over it preventing infection entirely.
Yesterday the CDC's MMWR published their interim flu vaccine effectiveness report for the 2023-2024 flu season. While the final numbers won't be available until next summer, this year's vaccine appears to provide moderate protection against serious (i.e. medically attended) influenza virus infection.
Interim Estimates of 2023–24 Seasonal Influenza Vaccine Effectiveness — United States
Weekly / February 29, 2024 / 73(8);168–174
Aaron M. Frutos, PhD1,2; Ashley M. Price, MPH1; Elizabeth Harker, MPH1; Emily L. Reeves, MPH1; Haris M. Ahmad, MPH1; Vel Murugan, PhD3; Emily T. Martin, PhD4; Stacey House, MD, PhD5; Elie A. Saade, MD6; Richard K. Zimmerman, MD7; Manjusha Gaglani, MBBS8,9,10; Karen J. Wernli, PhD11,12; Emmanuel B. Walter, MD13; Marian G. Michaels, MD7,14; Mary A. Staat, MD15,16; Geoffrey A. Weinberg, MD17; Rangaraj Selvarangan, PhD18,19; Julie A. Boom, MD20,21; Eileen J. Klein, MD22; Natasha B. Halasa, MD23; Adit A. Ginde, MD24; Kevin W. Gibbs, MD25; Yuwei Zhu, MD23; Wesley H. Self, MD23; Sara Y. Tartof, PhD12,26; Nicola P. Klein, MD, PhD27; Kristin Dascomb, MD, PhD28; Malini B. DeSilva, MD29; Zachary A. Weber, PhD30; Duck-Hye Yang, PhD30; Sarah W. Ball, ScD30; Diya Surie, MD31; Jennifer DeCuir, MD, PhD31; Fatimah S. Dawood, MD31; Heidi L. Moline, MD31; Ariana P. Toepfer, MPH31; Benjamin R. Clopper, MPH31; Ruth Link-Gelles, PhD31; Amanda B. Payne, PhD31; Jessie R. Chung, MPH1; Brendan Flannery, PhD1; Nathaniel M. Lewis, PhD1; Samantha M. Olson, MPH1; Katherine Adams, MPH1; Mark W. Tenforde, MD, PhD1; Shikha Garg, MD1; Lisa A. Grohskopf, MD1; Carrie Reed, DSc1; Sascha Ellington, PhD1; CDC Influenza Vaccine Effectiveness Collaborators (VIEW AUTHOR AFFILIATIONS)View suggested citation
Summary
What is already known about this topic?
Influenza vaccines are reviewed biannually and updated as needed. In the United States, annual influenza vaccination is currently recommended for all persons aged ≥6 months.
What is added by this report?
Analysis of data from four vaccine effectiveness (VE) networks estimated interim pediatric influenza VE was 59%–67% in outpatient settings and 52%–61% against influenza-associated hospitalization. Interim adult influenza VE was 33%–49% in outpatient settings and 41%–44% against influenza-associated hospitalization.
What are the implications for public health practice?
These findings indicate that the 2023–24 seasonal influenza vaccine is effective at reducing the risk of influenza-associated outpatient visits and hospitalization. All eligible persons aged ≥6 months should receive annual influenza vaccination.
Abstract
In the United States, annual influenza vaccination is recommended for all persons aged ≥6 months. Using data from four vaccine effectiveness (VE) networks during the 2023-24 influenza season, interim influenza VE was estimated among patients aged ≥6 months with acute respiratory illness-associated medical encounters using a test-negative case-control study design.
Among children and adolescents aged 6 months–17 years, VE against influenza-associated outpatient visits ranged from 59% to 67% and against influenza-associated hospitalization ranged from 52% to 61%.
Among adults aged ≥18 years, VE against influenza-associated outpatient visits ranged from 33% to 49% and against hospitalization from 41% to 44%.
VE against influenza A ranged from 46% to 59% for children and adolescents and from 27% to 46% for adults across settings. VE against influenza B ranged from 64% to 89% for pediatric patients in outpatient settings and from 60% to 78% for all adults across settings.
These findings demonstrate that the 2023–24 seasonal influenza vaccine is effective at reducing the risk for medically attended influenza virus infection. CDC recommends that all persons aged ≥6 months who have not yet been vaccinated this season get vaccinated while influenza circulates locally.
The CDC also released the following statement yesterday:
This Season’s Flu Vaccines Reduced Flu Medical Visits and Hospitalizations Across All Ages
February 29, 2024 — Interim flu vaccine effectiveness estimates show that so far during the 2023-2024 flu season, flu vaccines have worked, substantially reducing the risk of flu-related medical visits and hospitalizations across all age groups, with some estimates higher than have been previously observed, even during well-matched seasons. Additionally, the data show that flu vaccination is working against both the influenza A and B viruses that have spread most commonly so far this season. CDC continues to recommend getting a yearly flu vaccine as long as flu viruses are spreading in the community.
Specifically, flu vaccination has reduced the risk of flu medical visits by about two-thirds and flu-related hospitalization by about half for vaccinated children and flu medical visits by half and hospitalization by about 40% for vaccinated adults. This is the first time that interim flu vaccine effectiveness estimates for children and adults from four major networks are available at the same time, providing extensive data on how well flu vaccines are working this season across influenza virus types, by age, and across a spectrum of illness severity.
A side benefit we've looked at repeatedly over the years (see here, here, and here) has been the flu vaccine's apparent impact on reducing heart attacks, strokes, and possibly even dementia later in life.
Last November, in Nature: Influenza Vaccination & Major Cardiovascular Risk: a Systematic Review & Meta-Analysis. the authors wrote that they found:
- `. . . compelling evidence that influenza vaccination is associated with a decreased risk of major cardiovascular events'.
- Specifically, the authors found that ". . . patients who received the influenza vaccine experienced a remarkable risk reduction of over 20% in cardiovascular death". This particular analysis focused on patients with an existing diagnosis of CVDs (Cardiovascular diseases).
While a VE of less than 50% may be disappointing, a fifty percent reduction in severe flu is still significant - and when you add in the reduced risk of cardiovascular events - the flu shot undoubtedly saves a lot of lives every year.
Which is why I get the flu shot every year, and urge others to do the same.
