Recent RWE Data Show Cell-Based Influenza Vaccines Offer 20% Greater Protection in the Prevention of Test-Confirmed Influenza in Pediatric and Adult Populations Relative to Standard Egg-Based Influenza Vaccines

Rhea-AI Summary

CSLLY (CSL Seqirus) announced real‑world evidence from the U.S. 2023/24 season showing cell‑based quadrivalent influenza vaccines had an estimated 19.8% relative vaccine effectiveness (rVE) versus standard egg‑based quadrivalent vaccines to prevent test‑confirmed influenza in ages six months to 64 years (106,779 vaccinated and tested patients; 95% CI 15.7–23.8%).

Sensitivity analyses and subgroup results (pediatric, adult, outpatient, high‑risk) were consistent. A modeled estimate suggests 14,930 additional hospitalizations could have been prevented if all vaccinated 6 months–64 years received cell‑based vaccines. Data were published and presented at IDWeek Oct 19–22, 2025.

Positive

• rVE 19.8% vs egg‑based vaccines (95% CI 15.7–23.8%)
• Analysis of 106,779 vaccinated, influenza‑tested patients
• 14,930 estimated additional hospitalizations averted in 2023/24 model
• Consistent protection across pediatric, adult, outpatient, high‑risk subgroups
• Findings represent fifth season of retrospective studies showing benefit

Negative

• None.

• Real-world evidence (RWE) data estimate 19.8% relative vaccine effectiveness (rVE) of cell-based quadrivalent influenza vaccines (QIVc) versus standard egg-based quadrivalent vaccines (QIVe) in the prevention of test-confirmed influenza during the 2023/24 U.S. season for populations six months to 64 years. ¹
• Consistent rVE results were observed for pediatric, adult, outpatient and high-risk subgroups. ¹
• This latest RWE represents the fifth season of large retrospective test-negative studies showing greater protection of cell-based influenza vaccines relative to standard egg-based influenza vaccines, elevating the standard of care for influenza prevention for pediatrics six months and older and adults up to and including 64 years. ¹

SUMMIT, N.J., Oct. 21, 2025 /PRNewswire/ -- CSL Seqirus, a business of CSL (ASX: CSL), announced recent data showing that cell-based influenza vaccines offer greater protection in preventing test-confirmed influenza compared to standard egg-based vaccines in a study among individuals six months to 64 years of age.¹

These data were published in Infectious Diseases and Therapy earlier this month and will be presented at the IDWeek annual meeting taking place in Atlanta, Georgia from October 19-22, 2025.

In collaboration with a leading infectious disease epidemiologist, CSL Seqirus performed a retrospective test-negative study of the relative vaccine effectiveness (rVE) of cell-based influenza vaccines relative to standard egg-based influenza vaccines in individuals six months to 64 years of age during the U.S. 2023/24 influenza season.¹

The real-world evidence (RWE) study analyzed data from 106,779 vaccinated and influenza-tested patients, with analyses performed in the full population, as well as pediatric (six months to 17 years), adult (18 to 64 years), outpatient and high-risk subgroups. rVE was estimated using linked data from electronic health records, medical and pharmacy claims and laboratory tests in the U.S.¹

Findings from the full population showed greater protection of cell-based influenza vaccines compared to standard egg-based influenza vaccines in the prevention of test-confirmed influenza, with estimated rVE of 19.8% (95% CI; 15.7 – 23.8%). Improved protection versus standard-dose egg vaccines was consistently observed across pediatric and adult age subgroups as well as for at-risk populations. Sensitivity analyses supported the robustness of the analysis.¹

"These data from the 2023/24 season show the clinical benefit of cell-based influenza vaccines over standard egg-based vaccines, and they continue to be relevant as we enter this year's influenza season," said Gregg Sylvester, MD, Chief Health Officer and Head of Medical Affairs, CSL Seqirus. "Last year, we saw the worst flu season in 15 years and the highest number of pediatric deaths since the 2009 pandemic.^2,3,4 Behind every statistic is a person, a family's loved one, and we must continue to consider real-world evidence as an educational tool to encourage safe and effective vaccination for all eligible people six months and older."

According to an estimation using a compartmental influenza burden averted model, if all vaccinated individuals aged six months to 64 years during the 2023/24 influenza season received cell-based influenza vaccines over standard egg-based, an additional estimated 14,930 hospitalizations would have been prevented.¹

"Given the ever-changing nature of the influenza virus, RWE studies allow us to assess influenza vaccine effectiveness each season, which we can't reasonably do using clinical trials. This study adds a fifth season showing cell-based influenza vaccines over greater protection against medically-attended disease than standard egg-based influenza vaccines," said Sheena Sullivan, MPH, PhD, Associate Professor, Monash University, Melbourne, Australia.

Cell-based technology provides an exact antigenic match to the vaccine strain selected by the World Health Organization.^5,6

"At CSL Seqirus, we're continually working to improve vaccine effectiveness and help safeguard the public. These RWE data suggest that the greater protection of cell-based vaccines compared to standard egg-based alternatives may allow healthcare providers to further reduce the burden of influenza on their patients," said Stefan Merlo, Vice President of Commercial Operations, North America, CSL Seqirus.

The CSL Seqirus Holly Springs, North Carolina facility is the largest cell-based influenza vaccine manufacturing facility in the world, with capacity for formulation, fill and finish manufacturing of seasonal influenza vaccines for global markets.

About the Study
Multiple real-world data sources were linked, including electronic health records, medical and pharmacy claims, and laboratory tests in the United States. A retrospective test-negative design (TND) was applied among individuals aged six months to 64 years vaccinated with either QIVc or QIVe in 2023/24 who had an influenza test obtained within seven days of a documented acute respiratory or febrile illness.¹ rVE was estimated by comparing the odds of testing positive for influenza between QIVc and QIVe recipients. Relative vaccine effectiveness measures vaccine effectiveness by comparing people who have received one vaccine type or regimen to those who received a different vaccine type or regimen.⁷ Analyses were performed in the full population and subgroups: pediatric (six months to 17 years), adult (18 to 64 years), outpatient and high-risk.¹

Study Limitations
The above study featuring RWE was subject to the typical limitations associated with retrospective analyses of RWE. Observational studies have limitations including the potential for selection bias and residual confounding. TND studies offer advantages over other observational studies and can provide reliable inferences on vaccine effectiveness in the real-world setting; however, they are not as definitive as Randomized Controlled Trials (RCTs) in establishing causality and are a complementary approach to clinical trials to assess seasonal influenza vaccine effectiveness. We recommend interpreting these results within a comprehensive evidence framework that includes various study designs for a well-rounded perspective on vaccine efficacy/effectiveness.

About Seasonal Influenza 
Influenza is a common, contagious seasonal respiratory disease that may cause severe illness and life-threatening complications in some people.⁸ Influenza can lead to clinical symptoms varying from mild to moderate respiratory illness to severe complications, hospitalization and in some cases, death.⁸ Because transmission of influenza viruses to others may occur one day before symptoms develop and up to five to seven days after becoming sick, the disease can be easily transmitted to others.⁹ Preliminary estimates from the Centers for Disease Control and Prevention (CDC) report that during the 2024/25 influenza season, there were an estimated 610,000-1,300,000 influenza-related hospitalizations in the U.S.³ The CDC recommends annual vaccination for individuals aged six months and older, who do not have any contraindications.¹⁰ Since it takes about two weeks after vaccination for antibodies to develop in the body that help protect against influenza virus infection, it is recommended that people get vaccinated before influenza begins spreading in their community.¹⁰ The CDC recommends that most people get vaccinated by the end of October.¹⁰

About CSL Seqirus
CSL Seqirus is part of CSL (ASX: CSL). As one of the largest influenza vaccine providers in the world, CSL Seqirus is a major contributor to the prevention of influenza globally and a transcontinental partner in pandemic preparedness. With state-of-the-art production facilities in the U.S., the U.K. and Australia, CSL Seqirus utilizes egg, cell and adjuvant technologies to offer a broad portfolio of differentiated influenza vaccines in more than 20 countries around the world.

For more information about CSL Seqirus, visit CSL.com.

About CSL
CSL (ASX: CSL; USOTC: CSLLY) is a global biotechnology company with a dynamic portfolio of lifesaving medicines, including those that treat haemophilia and immune deficiencies, vaccines to prevent influenza, and therapies in iron deficiency and nephrology. Since our start in 1916, we have been driven by our promise to save lives using the latest technologies. Today, CSL – including our three businesses: CSL Behring, CSL Seqirus and CSL Vifor – provides lifesaving products to patients in more than 100 countries and employs 32,000 people. Our unique combination of commercial strength, R&D focus and operational excellence enables us to identify, develop and deliver innovations so our patients can live life to the fullest. For inspiring stories about the promise of biotechnology, visit csl.com/we-are-csl/vita-original-stories and follow us on x.com/CSL.

For more information about CSL, visit www.CSL.com.

Intended Audience 
This press release is issued from CSL Seqirus in Summit, New Jersey, USA and is intended to provide information about our global business. Please be aware that information relating to the approval status and labels of approved CSL Seqirus products may vary from country to country. Please consult your local regulatory authority on the approval status of CSL Seqirus products. 

Forward-Looking Statements 
This press release may contain forward-looking statements, including statements regarding future results, performance or achievements. These statements involve known and unknown risks, uncertainties and other factors which may cause our actual results, performance or achievements to be materially different from any future results, performances or achievements expressed or implied by the forward-looking statements. These statements reflect our current views with respect to future events and are based on assumptions and subject to risks and uncertainties. Given these uncertainties, you should not place undue reliance on these forward-looking statements. 

FLUCELVAX^® (Influenza Vaccine)

IMPORTANT SAFETY INFORMATION

What is FLUCELVAX^® (Influenza Vaccine)?
FLUCELVAX is a vaccine that helps protect people aged 6 months and older from the flu. Vaccination with FLUCELVAX may not protect all people who receive the vaccine.

Who should not receive  FLUCELVAX?
You should not receive FLUCELVAX if you have a history of severe allergic reactions (e.g., anaphylaxis) to any component of the vaccine.

Before receiving FLUCELVAX, tell your healthcare provider about all medical conditions, including if you:

• have ever had Guillain-Barré syndrome (severe muscle weakness) within six weeks after getting a flu vaccine. The decision to give FLUCELVAX should be made by your healthcare provider, based on careful consideration of the potential benefits and risks.
• have problems with your immune system or are taking certain medications that suppress your immune system, as these may reduce your immune response to the vaccine
• have ever fainted when receiving a vaccine

What are the most common side effects of FLUCELVAX?

• pain and/or redness where the vaccine was given
• headache
• extreme tiredness
• muscle aches

Additional side effects seen in children include:

• tenderness, bruising and/or a raised hardened area where the vaccine was given
• sleepiness
• irritability
• diarrhea
• changes in eating habits
• feeling unwell (malaise)

These are not all of the possible side effects of FLUCELVAX.
You can ask your healthcare provider for more information and for advice about any side effects that concern you.

What do I do if I have side effects?
Report any severe or unusual side effects to your healthcare provider.

To report SUSPECTED ADVERSE REACTIONS, contact CSL Seqirus at 1 - 855 - 358-8966 or VAERS at 1‐800‐822‐7967 or www.vaers.hhs.gov.

Before receiving this vaccine, please see the full US Prescribing Information for FLUCELVAX . You can ask your healthcare provider or pharmacist for information about FLUCELVAX that is written for healthcare professionals.

MEDIA CONTACT
Melanie Kerin
+44 7345 433260
Melanie.Kerin@Seqirus.com 

USA-QIVc-25-0001

¹ Stein, A., et al. Estimated Relative Effectiveness and Public Health Impact of Cell-Based Versus Egg-Based Influenza Vaccines During the 2023–2024 Season in the United States. Presentation at IDWeek, October 21, 2025.
² U.S. Centers for Disease Control and Prevention (CDC). 2024-2025 United States Flu Season: Preliminary In-Season Severity Assessment. Retrieved from: https://www.cdc.gov/flu/php/surveillance/in-season-severity.html. Accessed October 2025.
³ CDC. Preliminary Estimated Flu Disease Burden 2024-2025 Flu Season. Retrieved from: https://www.cdc.gov/flu-burden/php/data-vis/2024-2025.html. Accessed October 2025.
⁴ CDC. Influenza Activity in the United States during the 2024–25 Season and Composition of the 2025–26 Influenza Vaccine. Retrieved from: https://www.cdc.gov/flu/whats-new/2025-2026-influenza-activity.html. Accessed October 2025.
⁵ Rockman, S., Laurie, K., Ong, C., Rajaram, S., McGovern, I., Tran, V., & Youhanna, J. (2023). Cell-Based Manufacturing Technology Increases Antigenic Match of Influenza Vaccine and Results in Improved Effectiveness. Vaccines, 11(1), 52. https://doi.org/10.3390/vaccines11010052.
⁶ CDC. Cell-based flu vaccines. https://www.cdc.gov/flu/vaccine-types/cell-based.html. Accessed October 2025.
⁷ CDC. Terms for vaccine effectiveness. Retrieved from: https://www.cdc.gov/covid/php/surveillance/vaccine-effectiveness-studies.html. Accessed October 2025.
⁸ CDC. Key Facts about Influenza. Retrieved from: https://www.cdc.gov/flu/vaccines/keyfacts.html. Accessed October 2025.
⁹ CDC. About Influenza. Retrieved from: https://www.cdc.gov/flu/about/index.html. Accessed October 2025.
¹⁰ CDC. Who Needs a Flu Vaccine and When. Retrieved from: https://www.cdc.gov/flu/vaccines/vaccinations.html. Accessed October 2025.

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SOURCE CSL Seqirus

FAQ

What did CSL Seqirus report about CSLLY cell‑based vaccine effectiveness for the 2023/24 U.S. season?

The company reported an estimated 19.8% rVE for cell‑based QIV versus standard egg‑based QIV to prevent test‑confirmed influenza in ages six months to 64 years (95% CI 15.7–23.8%).

How large was the study behind CSLLY's 2023/24 vaccine effectiveness result?

The retrospective test‑negative study analyzed linked data from 106,779 vaccinated and influenza‑tested patients in the U.S.

Did the CSLLY data show consistent benefits across age and risk groups in 2023/24?

Yes; the report states improved protection was consistently observed for pediatric, adult, outpatient, and high‑risk subgroups.

What public health impact did CSLLY estimate if all 6 months–64 years had received cell‑based vaccines in 2023/24?

A compartmental model estimated an additional 14,930 hospitalizations would have been prevented.

Where and when were the CSLLY 2023/24 RWE data published and presented?

The data were published in Infectious Diseases and Therapy earlier in October 2025 and presented at IDWeek, Oct 19–22, 2025 in Atlanta.

How many seasons of retrospective studies support CSLLY's claim of greater protection for cell‑based vaccines?

The company states these data represent the fifth season of large retrospective test‑negative studies showing greater protection versus standard egg‑based vaccines.