CSL Seqirus reports stronger immune response with enhanced flu vaccine in adults over 50

CSL Seqirus, the influenza vaccine business of CSL Limited (ASX: CSL; USOTC: CSLLY), has published pivotal Phase III results for its MF59-adjuvanted, cell-derived, higher-dose quadrivalent influenza vaccine in The Lancet Infectious Diseases. The randomized observer-blind study enrolled 7,699 adults aged 50 years and older across eight countries and compared the investigational vaccine, known as aQIVc, with an adjuvanted egg-based quadrivalent influenza vaccine and a recombinant quadrivalent vaccine. aQIVc produced superior immune responses to the egg-based comparator across all four influenza strains and age groups studied, while meeting non-inferiority criteria versus the recombinant vaccine for three of four strains among all participants aged at least 50 and all four strains among those aged 65 and older. Those findings are clinically important, but the pivotal study was designed around immunogenicity and safety rather than proving that aQIVc prevented more laboratory-confirmed influenza cases than the comparators.

The commercial formulation has already moved further than the publication alone suggests. A trivalent version is approved in the United Kingdom and European Union for adults aged 50 and older under the AUJEMFLU brand, while CSL Seqirus says additional regulatory submissions are under way in multiple markets. Each dose contains 45 micrograms of haemagglutinin per influenza strain and 19.5 mg of the MF59 adjuvant, combining higher antigen content, cell-based manufacturing and adjuvant technology in one product.

What did the 7,699-person Phase III trial actually prove about aQIVc?

The study’s strongest finding was immunological superiority against the adjuvanted egg-based comparator across all four strains evaluated. Participants receiving aQIVc generated antibody responses that met the trial’s statistical superiority criteria, while responses persisted through the influenza season.

Against the recombinant vaccine, the result was more nuanced. aQIVc achieved non-inferiority for three of four strains across the full population aged 50 and older, while meeting non-inferiority for all four strains in the subgroup aged at least 65. That prevents the study from being summarized accurately as universal superiority over both enhanced comparators.

Safety findings were also supportive. Most reported reactions were mild or moderate and resolved within three days, while the study evaluated both manufacturing consistency and safety alongside immune response.

The distinction between superiority and non-inferiority matters for physicians and investors. CSL Seqirus has evidence that the combined vaccine architecture produces a strong immune response, particularly versus egg-based technology, but the data do not establish that every comparator is inferior across every strain and population.

Why might combining cell-based manufacturing, MF59 and higher antigen dose improve influenza immune response?

Traditional egg-based influenza vaccine production can introduce adaptations as viruses are grown in eggs, potentially changing parts of the antigen relative to circulating strains. Cell-based manufacturing is intended to reduce that source of mismatch by avoiding egg adaptation during production.

MF59 serves a different purpose. The adjuvant is designed to enhance the immune system’s response to vaccine antigen, while the higher-dose component increases the amount of haemagglutinin presented for each strain. CSL Seqirus has therefore combined three separate approaches rather than relying on one mechanism to compensate for age-related decline in immune responsiveness.

Phase II development helped determine the final dose. Earlier randomized studies found that formulations containing 45 micrograms of haemagglutinin per strain plus double the standard MF59 amount generated the strongest immune responses among the doses tested, with no new safety concern preventing advancement into Phase III.

The Phase III result validates that combination at a much larger scale from an immunogenicity perspective. It does not establish that multiplying immune-enhancement technologies always translates proportionally into fewer infections or hospitalizations.

Why is superior immunogenicity not the same as superior real-world vaccine effectiveness?

Influenza vaccine trials can use antibody responses as established immunogenicity measures because conducting very large efficacy trials across unpredictable influenza seasons can be difficult. Higher antibody titres can support regulatory and scientific conclusions about immune response, but they are not identical to counting how many vaccinated people ultimately contract laboratory-confirmed influenza.

Real-world effectiveness depends on additional factors, including how well vaccine strains match circulating viruses, the age and health of recipients, influenza-season severity and immune responses not completely captured by traditional antibody measures.

The aQIVc Phase III programme enrolled approximately 7,700 participants specifically as an immunogenicity and safety study. ClinicalTrials.gov describes its primary framework around immune response, lot consistency and safety rather than head-to-head prevention of confirmed influenza disease.

That does not reduce the value of the result. It defines what the evidence can support. AUJEMFLU now has a strong comparative immune-response dataset, while observational effectiveness studies or additional clinical programmes will be important for demonstrating whether that biological advantage produces materially better outcomes in routine vaccination campaigns.

Why does AUJEMFLU matter financially when CSL Seqirus revenue declined in fiscal 2026?

CSL Seqirus reported fiscal 2026 segment revenue of US$2.031 billion, down from US$2.166 billion the previous year, a decline of about 6.2%. Its segment operating result fell to US$899 million from US$1.027 billion, while the operating margin declined to 44.3% from 47.4%.

That makes product differentiation strategically important. Influenza vaccination is a mature market in which seasonal demand, immunization rates and competitive positioning can affect revenue even for established manufacturers.

CSL expects Seqirus revenue to return to low single-digit growth in fiscal 2027 despite anticipating another decline in United States immunization rates, albeit at a slower pace than in recent seasons. A differentiated enhanced vaccine for adults over 50 can support that strategy if national immunization advisory bodies recommend it and payers provide access.

Approval alone therefore does not determine the commercial outcome. The critical variables are national recommendations, reimbursement, procurement contracts and the share of older adults who receive AUJEMFLU instead of competing enhanced vaccines.

What did CSL shares do after the Phase III publication became public?

CSL shares closed at A$167.10 on September 11, down 1.42% from A$169.50 the previous day and approximately 4.8% below the September 3 close of A$175.50. The broader S&P/ASX 200 fell 0.9% on September 11, with healthcare among the weaker sectors, so the CSL decline should not be assigned solely to the vaccine publication.

The stock remains affected by much larger investor questions around CSL’s fiscal 2026 impairments, earnings outlook and broader portfolio performance. A publication supporting one Seqirus vaccine is unlikely to override those group-level issues immediately.

For the vaccine franchise, the evidence milestone is still useful. The Phase III publication provides peer-reviewed support for regulatory and reimbursement discussions at a time when CSL Seqirus is seeking renewed revenue growth.

The next commercial proof will therefore come less from another antibody chart and more from recommendations, launches and market share. If AUJEMFLU gains preference in older populations, the scientific differentiation can begin translating into financial differentiation.

Key takeaways on CSL Seqirus’ aQIVc Phase III results

• CSL Seqirus published pivotal Phase III aQIVc results in The Lancet Infectious Diseases.
• The study enrolled 7,699 adults aged 50 and older.
• Participants came from eight countries.
• aQIVc was superior to an adjuvanted egg-based comparator across all four strains and age groups studied.
• It was non-inferior to a recombinant vaccine for three of four strains in the full 50-plus population.
• Non-inferiority versus the recombinant comparator was achieved for all four strains among participants aged at least 65.
• The study evaluated immunogenicity and safety, not direct comparative clinical influenza prevention.
• The trivalent formulation is approved in the UK and EU as AUJEMFLU.
• CSL Seqirus fiscal 2026 revenue declined about 6.2% to US$2.031 billion.
• Recommendations, reimbursement and real-world effectiveness will determine the vaccine’s commercial impact.