Pfizer, Inc., announced positive top-line immunogenicity and safety data from the ongoing pivotal clinical trial MONeT (RSV IMmunizatiON Study for AdulTs at Higher Risk of Severe Illness). This trial is investigating a single dose of the FDA-approved RSV vaccine, Abrysvo, versus placebo in adults aged 18 to 59 years at risk of developing severe RSV-associated lower respiratory tract disease (LRTD).

MONeT is a multicenter clinical trial investigating the safety, tolerability, and immunogenicity of Abrysvo in adults at risk of RSV-associated disease, including adults with certain chronic medical conditions (substudy A) and adults who are immunocompromised (substudy B).

Substudy A is a double-blinded study that randomized 681 adults aged 18 to 59 years with chronic conditions, with 2:1 to receive a single dose of Abrysvo or placebo.

Substudy B is an open-label study that enrolled around 200 immunocompromised adults aged 18 years or older; approximately 50% of the cohort were aged 60 years or older, and subjects received two doses of Abrysvo 1 month apart.

Data reveal that the MONeT study achieved its coprimary immunogenicity endpoints and primary safety endpoint:

• Participants demonstrated RSV-A and RSV-B subgroup neutralizing responses noninferior to the response observed in the phase III RENOIR study of Abrysvo in more than 34,000 adults aged 60 years or older where vaccine efficacy was previously demonstrated
• Participants also achieved at least a fourfold increase in serum neutralizing titers for RSV-A and RSV-B 1 month after receiving Abrysvo compared with prevaccination.

During the trial, Abrysvo was well-tolerated, and safety findings were consistent with those from previous investigations of Abrysvo in other populations.

Pfizer intends to submit these data to regulatory agencies and request expansion of the age group from the current indication to age 18 years and older. The use of immunobridging studies to extrapolate efficacy from older to younger adults is an established regulatory pathway. The company also intends to publish these findings in a peer-reviewed scientific journal and share them at an upcoming scientific conference.

Annaliesa Anderson, PhD, senior VP and head of vaccine research and development at Pfizer, stated, “These encouraging results provide evidence that Abrysvo can help protect adults with increased risk against RSV-associated illness. We are excited to address a significant unmet need, pending regulatory authority approval, as Abrysvo has the potential to become the first and only RSV vaccine for adults 18 years and older.”

In addition to the MONeT trial, a clinical trial evaluating Abrysvo in children aged 2 years to younger than 18 years who are at higher risk for RSV disease has also been initiated.

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