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

The FDA initially approved Abrysvo in May 2023 for the prevention of prevention of LRTD caused by RSV in individuals aged 60 years or older.

In the MONeT trial, researchers are examining 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 about 200 immunocompromised adults aged 18 years or older, nearly 50% of whom are aged 60 years or older and received two doses of Abrysvo 1 month apart.

The results from the MONeT trial revealed that both the coprimary immunogenicity endpoints and primary safety endpoint were reached. Other results revealed that participants demonstrated RSV-A and RSV-B subgroup neutralizing responses that were 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. The participants also achieved at least a fourfold increase in serum-neutralizing titers for RSV-A and RSV-B 1 month following receipt of Abrysvo compared to prevaccination.

During the trial, Abrysvo was well-tolerated, and safety findings were consistent with those from previous investigations of Abrysvo in other populations. The manufacturer plans 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, and the manufacturer also intends to publish these findings in a peer-reviewed scientific journal and share these findings 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.”

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