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The FDA expanded the approval of the to include the prevention of lower respiratory tract disease (LRTD) caused by respiratory syncytial virus (RSV) in high-risk adults ages 18 to 59 years, Pfizer announced on Tuesday.

The approval expands on the vaccine's existing indications for the prevention of RSV-associated LRTD in adults ages 60 years and older and for pregnant individuals to protect infants at birth.

"RSV represents a significant threat to younger adults with certain chronic conditions," Aamir Malik, chief U.S. commercial officer and executive vice president at Pfizer, said in a press release. "We now have the opportunity to help alleviate the burden of RSV in this high-risk adult population."

The FDA's decision was based on inferred efficacy from the , which investigated the safety, tolerability, and immunogenicity of the RSV vaccine in adults at risk of RSV-associated disease.

Pfizer announced some positive from MONeT in April. The double-blind trial of 681 adults ages 18 to 59 with chronic conditions demonstrated that a single dose of the RSV vaccine resulted in RSV-A and RSV-B subgroup neutralizing responses noninferior to the response seen in the phase III RENOIR study. The RENOIR study previously demonstrated the RSV vaccine's efficacy in a population of adults ages 60 or older.

Participants in MONeT also achieved at least a fourfold increase in serum neutralizing titers for RSV-A and RSV-B 1 month following receipt of the RSV vaccine compared to pre-vaccination. During the trial, the vaccine was well-tolerated, and safety findings were consistent with those from previous investigations of the vaccine in other populations.

In August, Pfizer also announced topline results from , an open-label part of the trial that enrolled approximately 200 immunocompromised adults ages 18 or older -- roughly half of whom were ages 60 or older -- who received two doses of the vaccine 1 month apart. A single 120 µg dose of the vaccine generated a strong neutralizing response against both subtypes of RSV -- RSV-A and RSV-B -- across all cohorts and age groups in the study.

The company intends to submit results from MONeT for publication in a peer-reviewed scientific journal.

There are currently two other RSV vaccines available -- GSK's adjuvanted RSV prefusion F protein-based vaccine (Arexvy) and Moderna's mRNA RSV vaccine (mResvia). The FDA expanded the approval of the GSK vaccine in June to include adults ages 50 to 59 at risk of RSV-LRTD due to underlying conditions. The Moderna vaccine is approved for people ages 60 and older to protect against LRTD.

Of note, the CDC currently recommends that all adults ages 75 years and older should receive a single dose of any RSV vaccine, and adults ages 60 to 74 years who are at increased risk of severe RSV disease should receive a vaccine.

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