Phase III Trial Data Show Efficacy of Abrysvo in Adults Aged 18 to 59 Years with Elevated Risk of Severe RSV


Findings from MONeT clinical trial show positive safety and efficacy of Abrysvo in individuals aged 18 to 59 years with an increased risk of developing severe respiratory syncytial virus-associated lower respiratory tract disease.

Image credit: jarun011 |

Image credit: jarun011 |

Pfizer’s bivalent respiratory syncytial virus (RSV) prefusion F (RSVpreF) vaccine Abrysvo produced positive safety and efficacy in individuals aged 18 to 59 years with an increased risk of developing severe RSV-associated lower respiratory tract disease (LRTD). Findings from the pivotal, multicenter Phase III MONeT (RSV IMmunizatiON Study for AdulTs at Higher Risk of Severe Illness) clinical trial (NCT05842967) will be the basis of filings with regulatory agencies seeking approval of Abrysvo for this age group, which does not currently have an approved vaccine to protect against RSV.

“These encouraging results provide evidence that Abrysvo can help protect adults with increased risk against RSV-associated illness,” said Annaliesa Anderson, PhD, senior vice president and head, Vaccine Research and Development, Pfizer. “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.”1

RSV develops in 3% to 7% of healthy older adults in the United States and Europe annually, with an estimated 177,000 hospitalizations and 14,000 deaths each year in the United States. Among older adults, RSV is a common cause of LRTD, which affects the lungs and can cause life-threatening pneumonia and bronchiolitis. The CDC has found that RSV leads to approximately 60,000-120,000 annual hospitalizations and 6,000-10,000 annual deaths among adults 65 years of age and older.2

Individuals with certain underlying chronic conditions, such as asthma, diabetes, and chronic obstructive pulmonary disease, have a greater risk of becoming infected with RSV, as well as hospitalization for RSV-associated LRTD. Among those aged 18 to 49 years, 9.5% have a chronic condition that elevates their risk for severe RSV disease, which increases to 24.3% in those aged 50 to 64 years.1 Despite the elevated risk, there has yet to be an approved RSV vaccine for this age group, which represents a significant unmet need.

Abrysvo was approved by the FDA in June 2023 to prevent LRTD caused by RSV in individuals 60 years of age and older.3 Abrysvo is based on foundational basic science discoveries, such as findings from the National Institutes of Health (NIH), detailing the crystal structure of prefusion F, a vital form of the viral fusion protein that RSV employs to attack human cells. NIH research indicates that antibodies specific to the prefusion form had high efficacy preventing viral infection, which indicates that a prefusion F-based vaccine may provide optimal protection against RSV, according to Pfizer.3

The MONeT trial evaluated the safety, tolerability, and immunogenicity of Abrysvo in adults with a greater risk of RSV-associated disease. The double-blinded substudy A of the trial included 681 individuals, aged 18 to 59 years with certain chronic medical conditions, who were randomly assigned at a 2:1 ratio to receive a single dose of Abrysvo or placebo. The open label substudy B enrolled approximately 200 immunocompromised individuals aged 18 years or older, of whom approximately half were 60 years of age or older, to receive two doses of Abrysvo one month apart.

Data show that Abrysvo achieved the trial’s the co-primary immunogenicity endpoints and its primary safety endpoint. Those administered Abrysvo demonstrated RSV-A and RSV-B subgroup neutralizing responses that were non-inferior to what was observed in the Phase III (NCT05035212) RENOIR trial of the vaccine.

Individuals administered Abrysvo achieved at least a four-fold increase in serum neutralizing titers for RSV-A and RSV-B one month after administration of the vaccine compared to pre-vaccination. The vaccine was also found to be well-tolerated, with safety findings consistent with what was reported in prior clinical trials of Abrysvo in other populations.

Pfizer stated that it will submit the trial data to regulatory agencies seeking to expand the indication of Abrysvo from the current indication to include those 18 years of age and older.1


1. Pfizer Announces Positive Top-Line Results from Phase 3 Study of ABRYSVO® in Adults Aged 18 to 59 at Increased Risk for RSV Disease. Pfizer. News release. April 9, 2024. Accessed April 9, 2024.

2. FDA Approves First Respiratory Syncytial Virus (RSV) Vaccine. FDA. News release. May 3, 2023. Accessed April 9, 2024.

3. U.S. FDA Approves ABRYSVO™, Pfizer’s Vaccine for the Prevention of Respiratory Syncytial Virus (RSV) in Older Adults. Pfizer. News release. May 31, 2023. Accessed April 9, 2024.

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