On August 13, 2021, Pfizer (Pfizer/PFE.US) announced the safety and immunogenicity results of the sub-study B of the ongoing pivotal Phase 3 clinical trial MONeT, which showed that two doses of Abrysvo vaccine were well tolerated and immunogenic in adults aged 18 years and older who are at high risk of developing severe lower respiratory tract disease (LRTD) related to RSV.

RSV is a contagious virus that causes a common respiratory illness. It can affect the lungs and respiratory system of those infected and can lead to severe illness in infants, the elderly, and some chronic disease patients. RSV is divided into two main subtypes: RSV-A and RSV-B, both of which can cause disease.

Abrysvo is made using the prefusion F protein crystal structure of the RSV virus, without adjuvant. This prefusion protein is the main form of the F protein that the virus uses to enter human cells. Past studies have shown that targeting this prefusion protein can effectively block virus infection. Last May, the FDA approved the vaccine for use in adults aged 60 and older to prevent acute respiratory illness and LRTD caused by RSV. In August of the same year, the FDA approved the vaccine for use in pregnant women at 32-36 weeks of gestation to generate passive immunity in infants born to these women to prevent LRTD and severe LRTD caused by RSV in infants born to these women.