FDA Approves Pfizer's Maternal RSV Vaccine Abrysvo to Protect Infants

The FDA has approved the first ever maternal vaccine to prevent the development of lower respiratory tract disease caused by respiratory syncytial virus (RSV) in infants from birth to age 6 months. The bivalent RSV prefusion F (RSVpreF) vaccine from Pfizer is unadjuvanted and composed of 2 preF proteins selected to optimize protection against RSV A and B strains.

The vaccine, administered to preganant women through 32 and 36 weeks gestational age, is potentially a game-changer for infant health after the US experienced a particularly bad RSV season in 2022. The approval is an expanded indication of the company's RSVpreF vaccine, which was first approved in May for the prevention of severe LRTD in adults age 60 and older.

The FDA’s decision is based on data from the pivotal, randomized, double-blind, placebo-controlled phase 3 clinical trial MATISSE (NCT04424316), which evaluated efficacy, safety, and immunogenicity of the vaccine against LRTD and severe LRTD due to RSV in infants born to healthy individuals who were vaccinated during pregnancy. The trial, which was conducted in 18 countries over 4 RSV seasons, followed infants for up to 2 years.

Overall, 3682 maternal participants aged 49 years or younger, at 24-36 weeks’ gestation on the day of planned RSVpreF injection, received a single intramuscular injection of 120 μg of RSVpreF vaccine and 3676 received placebo, with investigators subsequently evaluating 3570 and 3558 infants, respectively.

Within 90 days of birth, medically attended severe RSV-associated lower respiratory tract illness occurred in 6 infants of women in the vaccine group and 33 infants of women in the placebo group, for a vaccine efficacy of 81.8%. At 180 days after birth, there were 19 cases of medically attended severe RSV-associated lower respiratory tract illness in the RSVpreF infants and 62 cases in the placebo group, meeting the primary efficacy end points.

The incidence of adverse events were similar between the active and placebo groups in both mothers and infants, with investigators concluding that the vaccine had a good safety profile.

After reviewing all available trial data during an FDA Vaccines and Related Biological Products Advisory Committee (VRBPAC) meeting in May, the committee voted 14 to 0 in favor of the efficacy of RSVpreF and 10 to 0 in favor of the safety.

“I'm a pediatrician,” said study author and investigator Iona M. Munjal, MD, director of clinical research and development at Pfizer, and “if I could prevent half of the babies coming into the hospital, I would. 82% [efficacy] for me is a really, really amazing achievement for the vaccine. And we're very excited.”

Pfizer is continuing to study the efficacy and safety of Abrysvo for RSV, including a trial being conducted in children age 2 to 18 at higher risk for RSV disease, as well as a second trial is evaluating adults ages 18 to 60 at higher risk for RSV due to underlying medical conditions including asthma, diabetes and COPD, and adults ages 18 and older who are immunocompromised and at high-risk for RSV. The company also plans on conducting post-marketing studies to continue to monitor the safety of the vaccine.