RESPIRATORY SYNCYTIAL VIRUS, SUBGROUP B, STABILIZED PREFUSION F PROTEIN 847B

Table of contents

• What is Respiratory Syncytial Virus?
• How does Respiratory Syncytial Virus work?
• Which conditions is Respiratory Syncytial Virus used or studied for?
• How is Respiratory Syncytial Virus given?
• What do the clinical trials show?
• What side effects are known?

What is respiratory syncytial virus?

Respiratory Syncytial Virus (RESPIRATORY SYNCYTIAL VIRUS, SUBGROUP B, STABILIZED PREFUSION F PROTEIN 847B) is a protein-based vaccine substance. It is one of two stabilised prefusion F protein components in the bivalent recombinant vaccine known as Abrysvo, together with the subgroup A component 847A.^[9] The substance is classified as a respiratory syncytial virus vaccine with ATC code J07BX05.^[9] Abrysvo is an authorised medicine in the European Union, with marketing authorisation held by Pfizer Europe MA EEIG.^[1] The World Health Organization (WHO) prequalified the single-dose presentation in March 2025 and the multi-dose vial presentation in September 2026.^[10]

How does respiratory syncytial virus work?

The vaccine contains the stabilised prefusion F protein of respiratory syncytial virus. This protein is the form the virus uses before it fuses with human cells, and it is the main target of protective antibodies. When the vaccine is given, the immune system learns to recognise this protein and produces antibodies against it. In pregnant women, these antibodies are transferred to the baby before birth, protecting the infant during the first six months of life.^[10] The vaccine is designed to prevent lower respiratory tract disease caused by RSV.

Which conditions is respiratory syncytial virus used or studied for?

• Respiratory syncytial virus infection — a common respiratory virus that can cause severe lung infections, especially in infants and older adults.
• Prevention of lower respiratory tract disease caused by RSV — studied in children at high risk of RSV disease.
• Allogeneic haematopoietic stem cell transplantation — studied in transplant recipients whose immune systems are rebuilding.
• Lung transplant — studied in lung transplant recipients.
• Inflammatory rheumatic disease — studied in patients taking antirheumatic treatments that may affect vaccine response.
• Bronchiolitis and respiratory tract infection in infants — studied through maternal vaccination during pregnancy.

How is respiratory syncytial virus given?

The vaccine is given as a solution for injection by intramuscular injection.^[1] The registered pharmaceutical form is a powder and solvent for solution for injection, supplied in single-dose and multi-dose presentations.^[1] In the trials, the vaccine is given at a dose of 120 micrograms or 0.5 millilitres, depending on the study. The WHO recommends that the maternal vaccine be given to pregnant women during the third trimester, from week 28 onwards.^[10]

What do the clinical trials show?

Eight clinical trials are registered for this substance, ranging from Phase 2 to Phase 4. All eight trials are currently authorised. The trials study the vaccine in different populations: adults aged 18 and older, pregnant women, infants, children at high risk of RSV disease, transplant recipients, and patients with inflammatory rheumatic disease.

Which conditions are studied in the trials?

The trials cover RSV infection, prevention of RSV-related lower respiratory tract disease, allogeneic haematopoietic stem cell transplantation, lung transplantation, inflammatory rheumatic disease, and prevention of RSV in infants through maternal vaccination. The largest trial, with 690,000 participants, evaluates whether Abrysvo reduces the risk of RSV-related hospitalisation in adults aged 18 and older.^[2] A Phase 4 trial with 204 participants studies vaccination in allogeneic haematopoietic stem cell transplant recipients.^[1]

What phases and status do the trials have?

There is one Phase 2 trial, five Phase 3 trials, and one Phase 4 trial. All trials are authorised. The Phase 2 trial enrols 400 lung and haematopoietic stem cell transplant recipients.^[5] A Phase 3 trial with 180 participants studies patients with inflammatory rheumatic disease.^[3] Another Phase 3 trial with 29,000 participants studies revaccination in immunocompromised adults.^[7]

What do the trials measure?

The trials measure antibody responses, including seroconversion rates and fold-changes in RSVpreF-specific IgG antibody levels. Some trials measure clinical outcomes such as hospitalisation for RSV-related respiratory tract disease and symptomatic RSV respiratory tract infection. A Phase 3 trial in children at high risk measures safety outcomes including local reactions, systemic events, and neutralising titres against RSV subgroups A and B.^[6]

Who can take part in the trials?

Eligibility varies by trial. Populations include adults aged 18 and older, pregnant women between 32 and 36 weeks of gestation, children aged 5 to under 18 years with chronic medical or immunocompromising conditions, allogeneic haematopoietic stem cell transplant recipients, lung transplant recipients, and patients with inflammatory rheumatic disease. Healthy control subjects aged 60 years or more are included in one Phase 4 trial.^[1]

What side effects are known?

A post-marketing safety study using a pregnancy registry is evaluating the safety of Abrysvo exposure during pregnancy. The study examines adverse pregnancy outcomes including preterm birth, hypertensive disorders of pregnancy, stillbirth, premature labour, premature rupture of membranes, and other maternal and neonatal outcomes.^[9] The study also monitors conditions such as Guillain-Barre syndrome, autoimmune demyelinating disease, polyneuropathy, atrial fibrillation, and thrombocytopenia.^[9] These events are being studied; the registry data will help determine whether they are associated with the vaccine.