Effectiveness of a second dose of RSV subgroup A 847A and subgroup B 847B versus primary dose to prevent respiratory syncytial virus infection in immunocompromised adults

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What is this study about?

Respiratory syncytial virus infection is a common viral illness that can cause inflammation of the airways and lungs, known as lower respiratory tract disease. Adults whose immune systems are weakened, described as immunocompromised, are at higher risk of severe symptoms and may need medical care when infected.

The study evaluates the vaccine Abrysvo, a product designed to protect against two types of the virus by delivering a small amount of protein (120 µg) into the muscle. Participants may receive a single initial dose or a second dose (revaccination) later in the same season, while others receive no vaccine, allowing comparison of outcomes.

The purpose is to assess how well the vaccine, with or without a second dose, reduces the chance of experiencing a medically attended RSV event or symptoms of lower respiratory tract disease during the first season after vaccination. After enrollment, participants are followed through the RSV season, and any visits to doctors, emergency departments, or hospital stays related to respiratory illness are recorded to determine the vaccine’s relative effectiveness.

1 enrollment and randomization

after you join the study, you give informed consent and are randomly assigned to either the revaccination group (second dose) or the single‑dose group (no revaccination).

the assignment determines whether you will receive one or two doses of abrysvo during the trial.

2 baseline visit

you attend a clinic visit where basic health information is recorded and a brief physical check is performed.

this visit establishes your health status before any vaccination is given.

3 primary vaccination

you receive a single 120 µg dose of abrysvo as an intramuscular injection in the upper arm.

the injection contains the bivalent recombinant respiratory syncytial virus vaccine, which targets two subgroups of the virus.

4 post‑injection observation

you remain in the clinic for a short period (typically 15–30 minutes) so that staff can watch for any immediate reactions such as redness, swelling, or faintness.

if no reaction occurs, you are allowed to leave the clinic.

5 routine follow‑up after primary dose

during the first respiratory syncytial virus (rsv) season after vaccination, you monitor your health and report any respiratory symptoms or medical visits related to rsv.

scheduled follow‑up visits may be arranged at intervals such as one month, three months, and six months, but the exact timing follows the study protocol.

6 revaccination (second dose) – if assigned

if you were placed in the revaccination group, you receive a second 120 µg dose of abrysvo as an intramuscular injection.

this second dose is given at the start of the next rsv season, approximately twelve months after the first dose.

7 post‑revaccination observation

as with the first dose, you stay in the clinic for about 15–30 minutes after the second injection so that staff can observe any immediate side effects.

8 continued follow‑up after revaccination

you continue to track and report any respiratory illness, medical visits, or hospitalizations for the remainder of the study, which includes the second and third rsv seasons after randomization.

the study follows you until the estimated end date of may 2032.

9 final study visit

at the conclusion of the trial, you attend a final clinic visit where a summary health assessment is performed and study data are collected.

no further study medication is given at this visit.

Who Can Join the Study?

Be 18 years of age or older at the time of enrollment or revaccination.
Have a weakened immune system (the study calls this immunocompromised), as defined in the trial protocol.
For the revaccination group: have received one dose of the RSV vaccine Abrysvo about three years earlier (for example, in late 2024, late 2025‑early 2026, or autumn/early winter 2026).
For the revaccination group: not have had a related serious adverse event (SAE) after the first dose of Abrysvo.
For the revaccination group: not have received any other RSV vaccine doses after the first Abrysvo dose.
For the primary‑vaccination sub‑study: have never received any RSV vaccine doses before joining the study.
Be able to understand written and spoken English or Danish, and, in the investigator’s judgment, be able to follow the study procedures.
Provide a signed and dated written informed consent form agreeing to take part in the study.

Who Cannot Join the Study?

Pregnancy – If you are pregnant, you cannot join the study because pregnant individuals may receive the vaccine through the regular national vaccination program.
Known hypersensitivity to the RSV vaccine (Abrysvo) or any of its excipients, or any other contraindication listed in the Summary of Product Characteristics (SmPC). Hypersensitivity means a strong allergic reaction. Excipients are inactive ingredients that help the vaccine work. Contraindication is a reason why a medicine should not be used. The Summary of Product Characteristics (SmPC) is the official document that describes who should not receive the vaccine.

Where you can join this trial?

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Verified Sites

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Other Sites

Site Name	City	Country	Status
Glzjoaaq Hwslzcnd	Hellerup	Denmark
Dtyqkc Ljgofl Vxjnxmzpqnfu Sftiwwz	Søborg	Denmark

Trial status

Country	Status	Recruitment Start
Denmark	Not yet recruiting	01.11.2026

Trial locations

Abrysvo is a vaccine that helps protect against respiratory syncytial virus (RSV), a virus that can cause serious lung infections. It contains two pieces of the virus (one from subgroup A and one from subgroup B) that are designed to trigger the body’s immune system to recognize and fight RSV. In the study, participants receive the vaccine by an injection into the muscle. The goal is to see how well a second dose of the vaccine works compared with not receiving another dose, and also to measure how well the first dose works compared with not being vaccinated at all, especially in adults whose immune systems are weakened.

Investigated Diseases:

Respiratory syncytial virus infection

Respiratory syncytial virus infection – Respiratory syncytial virus infection is a viral illness that primarily affects the airways of the lungs. It usually starts with symptoms such as runny nose, cough, and fever. In many people the infection stays limited to the upper airway, causing a mild cold. In some cases it spreads to the lower airway, leading to bronchiolitis or pneumonia, which can cause breathing difficulty. The illness often follows a seasonal pattern, with more cases in the fall and winter. Children, older adults, and people with weakened immune systems are more likely to develop the more extensive form.

Trial ID:

2026-527536-14-00

Trial Phase:

Therapeutic confirmatory (Phase III)

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Effectiveness of a second dose of RSV subgroup A 847A and subgroup B 847B versus primary dose to prevent respiratory syncytial virus infection in immunocompromised adults

What is this study about?

Who Can Join the Study?

Who Cannot Join the Study?