A Phase 2 interventional study of RV521 and Placebo in Respiratory Syncytial Virus Infections, sponsored by Pfizer. Completed at 1 site in United Kingdom. Open to participants aged 18 Years to 45 Years, including healthy volunteers. Per ClinicalTrials.gov, last updated 2025-07-31.
Sponsored by Pfizer · Phase 2, Interventional, and Treatment
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Among 13 studies in Respiratory Syncytial Virus Infections by Pfizer — 2 open to participants now, 3 under way, 1 of them phase 3. Respiratory Syncytial Virus Infections · Pfizer.
Among the 13 studies in Respiratory Syncytial Virus Infections by Pfizer, this one is the 4th largest by enrollment of the 11 interventional ones that give a count, is the only one that started in 2017, is one of 2 in phase 2, and was the 1st registered.
306 studies on the registry are indexed under Respiratory Syncytial Virus Infections; 49 are open to participants now.
Browse Respiratory Syncytial Virus Infections studies →2 of the 13 studies in Respiratory Syncytial Virus Infections by Pfizer are open now, in 4 countries. Recruiting first, the newest first:
Pfizer · Sites in United States, Argentina, South Africa and 1 more country
Ages 0+ · Healthy volunteers
Pfizer
Ages 0 yrs–12 mo
By country, each count listing its studies:
Of the 29 Respiratory Syncytial Virus Infections studies recruiting now, 14 (48%) set no upper age limit and 11 (38%) stop at 75 or younger; 13 take children under 18; 8 take healthy volunteers.
This study: 18 Years to 45 Years · all sexes · healthy volunteers too.
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13 studies; the count open now beside each. This study's row is marked.
A study in two phases is counted once above. Listed by phase, each with its combined phases: Phase 1 6 · Phase 2 3 · Phase 3 3
First in 2017; this study’s year is marked. 1 started in 2017.
This study's enrollment of 66 is below the median of 89 across 223 interventional studies indexed under Respiratory Syncytial Virus Infections.
Median enrollment 16 across the 11 interventional studies, 551 across the 2 observational. Of the 9 that completed or were terminated, 4 (44%) have results posted.
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Pfizer is the lead sponsor of 3,337 studies on the registry; 150 are open to participants now.
Of its 601 completed or terminated interventional studies of FDA-regulated products, 392 (65%) have results posted.
1 of the studies in Respiratory Syncytial Virus Infections by Pfizer that are under way reach primary completion within a year, 0 of them phase 3. Phase 3 first, then by date:
| Study | Primary completion |
|---|---|
| NCT06546800PfizerSPECIAL INVESTIGATION FOR ABRYSVO IN INDIVIDUALS AGED 60 YEARS OR OLDER | 2027-10-28 |
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Among the 13 studies in Respiratory Syncytial Virus Infections by Pfizer, from their registry updates:
Nothing recorded among them since 2026-09-25.
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The purpose of the study is to learn about the safety, pharmacokinetics and antiviral activity of the study medicine (called sisunatovir/RV521) for the potential treatment of respiratory syncytial virus (RSV). Sisunatovir will be given as multiple doses during the treatment period. RSV is a highly contagious virus that can lead to serious lung infections in patients with reduced ability to fight infection. Most vulnerable populations include babies, the elderly and patients that have received a bone marrow transplant.
This study is seeking healthy participants who are:
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
BMI (body mass index)
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Clinically significant
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
HIV
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
RV521 drug substance in capsule for oral administration
Drug: RV521
Micro-crystalline cellulose in capsule for oral administration
Drug: Placebo
RV521 drug in capsules
Placebo in capsules
Change in viral load
Time frame: Baseline to study day 12
Plan to share: Yes — Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical_trials/trial_data_and_results/data_requests.
Data sharing details ↗ · www.pfizer.com
Ran from Jul 2017 to primary completion in Oct 2017, 3 months.
Of 32 industry-sponsored phase 2 interventional studies under Respiratory Syncytial Virus Infections, half reached primary completion within 11 months of starting (the median), counting those still running; a quarter within 5 and three quarters within 27. This study's 3 months is below that.
Counting every sponsor, half of 37 studies reached it within 11 months; a quarter within 5 and three quarters within 27.
How these figures are worked out, and which studies are left out of them.
This study is completed, as verified in Jul 2025. You cannot join it, but the record below documents what was studied.
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Studies of the drug tested here: Sisunatovir
Respiratory Syncytial Virus Infections →