A Study to Learn About the Effects of Sisunatovir in Infants With Respiratory Syncytial Virus Lower Respiratory Tract Infection.: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study to Learn About the Effects of Sisunatovir in Infants With Respiratory Syncytial Virus Lower Respiratory Tract Infection. (NCT04225897), a randomized phase 2 study. The primary outcome is Part A: Number of Participants With Treatment Emergent Adverse Events (TEAEs), Serious Adverse Events (SAEs) and TEAEs Leading to Permanent Discontinuation of IMP.

2. Clinical Question

In statistical terms, the central question is descriptive: what adverse events were recorded among participants who received study medicine, and how were those counts distributed across the dose and cohort groups? The endpoint is a safety measure, not a direct measure of antiviral effect or clinical recovery. The record also lists pharmacokinetic outcomes, including maximum observed plasma concentration, time to maximum concentration, and area under the plasma concentration-time curve. Those outcomes address drug exposure, which is distinct from whether participants experienced adverse events.

The trial’s sequential structure matters to interpretation. Part A was intended to inform dosing for Part B, and Part B was intended to inform the regimen for Part C. That design creates several cohorts and treatment groups, but does not by itself establish that every group can be compared as if it were part of one fixed, concurrently randomized contrast. The registry record does not report the detailed comparisons planned for each part.

3. Trial Design

The registry record identifies this as a Phase 2 study with randomized allocation, triple masking, and a sequential model. The official title describes an open-label part followed by a double-blind, placebo-controlled part. These descriptions indicate that the study included distinct stages and that masking was not necessarily the same throughout. The precise allocation and masking arrangements for every cohort are not reported in the registry record.

The study was planned around three parts. In Part A, participants aged 6 months to 3 years were described as receiving a single dose of 2.5 mg/kg in Cohort 1. A second Part A cohort, aged 1 month to 6 months, was described as receiving a single dose of 2 mg/kg after Cohort 1 was completed. The brief description gives a planned Part A enrollment range of 12-24 participants. The posted safety groups also include Part A Cohort 1 groups labelled 1.0 mg/kg and 2.0 mg/kg, as well as 2.5 mg/kg, and a Cohort 2 group labelled 2.0 mg/kg. The reasons for the differences between the brief study description and the posted group labels are not reported in the registry record.

Part B was described as enrolling participants aged 1 month to 36 months to receive sisunatovir or placebo every 12 hours for 5 days, with doses determined after Part A. The brief description gives a planned enrollment range of 24-40 participants. The posted Part B safety groups are arranged across Cohorts 3, 4, and 5 and include placebo and multiple dose labels. Part C was described as a later sisunatovir-or-placebo part with a regimen to be determined after Part B, and an approximate planned enrollment of 120 participants.

The record reports enrollment of 51 and status of terminated. The start date was 2019-11-13, and primary completion was 2022-12-05. The reason for termination, whether all planned parts were conducted, and the number enrolled in each part are not reported in the registry record. The overall enrollment and the planned enrollment descriptions should therefore not be read as evidence that every planned stage reached its intended size.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: This study was planned to be conducted in 3 parts: Part A, B and optional part C. Part C was not conducted as part of a reassessment of the clinical development plan for RV521 (sisunatovir); hence, data is not reported for Part C in any section of the results. A total of 51 participants were enrolled in the study (Part A=19 and Part B=32).

Only the first 6 of 12 groups are shown.

Part A (Screening Visit to Day 7)

Reasons for not completing this period:

Part B (Screening Visit to Day 12)

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Safety population included all participants who received at least 1 dose of IMP.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Safety population included all participants who received at least 1 dose of investigational medicinal product (IMP).

Only the first 6 of 13 groups are shown.

6. Primary Endpoint

The registry lists the primary outcome as Part A: Number of Participants With Treatment Emergent Adverse Events (TEAEs), Serious Adverse Events (SAEs) and TEAEs Leading to Permanent Discontinuation of IMP, measured over From start of IMP on Day 1 up to Day 7.

The primary endpoint was the number of participants in Part A with treatment-emergent adverse events, serious adverse events, and treatment-emergent adverse events leading to permanent discontinuation of investigational medicinal product. The observation window ran from the start of treatment on Day 1 up to Day 7. The safety population was defined as participants who received at least one dose of investigational medicinal product.

A treatment-emergent adverse event was defined as an adverse event that started or worsened after the first dose. The record defines an adverse event broadly as an untoward medical occurrence in a participant who received a medicinal product; an event did not need to be judged as caused by that product to meet the definition. A serious adverse event was defined by outcomes such as death, a life-threatening event, required or prolonged inpatient hospitalization, persistent or significant disability or incapacity, or another important medical event.

Counting participants with events is useful in a safety endpoint because it describes how many people experienced an event, rather than treating multiple events in one participant as if they came from different people. The serious-event category also distinguishes events meeting the record’s seriousness criteria from other events. These counts are descriptive: without a prespecified comparative analysis and its uncertainty, they do not establish whether observed differences between groups reflect treatment, chance, or differences among cohorts.

The registry record does not provide a single reduced comparison for this primary endpoint. It also does not report a numerical result for the specific category of treatment-emergent adverse events leading to permanent discontinuation in the endpoint description. The posted event counts should therefore be read as safety information by group, not as a complete inferential answer to every component named in the primary endpoint.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The methods that can be described from the registry record are primarily descriptive. It defines a safety population, a treatment-emergent event, and a serious event, and presents participant counts by group. The registry record does not report the statistical analysis plan, the inferential method for comparing groups, or a model used to adjust for differences among cohorts.

What does a participant count represent?

A count of participants with an event records whether a person experienced the specified event category during the defined observation period. This differs from counting every event occurrence: one participant can experience more than one event, but a participant count is intended to count that person once within the category. The registry record’s primary endpoint is framed in terms of participants, and the safety population includes those who received at least one dose of study medicine.

The numerator and the number of participants at risk provide useful context for each group. A count of one event among a small group has a different degree of precision from the same count among a much larger group. In small groups, an individual participant can materially alter the observed fraction. The posted counts therefore describe the enrolled sample; they should not be treated as exact estimates of the event frequency in all infants who might meet the eligibility criteria.

Why separate serious events?

Seriousness is a classification based on specified consequences or medical importance, not simply a judgment that an event was caused by the study medicine. The record’s adverse-event definition explicitly does not require a causal relationship. A serious event can therefore be relevant to safety monitoring even when its cause is uncertain. Conversely, a count of serious events does not by itself indicate causality, severity beyond the stated classification, or whether an event was expected.

The primary endpoint names treatment-emergent adverse events, serious adverse events, and events leading to permanent discontinuation as distinct categories. The posted counts should be interpreted in light of those distinctions. The registry record does not supply a separate causal assessment or detailed event narratives for the groups in the material summarized by these counts.

What can randomization and masking tell us?

The record identifies randomized allocation and triple masking, while the official title describes an open-label part followed by a double-blind, placebo-controlled part. Randomization, when implemented as planned, is a design strategy for balancing characteristics across comparison groups on average. Masking is intended to limit the influence of knowledge about assigned treatment on care, reporting, or assessment. These general principles do not establish that every cohort in this sequential study had the same allocation or masking arrangements. The detailed procedures by part are not reported in the registry record.

Sequential designs use information from an earlier stage to guide a later stage. Here, the brief description says later dose regimens were to be determined after preceding parts. This can be useful in a developing study, but it also means that groups may represent different stages, dose choices, or participant cohorts. A straightforward pooled comparison could be misleading if it ignored those distinctions. The registry record does not report whether results were pooled, stratified, or adjusted.

How should uncertainty be considered?

A confidence interval is a general statistical way to express the range of estimates compatible with data and a stated method, while reflecting sampling uncertainty. A narrow interval indicates more precision than a wide interval under that method. The registry record does not report confidence intervals for the adverse-event comparisons. Consequently, readers cannot use the posted counts alone to quantify the uncertainty around a group contrast.

A p-value is a general measure, under a specified statistical model and null hypothesis, of how unusual data at least as incompatible with that hypothesis would be. It is not the probability that a treatment works, nor does it measure the size or clinical importance of an effect. No p-value for a primary endpoint comparison is reported in the registry record. With sparse counts across several cohorts, the choice of a suitable analysis would matter; the actual method is not reported.

What are the sample-size illustrations?

The planning illustrations use the observed effect as a reference, assume a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. The stated effect to detect is a fraction of the observed effect. Such calculations are hypothetical planning exercises, not evidence about the study’s achieved power, and their interpretation depends on assumptions that may not match this sequential multi-cohort design. For non-time-to-event outcomes, the stated sample size convention is participants per arm; time-to-event calculations use a total event count. The primary endpoint is not reduced to a single comparison in the registry record, so a single direct planning contrast for it is not available from the posted description.

These illustrations do not establish the rationale for the actual enrollment or prove that the study could answer a specific safety question. The protocol’s sample-size justification and the analysis plan are not reported in the registry record. The illustrations should not be mistaken for observed trial results or for a substitute for uncertainty intervals around the actual event counts.

10. Multiplicity, Missing Data and Analysis Populations

The study has multiple parts, cohorts, dose groups, safety categories, and pharmacokinetic outcomes. When many outcomes or comparisons are examined, the chance of seeing an apparently notable result by chance can increase. Statistical approaches to multiplicity can include prioritizing a limited set of hypotheses, adjusting significance thresholds, or describing results as exploratory. The registry record does not report a multiplicity strategy, a hierarchy of hypotheses, or which comparisons were confirmatory.

The primary endpoint itself names several event categories, while the posted safety results are organized across several group labels. The record does not state how those categories were combined or whether formal comparisons were planned between particular doses and placebo. It also does not report interim analyses, stopping rules, or how the sequential decisions between parts were statistically governed. The reason for the terminated status is not reported in the registry record.

Missing-data handling is also not reported in the registry record. The safety population is defined as participants who received at least one dose, but the record does not describe how participants with incomplete follow-up, missing event information, or early discontinuation were handled in each endpoint analysis. The endpoint timeframe is from treatment start on Day 1 up to Day 7, but completeness of observation for each participant is not described.

The listed pharmacokinetic outcomes include concentration and exposure measures, but analysis populations and missing-sample procedures are not reported in the registry record. As a result, the posted safety counts should not be assumed to represent a prespecified multiplicity-adjusted analysis or to include a particular missing-data procedure. The absence of these details limits assessment of how selective reporting, incomplete follow-up, or multiple testing may affect interpretation.

11. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: pyrexia (14.3%).

The safety groups list the number of participants at risk and the number affected by serious adverse events, other adverse events, and deaths. In Part A Cohort 1 labelled RV521 1.0 mg/kg, serious adverse events affected 0 of 3 participants and other adverse events affected 2 of 3; deaths were 0. For RV521 2.0 mg/kg in Part A Cohort 1, the corresponding figures were 1 of 7 for serious adverse events, 4 of 7 for other adverse events, and 0 deaths. For RV521 2.5 mg/kg in Part A Cohort 1, they were 0 of 3, 3 of 3, and 0 deaths. Part A Cohort 2 labelled RV521 2.0 mg/kg had 0 of 6 serious adverse events, 1 of 6 other adverse events, and 0 deaths.

In Part B, serious adverse events and deaths were recorded as zero in each listed group. Other adverse events affected 1 of 3 participants in Cohort 3 placebo, 2 of 3 in the group labelled RSV1 2.5 mg/kg, 1 of 4 in the RV521 3.5 mg/kg group, and 1 of 3 in the RV521 5 mg/kg group. In Cohort 4, the figures were 1 of 1 for placebo and 2 of 4 for RV521 2.5 mg/kg. In Cohort 5, they were 3 of 5 for placebo and 1 of 8 for RV521 2.5 mg/kg. These are observed participant counts, not comparative risk estimates.

The record lists Pyrexia at 14.3% among the top serious adverse event terms. The registry record does not report a reason for termination or a causal attribution for the listed events. Zero observed events in a group should not be read as proof of zero risk.

12. What the Result Does, and Does Not, Mean

The registry record documents a Phase 2 study of sisunatovir in infants with RSV lower respiratory tract infection, with safety and pharmacokinetic outcomes. The posted safety information gives counts by cohort and dose group, including serious adverse events, other adverse events, and deaths. These figures describe what was recorded among participants in the listed groups. They do not, on their own, estimate a causal difference between sisunatovir and placebo.

The study’s sequential structure and multiple dose groups are important. Cohorts may differ in age, dose, stage of the study, and other features. The registry record does not report a single adjusted comparison or uncertainty interval for the primary safety endpoint. Raw counts from small groups cannot establish that one dose has a higher or lower underlying event rate than another. Nor can a zero count demonstrate that an event is impossible or that a treatment has no safety risk.

The record also does not provide an efficacy result that answers whether sisunatovir reduces RSV illness or improves clinical outcomes. The listed pharmacokinetic measures concern drug concentrations and exposure, which are not the same as clinical benefit. The primary endpoint is a safety measure and should not be used to claim antiviral effectiveness.

The trial status is terminated, and the reason for termination is not reported in the registry record. Planned Part C enrollment is described, but adverse-event groups are listed only for Parts A and B. The available counts therefore do not show that all intended parts were completed or that the intended participant numbers were reached. The defensible interpretation is limited to the posted records: events were observed in the stated groups, and the reported counts do not supply a definitive between-treatment estimate or a broad conclusion about safety or efficacy.

13. Why This Trial Matters Statistically

This study is statistically informative as an example of why trial structure matters when interpreting safety counts. A label such as randomized does not make every listed group a direct comparison. The record describes a sequential program in which later dosing depended on earlier parts, alongside multiple cohorts and treatment labels. Comparing raw counts without accounting for those design features could suggest more certainty than the data support.

Safety endpoints are often best understood first as participant-level descriptions. The distinction between an event count and a count of participants with an event is important, especially when participants may experience more than one event. The endpoint also distinguishes treatment-emergent events from serious events and events leading to discontinuation. Those categories provide different information and cannot be substituted for one another.

The study also illustrates the limits of sparse data. Several posted groups contain few participants, so each observation has substantial influence on the group count. General statistical methods can quantify uncertainty or support adjusted comparisons, but the registry record does not report those results for the primary endpoint. Without them, the counts are useful for transparent description but do not support a precise comparative conclusion.

Finally, a terminated study with a planned later part raises questions about completeness and analysis that cannot be answered from status alone. The reason for termination, missing-data procedures, and analysis strategy are not reported in the registry record. Recognizing these gaps is part of sound interpretation: the available safety observations remain relevant, while claims about treatment differences, planned study completion, or efficacy must remain within the boundaries of what the record documents.

14. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record does not report the reason the study was terminated, whether every planned part was completed, or the number enrolled in each part. Although the overall enrollment is 51, the brief description includes planned enrollment ranges for Parts A and B and approximately 120 participants for Part C. The posted safety groups cover Parts A and B, so the relationship between the planned program and the posted results is not fully documented.

There is an apparent difference between the brief description of Part A dosing and some posted Part A group labels. The brief description mentions a single dose of 2.5 mg/kg for Cohort 1 and 2 mg/kg for Cohort 2, while the adverse-event groups include Cohort 1 labels of 1.0 mg/kg, 2.0 mg/kg, and 2.5 mg/kg. The reason for this difference is not reported in the registry record. Part B includes a group label of RSV1 2.5 mg/kg; whether that label is intended to identify RV521 is not reported in the registry record.

The registry record does not provide confidence intervals, p-values, an adjusted group comparison, or a complete statistical analysis plan for the safety endpoint. It also does not state the missing-data approach, multiplicity strategy, interim analysis procedures, or protocol sample-size justification. Detailed event-level descriptions and causal assessments are not reported in the registry record.

These limitations mean the adverse-event counts should be interpreted as a record of observed participants, not as a complete comparative analysis. The registry does not provide enough information to resolve every difference in group labels or to establish how fully each planned analysis was carried out.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: A Study to Learn About the Effects of Sisunatovir in Infants With Respiratory Syncytial Virus Lower Respiratory Tract Infection. — NCT04225897.
• Registry record last updated 2024-10-23; status last verified 2024-08.
• Results first posted 2024-10-23.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

18. Record Summary

This Phase 2 study evaluated sisunatovir, also called RV521, in infants hospitalized with RSV lower respiratory tract infection. The registry record describes a sequential study with open-label and placebo-controlled parts, randomized allocation, and triple masking. Enrollment was 51, and status is terminated. The reason for termination is not reported in the registry record.

The primary endpoint concerned Part A participants with treatment-emergent adverse events, serious adverse events, and treatment-emergent adverse events leading to permanent discontinuation, from Day 1 up to Day 7. The safety population included participants who received at least one dose. Posted safety groups include Part A and Part B cohorts; the registry record does not provide a single comparative estimate or uncertainty interval for the endpoint. The listed counts are descriptive and should not be treated as proof of differences or equivalence between treatments.