A Study on the Safety and Immune Response to an Unadjuvanted RSV Maternal Vaccine, in High Risk Pregnant Women Aged 15 to 49 Years and Infants Born to the Vaccinated Mothers: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study on the Safety and Immune Response to an Unadjuvanted RSV Maternal Vaccine, in High Risk Pregnant Women Aged 15 to 49 Years and Infants Born to the Vaccinated Mothers (NCT04980391), a randomized phase 3 study. The primary outcome is Percentage of Maternal Participants Reporting Any Solicited Administration Site Events.

2. Clinical Question

The study was intended to assess safety, reactogenicity and immune response following maternal vaccination, with assessment also involving infants born to vaccinated mothers. Its primary endpoint is a short-term maternal safety measure: the occurrence of at least one of the specified administration site events during the stated period. It does not, by itself, answer whether the vaccine prevents RSV illness in infants or establish an immune response. The registry lists infant respiratory illness and hospitalization measures among its secondary outcomes, but their results are not reported in the registry record.

The record reports that enrollment and vaccination were stopped following a recommendation from the Independent Data Monitoring Committee of NCT04605159. Participants already ongoing at that time continued to be monitored as part of the study. The record does not provide the monitoring committee’s reasoning or a result-based explanation for stopping this study. The study status is recorded as terminated, so the intended comparison should be interpreted in light of that early stoppage and the absence of reported primary outcome results.

3. Trial Design

This was a Phase III, randomized, parallel-group study comparing a single dose of RSV MAT with placebo in high-risk pregnant women and assessing outcomes in the mothers and their infants. The registry records enrollment as 384. It gives the study start date as 2021-08-03 and the primary completion date as 2023-05-30. The status is terminated.

The maternal vaccine group received one dose between 24 and 36 weeks of gestation, at Day 1; the control group received placebo on the same schedule. Infant groups are defined by the mother’s assigned group, rather than as infants independently receiving the maternal intervention. The brief summary states that dosing was intended in the second or third trimester. The registry’s study title calls the study double-blind, while its masking field is recorded as none. This difference in the record cannot be resolved from the information reported.

The study’s stated purpose covered maternal safety, reactogenicity and immune response, as well as assessments in infants born to vaccinated mothers. The listed secondary outcomes include maternal serious adverse events, medically attended events, worsening of pre-existing medical conditions or obstetric complications, maternal RSV-associated illness, infant RSV-associated lower respiratory tract illness and hospitalization, and maternal RSV MAT IgG-specific antibody concentrations. These are listed as outcome topics, not as results.

The brief summary says GSK decided to stop enrollment and vaccination following a recommendation from the Independent Data Monitoring Committee of NCT04605159, also identified in the record as RSV MAT 009. Ongoing participants continued to be monitored. The reason for that committee recommendation, the timing of the stoppage, and the expected enrollment under the original plan are not reported in the registry record. Randomization describes the intended allocation process; it does not establish that the planned recruitment or follow-up was completed.

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: Out of 384 participants who completed the informed consent process, 169 maternal participants were vaccinated, and 198 infants, including twins, were born to those exposed mothers (RSV\_MAT Group-Mother and Control Group-Mother). Therefore, 367 participants were considered exposed.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Analysis was performed on the Solicited Safety Set, which included all maternal participants who received 1 dose of a study intervention and who had solicited safety data available during the specified period.

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.

6. Primary Endpoint

The registry lists the primary outcome as Percentage of Maternal Participants Reporting Any Solicited Administration Site Events, measured over From Day 1 to Day 7 included.

The primary endpoint is the percentage of maternal participants reporting any solicited administration site event from Day 1 to Day 7 included. The specified events are erythema, pain and swelling. Any pain is defined as occurrence of the symptom regardless of intensity grade. Erythema or swelling qualifies when the reported surface diameter is at least 20 millimeters.

This endpoint is used to describe short-term local reactogenicity after the maternal dose. A percentage expresses how common the defined event was among the participants included in the endpoint analysis. It does not convey the intensity or duration of symptoms beyond what the endpoint definition captures, and it should not be read as a measure of infant RSV illness or vaccine immune response.

The registry describes the analysis population as the Solicited Safety Set: maternal participants who received one dose of a study intervention and had solicited safety data available during the specified period. Thus, the stated population is based on receipt of intervention and availability of safety data for that period. How participants without available solicited safety data were handled in the reported analysis is not reported in the registry record.

The endpoint lists a 95% confidence interval as its dispersion measure. However, the percentage and its interval are not reported in the registry record. Without those estimates, the size and precision of the observed primary endpoint cannot be assessed from the record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes a randomized parallel-group design. Randomization is a method for assigning participants to study groups by chance, which can help make groups comparable at the start of a trial. It does not guarantee that the groups are identical, and it does not replace reporting the outcomes and analysis. The record gives the allocation as randomized and the model as parallel. It does not report the randomization sequence method, allocation concealment procedures, or any stratification factors.

What does a percentage endpoint represent?

The primary measure is the percentage of maternal participants reporting any of the specified administration site events during the stated period. For a binary outcome, each participant is classified according to whether the event occurred under the endpoint definition. The percentage describes the proportion with the event in the analysis population. It is different from a count of event episodes: the endpoint is about participants reporting any event, not the total number of symptoms or repeated reports.

The definition combines erythema, pain and swelling into an “any” outcome. Pain qualifies regardless of intensity grade, while erythema and swelling qualify at the stated surface-diameter threshold. A combined endpoint can provide a compact summary of several specified events, but the combined result would not identify which component accounted for reports. Component-specific results are not reported in the registry record.

How should a confidence interval be understood?

In general, a confidence interval describes statistical uncertainty around an estimate under the assumptions of the analysis. A narrower interval indicates greater precision than a wider interval, while an interval does not remove limitations in the data or analysis. The endpoint record specifies a 95% confidence interval as the dispersion measure. The actual estimate and interval are not reported in the registry record, so their precision cannot be evaluated.

The record says the primary analysis population is the Solicited Safety Set, made up of maternal participants who received one dose and had solicited safety data available in the relevant period. This is important because the denominator for a percentage depends on who is included. The registry does not report how missing solicited data were handled or whether any additional analysis populations were used for the primary comparison.

What do the posted safety counts show?

The serious adverse event figures present affected participants and participants at risk for maternal and infant groups. In general, counts and denominators are needed to understand how frequently events were recorded; a count on its own is not a percentage. Even with counts and denominators, interpretation depends on the event definitions, follow-up, and analysis approach. The registry does not report a statistical comparison or confidence intervals for these serious adverse event figures. No adjusted or causal interpretation can be assigned to them from the counts alone.

The other adverse event figures are listed separately, but the event-category definition and analysis details are not reported in the registry record. The serious-event terms listed include neonatal jaundice, premature baby, pre-eclampsia, low birth weight baby, small for dates baby, and neonatal sepsis, with percentages reported for each term. The record does not give denominators or further context for those term-specific percentages. They should therefore be read as descriptive entries, not as a comparative analysis.

What analysis plan is reported?

The registry lists no analyses. It does not report the statistical test for the primary comparison, whether any adjusted model was planned, the handling of multiple comparisons, or any interim analysis plan. The sample-size assumptions described for planning illustrations are not the trial’s own design justification: they use a two-sided significance level of 0.05, equal allocation, and a normal approximation, with an effect defined as a stated fraction of the observed effect. Such planning illustrations are not observed power, and they do not supply a missing trial result. The trial’s own sample-size justification is not reported in the registry record.

Statistical interpretation must therefore distinguish design information from analysis results. The randomized comparison and endpoint definition describe how the question was framed; they do not provide an estimated effect. With no analyses listed and no primary percentage or confidence interval reported, the primary comparison’s magnitude and precision remain unavailable.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 46 outcome measures, including the primary maternal administration site endpoint and multiple secondary maternal and infant outcomes. When a study evaluates many outcomes, the chance of finding an apparently unusual result can increase if each is tested separately. Statistical methods may address this through a prespecified hierarchy, adjustment, or other approach. The multiplicity strategy for this study is not reported in the registry record, and no analyses are listed. It is therefore not possible to determine how any family of comparisons was intended to be interpreted.

The primary endpoint population is defined as maternal participants who received one dose and had solicited safety data available during the specified period. The record does not describe the handling of missing endpoint data, incomplete follow-up, or participants who did not meet that analysis-set description. It also does not report sensitivity analyses or imputation methods. These omissions matter because percentages depend on the analysis population and the treatment of unavailable observations.

The registry lists the number of outcome measures but does not supply reported results for the primary endpoint or analyses for the listed outcomes. It also does not report a hierarchy of primary and secondary hypotheses beyond identifying the primary outcome. Consequently, the secondary outcomes should be understood as registered assessment topics, not as confirmatory findings.

The planning illustrations described in the statistical assumptions are distinct from the original study design and are not evidence of achieved power. The trial’s sample-size rationale is not reported in the registry record. Likewise, an analysis plan, any interim review of trial data, and any missing-data plan are not reported. The record’s statement that enrollment and vaccination stopped following a committee recommendation does not itself explain whether or how any planned statistical procedures changed.

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: jaundice neonatal (10.6%), premature baby (10.6%), pre-eclampsia (6.2%), low birth weight baby (6.1%), small for dates baby (6.1%).

The registry lists serious adverse event counts separately for maternal and infant groups. For mothers, the RSV MAT group has 33 affected participants among 113 at risk, and the control group has 11 among 56 at risk. For infants, the RSV MAT group has 35 affected among 132 at risk, and the control group has 21 among 66 at risk. The record also lists deaths as 0 for each maternal group and 1 for each infant group.

These are descriptive counts. The registry does not report a statistical comparison, confidence intervals, or adjustment for these figures. They should not be interpreted as establishing a difference between groups. Maternal and infant groups are distinct populations, and infant groups are defined by the maternal assignment. The event definitions, follow-up completeness, and details underlying the at-risk denominators are not reported in the registry record.

The record lists serious-event terms with percentages: neonatal jaundice, premature baby, pre-eclampsia, low birth weight baby, small for dates baby, and neonatal sepsis. The denominators and further context for these term-specific percentages are not reported in the registry record. These entries do not provide a complete safety profile or a causal assessment.

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

The study’s primary endpoint is a measure of short-term maternal administration site reactogenicity, defined using solicited reports of erythema, pain and swelling. The registry specifies how events qualify and identifies the analysis population, but it does not report the endpoint percentage or its confidence interval. The central primary comparison therefore cannot be quantified from the registry record.

The registry also describes a broad set of maternal and infant safety, illness, hospitalization and immune-response outcomes. Their presence in the record explains what the study set out to assess; it does not establish that any outcome occurred at a particular frequency or that the intervention changed it. No analyses are listed, so no comparative conclusion about these outcomes can be made from the posted results.

The serious adverse event figures provide counts for maternal and infant groups, and deaths are listed for the infant groups. Those entries are descriptive and are not accompanied by a reported statistical comparison or uncertainty interval. They should not be treated as proof of a difference, proof of equivalence, or a complete accounting of all safety findings.

The trial’s termination is part of the context for interpreting what was registered. The record says enrollment and vaccination stopped following a recommendation from the Independent Data Monitoring Committee of NCT04605159, while ongoing participants continued to be monitored. It does not state the recommendation’s rationale or attribute the stoppage to a result in this trial. Termination alone does not identify the direction or meaning of any treatment effect.

Planning illustrations based on an observed effect, where available in a separate planning display, are not observed power and do not replace an estimate with uncertainty. The illustrations are not the trial’s original sample-size rationale. In the absence of primary outcome results, the appropriate statistical reading is that the study question and methods are described, while the primary effect and its precision are not reported in the registry record.

13. Why This Trial Matters Statistically

The study illustrates the difference between a registered clinical question and a reported statistical result. The record gives a defined primary endpoint, a specified maternal safety population, randomized allocation, and the comparison with placebo. Those details allow readers to understand what was intended to be measured. Without the endpoint estimate and its confidence interval, however, readers cannot assess the observed frequency or precision of the primary outcome.

It also highlights the importance of keeping maternal and infant populations distinct. The infant groups are defined by the maternal group, while the primary endpoint concerns maternal participants. A count in one population cannot be substituted for an outcome in the other, and event definitions or denominators should not be assumed to be interchangeable.

The trial’s termination adds a second interpretive issue. Enrollment and vaccination stopped after a committee recommendation associated with NCT04605159, but the reason is not reported for this record. A reader cannot infer from termination alone whether the decision reflected safety, efficacy, operational considerations, or another factor. Reporting that context alongside the absence of analyses helps prevent an unsupported explanation of why the trial stopped.

Finally, the record distinguishes outcome listings from outcome findings. It names several maternal and infant measures, but provides no analyses. The serious adverse event counts and term-specific percentages offer descriptive entries, not a complete comparative analysis. This makes transparent reporting of denominators, missing-data handling, analysis populations, and uncertainty especially important for interpreting a trial that did not continue enrollment and vaccination as planned.

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 primary endpoint result and its specified confidence interval are not reported in the registry record. The registry lists no analyses, despite listing 46 outcome measures. This prevents assessment of the primary comparison’s magnitude, precision, or statistical uncertainty and limits interpretation of the secondary outcomes.

The record gives no explanation of the monitoring committee recommendation that led GSK to stop enrollment and vaccination. It says ongoing participants continued to be monitored, but does not report how many observations were available at each follow-up, whether follow-up was completed, or how stopping affected the outcome analyses. The reason for termination must not be inferred from the status alone.

There is a registry inconsistency in the description of masking: the official title calls the study double-blind, while the masking field records none. The information reported does not clarify which description reflects the conduct of the study or how masking was implemented.

For the posted serious adverse event figures, the record does not report a statistical comparison, uncertainty intervals, or detailed event and follow-up definitions. The other adverse event category is not further defined in the registry record. The listed serious-event terms have percentages but no stated denominators or additional context. These entries cannot replace detailed, population-specific safety analyses.

The registry does not report the original sample-size justification, multiplicity approach, missing-data methods, or a full statistical analysis plan. The stated Solicited Safety Set definition gives some information about the primary analysis population, but does not explain how unavailable data or other deviations were handled. Together, these gaps constrain reproducibility and the strength of interpretation possible from the record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: A Study on the Safety and Immune Response to an Unadjuvanted RSV Maternal Vaccine, in High Risk Pregnant Women Aged 15 to 49 Years and Infants Born to the Vaccinated Mothers — NCT04980391.
• Registry record last updated 2025-04-20; status last verified 2025-03.
• Results first posted 2024-02-06.
• 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

NCT04980391 is recorded as a Phase III randomized parallel-group study of a single intramuscular dose of unadjuvanted RSV MAT vaccine versus placebo in high-risk pregnant women aged 15 to 49 years, with assessments involving infants born to vaccinated mothers. The registry records enrollment as 384 and the status as terminated. The brief summary states that enrollment and vaccination stopped following a recommendation from the Independent Data Monitoring Committee of NCT04605159, while ongoing participants continued to be monitored.

The primary endpoint is the percentage of maternal participants reporting any solicited administration site event from Day 1 to Day 7 included. The registry describes its event criteria and analysis population but does not report the endpoint result. It lists 46 outcome measures and no analyses. Selected serious adverse event counts and terms are present, but comparative analyses are not reported. The record does not provide the rationale for stopping, the original sample-size justification, or details of missing-data and multiplicity methods.