A Study to Evaluate Different Dose Levels of GlaxoSmithKline (GSK) Biologicals' Investigational Respiratory Syncytial Virus (RSV) Vaccine (GSK3888550A), Based on the Vaccine Safety and the Antibodies (Body Defences) Produced Following Vaccine Administration, When Given to Healthy Non-pregnant Women: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study to Evaluate Different Dose Levels of GlaxoSmithKline (GSK) Biologicals' Investigational Respiratory Syncytial Virus (RSV) Vaccine (GSK3888550A), Based on the Vaccine Safety and the Antibodies (Body Defences) Produced Following Vaccine Administration, When Given to Healthy Non-pregnant Women (NCT03674177), a randomized phase 1 study. The primary outcome is Number of Subjects With Any and Grade 3 Solicited Local Adverse Events (AE) During a 7-day Follow-up Period.

2. Clinical Question

This was an early clinical evaluation of GSK3888550A, an investigational vaccine intended for maternal use. The registry summary explains that it was the first time the vaccine had been used in humans and that the study was conducted in non-pregnant women before testing in pregnant women. The central question was therefore about safety, reactogenicity, and immune response across different dose levels, rather than whether vaccination prevented RSV infection.

The primary endpoint focused on symptoms at the injection site: pain, redness, and swelling. It included any occurrence regardless of intensity and Grade 3 occurrences. The record defines Grade 3 pain as significant pain at rest or pain that prevented normal everyday activity. For redness or swelling, the listed definitions distinguish symptoms by surface diameter, with any redness or swelling defined as greater than 20 millimeters and Grade 3 redness or swelling as greater than 100 millimeters.

The randomized comparison included three vaccine groups and a saline control group. The registry record does not report a single combined result for the primary endpoint, so the clinical question cannot be answered from one overall comparison in the posted results. The endpoint definitions and the separate serious adverse-event counts still clarify what kinds of safety information the record contains.

3. Trial Design

This was a phase 1, randomized, triple-masked, parallel-group study sponsored by GlaxoSmithKline. Its status is listed as completed. Enrollment was 502. The registry record lists a study start date of 2018-10-30 and a primary completion date of 2019-04-16.

Participants were assigned to one of three investigational vaccine formulation groups or a control group. The vaccine groups received a single injection on Day 1: formulation 1 at 30 µg, formulation 2 at 60 µg, or formulation 3 at 120 µg. The control group received a single placebo injection of normal saline. In each group, the injection was intramuscular and given in the deltoid region of the non-dominant arm.

The reported group sizes in the serious adverse-event summary are 124 for formulation 1, 126 for formulation 2, 126 for formulation 3, and 126 for control. These are the denominators associated with that safety summary; the record does not describe how many participants contributed data to every other endpoint. The primary endpoint analysis population is defined as the Exposed Set, consisting of subjects with study vaccine administration documented.

The study’s stated purpose was to assess safety, reactogenicity, and immune response at different dose levels. The registry record names neutralizing antibody titers against RSV serotype A and anti-RSVPreF3 immunoglobulin G antibody concentrations among the secondary outcomes. It also lists the number of subjects with serious adverse events. The record does not provide the randomization method, allocation concealment procedures, or the identities of the parties included in triple masking. It also does not report the scheduled follow-up for the secondary outcomes in the information summarized in the registry record.

The primary safety window was defined as the day of vaccination and the six subsequent days. The endpoint was based on solicited local symptoms reported during that period. No broader claims about follow-up duration should be inferred from this endpoint window, because other follow-up schedules are not reported in the registry record.

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: Among 579 screened subjects in this study, 77 subjects were screen failure. 502 subjects were enrolled.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: The analysis was performed on the Exposed Set, which included all subjects with the study vaccine administration documented.

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 Number of Subjects With Any and Grade 3 Solicited Local Adverse Events (AE) During a 7-day Follow-up Period, measured over During a 7-day follow-up period (i.e., on the day of vaccination and 6 subsequent days).

The primary endpoint counts participants with solicited local adverse events during a 7-day follow-up period: the day of vaccination and six subsequent days. The symptoms assessed were pain, redness, and swelling at the injection site. The endpoint distinguishes any occurrence, regardless of intensity, from Grade 3 events.

For redness and swelling, an event counted as any symptom when its surface diameter was greater than 20 millimeters. Grade 3 redness or swelling was defined as a surface diameter greater than 100 millimeters. Grade 3 pain meant significant pain at rest or pain that prevented normal everyday activity. These definitions make the endpoint more informative than an undifferentiated count of symptoms: they distinguish whether a symptom occurred from whether it reached the specified severe-intensity threshold.

Solicited local symptoms are used to characterize reactogenicity, meaning short-term reactions assessed after vaccination. In general, a defined observation window and specified symptom criteria help make safety assessments more consistent across participants and treatment groups. This endpoint is a safety measure; it does not directly measure prevention of RSV infection or disease.

The registry record gives the endpoint definition but does not provide the category-specific participant counts or a single combined comparison for this primary outcome. Consequently, the magnitude and precision of the primary endpoint results cannot be described from the posted results summarized in the record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a participant count convey?

The primary endpoint is expressed as a count of participants with an event. For a binary outcome, each participant is classified according to whether the event occurred under the stated definition and observation period. A count is most interpretable alongside its denominator, because the same count can represent different event frequencies in groups of different sizes. The registry’s primary endpoint definition specifies the Exposed Set as the analysis population, while the serious adverse-event summary gives group-specific denominators.

In general, an event count is descriptive rather than a complete comparison. A comparison of groups would need to specify the outcome category, the analysis population, and the statistical method. For the primary endpoint, the registry record identifies any and Grade 3 local adverse events, but the posted result is not reduced to a single comparison. Category-specific results are not reported in the registry record.

Why distinguish event intensity?

The distinction between any symptom and Grade 3 symptoms separates occurrence from intensity. Any local symptom includes the event regardless of intensity, while Grade 3 uses the endpoint’s stated criteria for more severe symptoms. In general, reporting both can help readers understand different aspects of reactogenicity. The categories should not be treated as interchangeable, and one should not infer the number in either category from the other.

The outcome also specifies clinical criteria for pain, redness, and swelling. This helps define what counts as an event, including size thresholds for redness and swelling. The registry record states that redness or swelling greater than 20 millimeters qualifies as any symptom, while Grade 3 redness or swelling requires a surface diameter greater than 100 millimeters. These thresholds are part of the endpoint definition, not an estimate of how often such symptoms occurred.

What can randomization and masking contribute?

Random allocation is intended, in general, to support fair comparisons by assigning participants to groups without choosing a group based on their characteristics. Masking can reduce the influence of expectations on symptom reporting or assessment. This study is listed as randomized and triple-masked, but the registry record does not report the randomization sequence method, allocation concealment procedures, or which roles were masked. The design labels alone do not establish that every possible source of bias was eliminated.

The parallel-group model means the study groups were distinct rather than a crossover in which participants receive multiple assigned interventions in sequence. The record describes a single Day 1 injection for each group. It does not report further details about adherence or protocol deviations.

How should uncertainty be considered?

In general, an estimate describes the observed data, while its uncertainty communicates how much the estimate might vary under repeated sampling or related assumptions. A confidence interval is one common way to express uncertainty around an estimate. A p-value, when reported, addresses compatibility of data with a specified statistical null model; it is not a measure of clinical importance. The registry record lists no analyses and provides no primary-endpoint confidence interval or p-value in the results summarized for this account. Therefore, neither statistical significance nor the precision of a primary-endpoint comparison can be evaluated.

For binary outcomes, an analysis may compare participant proportions or model event status, but the selected approach and its assumptions matter. A normal approximation is one method sometimes used for planning or analysis; it relies on an approximation to the distribution of an estimate. The registry record’s planning description specifies a normal approximation, two-sided significance level 0.05, and equal allocation for its planning illustrations. Those assumptions describe the illustrations, not observed results or proof that the trial had a particular probability of detecting an effect.

What does over-dispersion mean in general?

Over-dispersion occurs when observed variability is greater than a statistical model expects. In general, ignoring it can make uncertainty estimates too small and statistical evidence appear stronger than warranted. It is relevant to some count-data models, particularly when events cluster or participants differ substantially in underlying event tendency. The endpoint in this trial is defined as participants with events, and the registry record does not report a model for over-dispersion or indicate that such a model was used.

How are planning illustrations different from trial findings?

In general, a sample-size planning illustration uses assumptions about an effect and a statistical procedure to show how many participants might be needed for a planned design. It is not an observed outcome, and it is not observed power. The registry record describes its planning illustrations as using a stated fraction of the observed effect, equal allocation between two arms, a two-sided significance level of 0.05, and a normal approximation. It states that non-time-to-event illustrations give participants per arm. The actual illustration values are not included in the registry entries summarized for this account, so no particular planning size or target can be stated.

Planning based on an observed effect can also be sensitive to the estimate used: in general, an effect observed in a study is uncertain and may not equal the effect anticipated in a future study. Such illustrations should be read as conditional calculations rather than evidence that a trial was adequately powered. The record does not report a sample-size justification or the assumptions used for the original trial design.

10. Multiplicity, Missing Data and Analysis Populations

The primary endpoint includes multiple local symptoms and separates any occurrences from Grade 3 occurrences. The registry record notes that the primary outcome has several categories or time points and is not reduced to one comparison. It does not report how the endpoint categories were prioritized for formal inference or how multiple comparisons were handled. No multiplicity adjustment or testing hierarchy is reported in the registry record.

The record names secondary outcomes that include serious adverse events, neutralizing antibody titers against RSV serotype A, and anti-RSVPreF3 immunoglobulin G antibody concentrations. It lists 15 outcome measures, but no analyses are listed. The statistical plan for these outcomes, including any control of false-positive findings across multiple outcomes, is not reported in the registry record. Without that information, any collection of outcome-specific findings would require cautious interpretation.

The endpoint analysis population is the Exposed Set, defined as all subjects with study vaccine administration documented. This is the analysis population stated for the primary endpoint; the registry record does not specify whether other analysis populations were used for secondary outcomes or immunogenicity analyses. The record also does not report the amount or pattern of missing endpoint data, reasons for missingness, rules for handling missing values, or any imputation method. Completeness of the 7-day symptom follow-up is not reported in the registry record.

In general, missing data can affect an estimate when the chance of missingness is related to participants’ outcomes or characteristics. The direction and size of any effect depend on the missing-data process and the analysis approach. Since those details are not reported, the degree to which missing data could affect the primary outcome cannot be assessed.

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: appendicitis (0.8%), diverticulitis (0.8%), umbilical hernia (0.8%).

The serious adverse-event summary gives counts and group denominators. It reports 0 of 124 participants affected in the RSV MAT Formulation 1 Group, 0 of 126 in the RSV MAT Formulation 2 Group, 1 of 126 in the RSV MAT Formulation 3 Group, and 2 of 126 in the Control Group. Deaths are reported as 0 in all four groups. These figures describe the entries in the registry safety summary; they are not adjusted comparisons.

Appendicitis, Diverticulitis, and Umbilical hernia are listed among the top serious adverse-event terms, each at 0.8%. The summary does not state which groups those terms occurred in, their timing, or whether the events were assessed as related to study intervention. These details are not reported in the registry record.

Serious adverse events should not be confused with the primary endpoint. The primary endpoint concerns solicited injection-site pain, redness, and swelling during the 7-day follow-up period, including any and Grade 3 events. The serious-event counts do not reveal the number of participants with those local symptoms. Nor do the counts alone show whether a difference between groups is meaningful: no inferential comparison or confidence interval for the serious-event figures is reported.

In general, rare-event counts are especially difficult to interpret from a single study without details about event definitions, follow-up, and uncertainty. The reported figures are useful as a descriptive record, but they cannot establish causation or provide a complete safety profile.

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

The record identifies the study as a randomized phase 1 evaluation of three dose levels of an investigational RSV maternal vaccine compared with saline in healthy non-pregnant women. Its primary endpoint was designed to count solicited local symptoms, with separate definitions for any and Grade 3 events during a 7-day period. The record also reports serious adverse-event counts by group and lists selected serious-event terms.

Those details describe the trial’s purpose and the available safety information. They do not provide the category-specific primary endpoint results or a single comparison for that outcome. As a result, the frequency of local reactions at each dose level and the precision of any dose-group difference cannot be stated from the registry information. The serious-event counts are a separate descriptive outcome and do not fill that gap.

The results also should not be interpreted as evidence that the vaccine prevents RSV infection. The primary endpoint is a safety and reactogenicity measure, and the registry record names antibody measurements as secondary outcomes without giving their numerical results in the information. No conclusion about protection, clinical benefit, or an optimal dose follows from the safety counts alone.

The study population also matters. The registry summary says the vaccine was being evaluated in healthy non-pregnant women before testing in pregnant women. The record therefore does not establish how the results apply to pregnant women or to other populations. Generalizability is limited by the population and trial context, and further information would be needed to characterize outcomes in other settings.

In general, interpretation depends on the outcome definition, analysis population, completeness of follow-up, and uncertainty around estimates. Several of those details are not reported for the primary endpoint in the registry record. The reported counts should be read as descriptive observations rather than definitive evidence for or against a treatment effect.

13. Why This Trial Matters Statistically

This trial is statistically notable for its early-development setting and its dose-ranging structure. The registry summary describes GSK3888550A as being used in humans for the first time and identifies the participants as healthy non-pregnant women before planned testing in pregnant women. A randomized comparison across three vaccine dose levels and a saline control can organize early safety and immune-response observations by assigned group.

The endpoint definition also illustrates why outcome specification matters. Local pain, redness, and swelling are not a single undifferentiated measure: the record distinguishes any symptom from Grade 3 events and defines thresholds for redness and swelling. Such distinctions can support consistent reporting, but they also create multiple outcome categories that require transparent reporting and careful interpretation.

The registry summary exposes an important distinction between a defined endpoint and a fully interpretable result. The primary outcome is clearly described, yet the available record does not reduce it to one comparison or give the category-specific participant counts in the information summarized. Without estimates and uncertainty, readers cannot judge the size or precision of group differences. This is a reporting limitation, not evidence that no analyses were conducted outside the registry record.

The serious adverse-event counts provide another statistical lesson: small counts are descriptive and should not be treated as proof of equivalence among groups. A zero count in a group does not establish that the underlying event risk is zero, and differences in sparse counts can be highly uncertain. The record does not report inferential estimates for these counts.

Finally, planning calculations are distinct from observed trial results. The registry describes illustrations based on the observed effect and specified statistical assumptions, but the original trial’s sample-size justification is not reported. Separating planning assumptions, observed outcomes, and inferential analyses is essential when evaluating what an early trial can establish.

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 main limitation for interpreting the primary endpoint is that its definition is reported without category-specific participant counts or a single comparison. The endpoint covers multiple symptoms and intensity categories, so the available information does not allow readers to establish how often each type of local event occurred or to quantify differences among the dose groups and control.

The registry record lists no analyses. It does not provide primary-endpoint effect estimates, confidence intervals, p-values, or a statistical model. It also does not report how multiplicity was addressed across endpoint categories or among the listed secondary outcomes. The absence of these details prevents assessment of statistical precision and the inferential framework; it does not establish that no analyses were performed outside the registry record.

The Exposed Set is defined for the primary endpoint, but the record does not describe the number of participants with complete symptom follow-up, the extent or reasons for missing data, or the handling of missing observations. The randomization method, allocation concealment, and the identities of the masked parties are also not reported in the registry record.

For serious adverse events, counts are provided, but the summary does not report event timing, relationship to intervention, or group attribution for the listed serious-event terms. The counts are sparse and no comparative uncertainty estimates are reported. This limits interpretation to description rather than causal conclusions.

The study population consists of healthy non-pregnant women, and the registry summary frames the trial as preceding testing in pregnant women. Findings cannot automatically be extended to pregnant women or other populations. Numerical results for the named antibody outcomes are not reported in the record information, so the immune-response objectives cannot be evaluated from these registry entries.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: A Study to Evaluate Different Dose Levels of GlaxoSmithKline (GSK) Biologicals' Investigational Respiratory Syncytial Virus (RSV) Vaccine (GSK3888550A), Based on the Vaccine Safety and the Antibodies (Body Defences) Produced Following Vaccine Administration, When Given to Healthy Non-pregnant Women — NCT03674177.
• Registry record last updated 2021-08-13; status last verified 2021-08.
• Results first posted 2020-04-15.
• 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

NCT03674177 is a completed, randomized, triple-masked, parallel-group phase 1 study sponsored by GlaxoSmithKline, with enrollment of 502. It evaluated three single-dose formulations of investigational RSV maternal vaccine GSK3888550A at 30 µg, 60 µg, and 120 µg against a saline placebo in healthy non-pregnant women.

The primary endpoint counted participants with solicited local adverse events, including any and Grade 3 pain, redness, or swelling during a 7-day follow-up period. The registry defines the endpoint and the Exposed Set but does not provide a single collapsed primary-outcome comparison in the results summarized. Serious adverse-event counts are reported by group, along with 0 deaths in each group. The record does not provide inferential analyses or numerical results for the named antibody outcomes in the information.