This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record. Numerical trial results on this page are restricted to the results posted in the ClinicalTrials.gov record; calculations added by Clinical Biostats are labelled as such.
1. Trial at a Glance
This page covers the registry results for Trial to Evaluate the Safety, Tolerability, and Immunogenicity of A Multivalent Group B Streptococcus Vaccine When Administered Concomitantly With Tdap in Healthy Nonpregnant Women (NCT04766086), a randomized phase 2 study. The primary outcome is Percentage of Participants Reporting Local Reactions Within 7 Days After Vaccination.
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT04766086 |
| Phase | Phase 2 |
| Condition | Group B Streptococcus Infections |
| Allocation | Randomized |
| Design model | Parallel |
| Masking | Double |
| Enrollment | 306 |
| Arms in the primary outcome | n/a |
| Lead sponsor | Pfizer |
| Results posted | Yes |
2. Clinical Question
Population
The trial enrolled healthy nonpregnant women 18 through 49 years of age. The safety population included participants who received at least 1 dose of a study intervention and had at least 1 valid post-dose safety assessment. For the primary outcome, the registry describes the analyzed number as the participants evaluable for that measure.
Intervention
The investigational intervention was a multivalent group B streptococcus vaccine, called GBS6 in the arm descriptions. One group received GBS6 together with Tdap. The study evaluated safety, tolerability, and immunogenicity when the vaccine was administered concomitantly with Tdap.
Comparator
The parallel groups were GBS6 with Tdap, GBS6 with placebo, and placebo with Tdap. These groups allow the study to assess outcomes in the setting of the vaccine combination and the two single-active-intervention regimens. The registry record does not report a primary-outcome comparison estimate between these groups.
Primary question
The listed primary outcome is the percentage of participants reporting local reactions from Day 1, the day of vaccination, through Day 7. Reactions included redness, swelling, and pain at the injection site of the left arm. The record describes how reactions were graded, but numerical results for this primary outcome and a statistical comparison are not reported in the registry record.
3. Trial Design
This was a phase 2B, placebo-controlled, randomized, observer-blinded trial with a parallel-group model. The registry labels masking as double. The study enrolled 306 participants and is listed as completed. The recorded start date is 2022-08-12, and the primary completion date is 2023-04-27. Pfizer is listed as the sponsor.
The three arm descriptions distinguish the vaccine combination from the regimens containing one active intervention: GBS6 with Tdap, GBS6 with placebo, and placebo with Tdap. This structure can support separate assessments of the investigational vaccine and Tdap in the assigned combinations. However, the record does not provide an allocation ratio, arm-specific enrollment totals for the whole trial, or details about how randomization was generated or implemented. Those details are not reported in the registry record.
The stated purpose was to evaluate safety, tolerability, and immunogenicity of the multivalent group B streptococcus vaccine administered concomitantly with Tdap in healthy nonpregnant women 18 through 49 years of age. The listed primary outcome specifically concerns local reactions during the 7 days after vaccination. It is therefore important to distinguish the broad study objectives from the outcome selected as primary in the posted record.
The registry lists 7 outcome measures and 10 analyses, but does not supply the primary endpoint results in the information reported. A detailed protocol, statistical analysis plan, sample-size justification, and explanation of the intended role of each analysis are not reported in the registry record. The available design description is sufficient to identify the randomized groups and primary safety question, but not to reconstruct the full analysis plan.
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: A total of 306 participants were enrolled and randomized in the study, of whom 304 were vaccinated.
Overall Study
| Stage | GBS6 + Tdap | GBS6 + Placebo | Placebo + Tdap |
|---|---|---|---|
| STARTED | 103 | 99 | 104 |
| Vaccinated | 102 | 99 | 103 |
| COMPLETED | 99 | 93 | 98 |
| NOT COMPLETED | 4 | 6 | 6 |
Reasons for not completing this period:
| Reason | GBS6 + Tdap | GBS6 + Placebo | Placebo + Tdap |
|---|---|---|---|
| Lost to Follow-up | 3 | 5 | 3 |
| Withdrawal by Subject | 1 | 1 | 2 |
| Other | 0 | 0 | 1 |
Analysis population for the primary outcome, as defined in the registry: Safety set included all participants who received at least 1 dose of the study interventions and had at least 1 valid post dose safety assessment. Here, "Number of Participants Analyzed" signifies number of participants evaluable for this outcome measure.
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 set included all participants who received at least 1 dose of the study interventions and had at least 1 valid post dose safety assessment.
| Characteristic | Unit | GBS6 + Tdap | GBS6 + Placebo | Placebo + Tdap | Total |
|---|---|---|---|---|---|
| Participants analysed | 102 | 99 | 103 | 304 | |
| Age, Continuous | Years | 35.4 (SD 8.54) | 34.8 (SD 8.60) | 35.5 (SD 8.59) | 35.3 (SD 8.55) |
| Sex: Female, Male: Female | Participants | 102 | 99 | 103 | 304 |
| Sex: Female, Male: Male | Participants | 0 | 0 | 0 | 0 |
| Ethnicity (NIH/OMB): Hispanic or Latino | Participants | 8 | 12 | 16 | 36 |
| Ethnicity (NIH/OMB): Not Hispanic or Latino | Participants | 93 | 86 | 86 | 265 |
| Ethnicity (NIH/OMB): Unknown or Not Reported | Participants | 1 | 1 | 1 | 3 |
| Race (NIH/OMB): American Indian or Alaska Native | Participants | 1 | 3 | 0 | 4 |
| Race (NIH/OMB): Asian | Participants | 3 | 5 | 1 | 9 |
| Race (NIH/OMB): Native Hawaiian or Other Pacific Islander | Participants | 0 | 0 | 0 | 0 |
| Race (NIH/OMB): Black or African American | Participants | 18 | 19 | 19 | 56 |
| Race (NIH/OMB): White | Participants | 80 | 68 | 75 | 223 |
| Race (NIH/OMB): More than one race | Participants | 0 | 4 | 4 | 8 |
| Race (NIH/OMB): Unknown or Not Reported | Participants | 0 | 0 | 4 | 4 |
6. Primary Endpoint
The registry lists the primary outcome as Percentage of Participants Reporting Local Reactions Within 7 Days After Vaccination, measured over Day 1 (day of vaccination) to Day 7.
| Element | Registry entry |
|---|---|
| Result type | Number |
| Measure of spread | 95% Confidence Interval |
| Unit | Percentage of participants |
| Endpoint classification | Binary Endpoint |
The primary endpoint is the percentage of participants reporting local reactions within 7 days after vaccination, from Day 1 through Day 7. The reactions were redness, swelling, and pain at the injection site of the left arm, recorded by participants in an electronic diary. Redness and swelling were measured using a measuring device, with 1 measuring device unit equal to 0.5 centimeter.
The grading rules distinguish reaction severity. Redness and swelling were graded according to measured size, while pain was graded by whether it interfered with activity, prevented daily activity, or led to an emergency room visit or hospitalization. Grade 4 reactions were classified by an investigator or medically qualified person. These definitions specify how participant reports and measurements map to the endpoint.
A percentage is useful for describing how common a defined event was among the participants evaluable for that outcome. The confidence interval, when reported, would convey uncertainty around that percentage rather than only presenting a point estimate. The registry record identifies a 95% confidence interval as the dispersion measure, but does not report the endpoint percentages or their confidence intervals. Consequently, the frequency and precision of local reactions cannot be described from the posted results available in this record.
7. Why No Re-analysis Is Shown
8. Statistical Interpretation
The trial’s primary outcome is a participant-level measure: whether a participant reported at least one local reaction during the specified post-vaccination period. The intended summary is the percentage of evaluable participants with an event, accompanied by a 95% confidence interval. The registry record supplies the definition and assessment window, but not the observed percentages, confidence intervals, or a between-group analysis for this outcome. Thus, the primary endpoint’s magnitude and precision are not reported in the registry record.
The three assigned groups are GBS6 with Tdap, GBS6 with placebo, and placebo with Tdap. Their design makes the treatment context relevant to interpreting a reaction rate: a reported group percentage would describe participants assigned to that particular combination. Without the primary endpoint results, however, no direction or size of a group difference can be described. The posted data do not support a statement about whether reaction frequency was higher or lower in any group.
The safety entries report serious adverse events in the three groups as 0 of 102, 0 of 99, and 1 of 103, respectively. Deaths are reported as 0 in each group. The record also lists bile duct stone at 1.0% among the top serious adverse event terms, without enough information in the available entry to assign that term to a particular arm. These are descriptive safety counts; they are not a substitute for the unreported primary local-reaction results.
The entries for other adverse events list 81 affected participants among 102 at risk in the GBS6 and Tdap group, 71 among 99 in the GBS6 and placebo group, and 79 among 103 in the placebo and Tdap group. The registry record does not provide the event terms or an analysis that would explain these aggregate counts. They should therefore be read as reported counts, not as a measure of a specific reaction or as a formal comparison.
For a binary percentage, the confidence interval describes uncertainty in the event proportion under the statistical method used. General interpretation: a wider interval indicates less precision, while a narrower interval indicates greater precision; neither alone establishes a treatment effect. No intervals for the primary outcome are reported in the registry record, so its precision cannot be assessed. The record also does not report p-values, adjusted comparisons, or a conclusion based on the primary endpoint. Interpretation must remain limited to the design and the safety counts that are actually posted.
9. Statistical Methods Explained
The registry identifies the primary outcome as a percentage of participants with local reactions and lists a 95% confidence interval as the dispersion measure. The outcome is binary at the participant level: a participant either met the event definition during the assessment window or did not. The posted description defines the window as Day 1 through Day 7 and specifies the types of local reaction assessed. The registry record does not provide the primary outcome counts or interval estimates, so the analysis result cannot be reconstructed from the available information.
What does a confidence interval convey?
General explanation: a confidence interval expresses the statistical uncertainty around an estimated quantity. For a percentage, it gives a range of values compatible with the observed data under the method’s assumptions. It is not a guarantee that the true value lies within the interval, and it does not by itself determine whether a difference is important. A point estimate and interval are most informative when read together with the outcome definition, denominator, and analysis population. Here, the registry names the intended interval level but does not report the primary estimate or interval.
Why does the denominator matter?
The primary outcome population is described as the safety set: participants who received at least 1 dose of a study intervention and had at least 1 valid post-dose safety assessment. The record further says that the number analyzed for this outcome means participants evaluable for that measure. These definitions matter because a percentage depends on which participants are included in its denominator. The record does not report the number evaluable for the primary outcome in each arm or explain how participants with incomplete diary data were handled.
How should the randomized groups be understood?
Randomization assigns participants to groups according to a chance-based process, helping reduce systematic differences in expectation between groups. General explanation: the benefits of randomization depend on the allocation process and on how participants are analyzed after assignment. The registry says the trial was randomized and parallel, but the allocation sequence, concealment procedures, and analysis strategy are not reported in the registry record. The listed arms are GBS6 with Tdap, GBS6 with placebo, and placebo with Tdap. Their labels describe assigned regimens; they do not establish any outcome result on their own.
What does masking contribute?
The record labels masking as double and describes the study as observer-blinded. General explanation: masking can reduce the influence of expectations on outcome assessment or reporting, particularly when outcomes depend on participant reports or clinical judgment. The local reactions were recorded by participants in an electronic diary, and the record describes investigator or medically qualified person classification for Grade 4 reactions. The registry does not specify which people were masked, how masking was maintained, or whether it was assessed. Those implementation details are not reported in the registry record.
What do the adverse event counts show statistically?
Counts and denominators are descriptive summaries. The serious adverse event entries give the number affected and the number at risk for each arm; the death entries give zero for every arm. General explanation: sparse event counts can make estimates of between-group differences imprecise, and the absence of an event in a group does not demonstrate that the underlying risk is zero. No confidence intervals or formal comparisons for these safety counts are reported in the registry record. The aggregate other-adverse-event counts also do not identify which events contributed to the totals.
What analysis choices remain unknown?
The registry record does not report the statistical model, the method used to construct the primary outcome confidence intervals, whether comparisons were adjusted, or any planned handling of multiple outcomes. It also does not report a sample-size justification or interim analyses. The posted sample-size assumptions describe a planning illustration based on an observed effect, using a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. That illustration is not the trial’s design justification, and its numerical outputs are not reported in the registry information. It must not be interpreted as observed power or as evidence that the trial could detect a particular effect.
General explanation: an analysis plan determines which comparison is primary, how uncertainty is quantified, and how missing observations or multiple tests are handled. Without those details and without the primary endpoint results, readers cannot verify the inferential procedure or determine whether a reported comparison would account for the design. The record allows a clear description of the endpoint and groups, but does not support a complete statistical reconstruction.
10. Multiplicity, Missing Data and Analysis Populations
The registry lists 7 outcome measures and 10 analyses. Multiple outcomes and analyses can raise multiplicity concerns when many hypotheses are tested, because the chance of at least one apparently notable result can increase across a collection of tests. General explanation: an analysis plan may specify one primary comparison, a hierarchy of outcomes, or an adjustment procedure. The registry record does not report whether multiplicity was controlled, which analyses were confirmatory, or how the listed analyses relate to the primary outcome.
The primary measure is described as a percentage of participants reporting local reactions during the post-vaccination assessment window. The record states that the number analyzed is the number evaluable for this outcome, but does not report arm-specific primary endpoint denominators or explain how missing diary entries were handled. The extent and pattern of missing outcome data are therefore not reported in the registry record. It is also not reported whether participants with partial diary completion were included, excluded, or handled under another rule.
The safety set is defined as participants who received at least 1 dose and had at least 1 valid post-dose safety assessment. That definition identifies a population for safety assessment, but it does not establish the analysis population used for every outcome or analysis. The registry record does not report an intention-to-treat population, a per-protocol population, or the relationship between any such populations and the reported analyses. Without the analysis populations and missing-data procedures, primary outcome percentages and comparisons cannot be interpreted beyond their stated endpoint definition.
11. Safety Results
The registry reports serious adverse events as affected participants divided by participants at risk.
| Arm | Participants with serious adverse events | Participants at risk | Percentage |
|---|---|---|---|
| GBS6 + Tdap | 0 | 102 | 0.0% |
| GBS6 + Placebo | 0 | 99 | 0.0% |
| Placebo + Tdap | 1 | 103 | 1.0% |
Deaths
| Arm | Deaths (participants affected of at risk) |
|---|---|
| GBS6 + Tdap | 0 of 102 |
| GBS6 + Placebo | 0 of 99 |
| Placebo + Tdap | 0 of 103 |
Other (non-serious) adverse events
Participants with at least one non-serious event at or above the registry's 0% reporting threshold.
| Arm | Participants affected of at risk |
|---|---|
| GBS6 + Tdap | 81 of 102 |
| GBS6 + Placebo | 71 of 99 |
| Placebo + Tdap | 79 of 103 |
Most frequent serious adverse event terms
| Event term | Organ system | GBS6 + Tdap | GBS6 + Placebo | Placebo + Tdap |
|---|---|---|---|---|
| Bile duct stone | Hepatobiliary disorders | 0 of 102 | 0 of 99 | 1 of 103 |
Most frequent non-serious adverse event terms
| Event term | Organ system | GBS6 + Tdap | GBS6 + Placebo | Placebo + Tdap |
|---|---|---|---|---|
| Fatigue (FATIGUE) | General disorders | 52 of 102 | 50 of 99 | 48 of 103 |
| Headache (HEADACHE) | Nervous system disorders | 36 of 102 | 48 of 99 | 40 of 103 |
| Injection site pain (PAIN) | General disorders | 40 of 102 | 45 of 99 | 34 of 103 |
| Myalgia (MUSCLE PAIN) | Musculoskeletal and connective tissue disorders | 31 of 102 | 24 of 99 | 27 of 103 |
| Diarrhoea (DIARRHEA) | Gastrointestinal disorders | 20 of 102 | 16 of 99 | 22 of 103 |
| Vomiting (NAUSEA/VOMITING) | Gastrointestinal disorders | 15 of 102 | 17 of 99 | 15 of 103 |
| Arthralgia (JOINT PAIN) | Musculoskeletal and connective tissue disorders | 18 of 102 | 15 of 99 | 13 of 103 |
| Injection site erythema (REDNESS) | General disorders | 8 of 102 | 2 of 99 | 0 of 103 |
| Injection site swelling (SWELLING) | General disorders | 4 of 102 | 4 of 99 | 2 of 103 |
| Pyrexia (FEVER) | General disorders | 1 of 102 | 4 of 99 | 3 of 103 |
Serious events with the highest arm-level rate: bile duct stone (1.0%).
The registry reports serious adverse event counts by group: 0 of 102 in the GBS6 and Tdap group, 0 of 99 in the GBS6 and placebo group, and 1 of 103 in the placebo and Tdap group. Deaths are reported as 0 in all three groups. The record lists bile duct stone at 1.0% among the top serious adverse event terms, but the available information does not identify the group associated with that term.
These figures are counts within the reported at-risk groups. General explanation: a count of zero means no event was recorded in that group during the reported assessment, not that the possibility of such an event is excluded. A single event also provides limited information for comparing groups, particularly when no uncertainty interval or statistical comparison is reported. The registry does not provide the serious-event definitions, observation duration, event narratives, or a comparative safety analysis in the available record.
The other-adverse-event entries report 81 affected participants among 102 at risk, 71 among 99, and 79 among 103, in the same group order. They are aggregate counts, and event terms for those totals are not reported in the registry record. They should not be treated as counts of local reactions or confused with the serious adverse event figures.
12. What the Result Does, and Does Not, Mean
The registry record establishes the trial’s purpose, design, assigned groups, and primary local-reaction outcome definition. It also reports selected adverse event counts. Those details permit a description of what was assessed and how the groups were arranged, but the primary endpoint’s numerical results are not reported in the registry record. No conclusion about the observed frequency of local reactions or differences between arms can be drawn from the information posted.
The trial’s title and summary state that safety, tolerability, and immunogenicity were evaluated. The primary outcome listed in the record addresses local reactions during the 7 days after vaccination; it is not itself a complete measure of every safety outcome or of immunogenicity. The available information does not provide numerical immunogenicity results, a broader set of safety results, or an overall analysis linking those objectives.
The serious adverse event counts and deaths are descriptive. They do not, without further information about event definitions, follow-up, and uncertainty, establish that a regimen is safer or less safe than another. Similarly, the aggregate other-event counts cannot be interpreted as a specific adverse reaction because the event terms are not reported alongside those totals. The bile duct stone entry is listed among top serious event terms, but the record does not identify its arm in the available information.
General explanation: a randomized design can support comparisons when outcomes, populations, and analysis methods are reported in sufficient detail. Randomization alone does not supply missing results, show that an observed difference is precise, or answer questions not captured by the endpoint. The registry record does not report the primary estimates, confidence intervals, p-values, or analysis plan needed to evaluate the primary question. The conclusions that can be drawn are therefore limited to the trial structure and the specific counts that are posted.
13. Why This Trial Matters Statistically
This trial is statistically informative as an example of why endpoint definitions and outcome reporting need to be read separately. The primary measure is not a general judgment of tolerability: it is a defined percentage of participants reporting local reactions at a specified injection site during a specified post-vaccination period. The record also describes grading rules for redness, swelling, and pain. Those details make clear what a reported endpoint would represent, but they cannot replace the missing numerical result.
The three-arm structure distinguishes the vaccine-plus-Tdap regimen from GBS6 with placebo and placebo with Tdap. This makes the comparator context central to any interpretation: an estimate for one arm is not automatically a direct comparison with every other arm. General explanation: when a trial includes multiple groups and outcomes, readers need to know which contrasts were planned and how multiplicity was addressed. The registry record does not report those planned contrasts or any multiplicity strategy.
The record’s safety population definition also highlights a practical issue in clinical trial analysis. A denominator should match the outcome and analysis population, and the registry says that the number analyzed for the primary outcome refers to participants evaluable for that measure. But arm-specific primary denominators and reaction counts are not reported. The aggregate adverse event counts cannot fill that gap because they do not identify the primary local-reaction outcome.
Finally, the reported serious event and death counts illustrate the limits of sparse descriptive data. General explanation: small counts can be clinically important to document while still providing little precision for comparing risks. The record does not report comparative estimates or uncertainty intervals for these safety entries. Statistical interpretation therefore depends not only on the randomized design, but also on complete outcome reporting and transparent analysis details.
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 central limitation for interpreting the primary question is that the registry record does not report the local-reaction percentages, confidence intervals, or between-group comparisons. The endpoint definition and 95% confidence interval measure are listed, but the numerical outcomes needed to assess magnitude and precision are absent. The reason those results are absent is not reported in the registry record.
The record lists 7 outcome measures and 10 analyses, but does not provide the full analysis plan, identify the planned primary contrasts, or describe any multiplicity handling. It does not report the sample-size justification, interim analyses, statistical models, or methods for constructing the confidence intervals. The sample-size assumptions are described as a planning illustration based on an observed effect, not as the trial’s own design justification; numerical planning outputs are not included in the available information.
Information about missing observations is also limited. Although the safety set and the evaluable-outcome count are defined, the number evaluable for the primary measure in each arm and the handling of incomplete diary records are not reported. The record does not describe whether outcome assessment differed across participants or how incomplete safety assessments were treated beyond the stated safety-set definition.
For adverse events, group counts are available, but the record does not provide event narratives, observation duration, comparative uncertainty estimates, or event terms for the aggregate other-event totals. The listed bile duct stone term is not assigned to an arm in the available entry. These gaps constrain interpretation of both the primary endpoint and the safety summaries; they do not establish that an unreported result was favorable or unfavorable.
15. Related Tutorials
16. Related Statistical Calculators
17. Sources
- ClinicalTrials.gov: Trial to Evaluate the Safety, Tolerability, and Immunogenicity of A Multivalent Group B Streptococcus Vaccine When Administered Concomitantly With Tdap in Healthy Nonpregnant Women — NCT04766086.
- Registry record last updated 2024-05-24; status last verified 2024-04.
- Results first posted 2024-05-24.
- 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.
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18. Record Summary
NCT04766086 is a completed randomized, placebo-controlled, observer-blinded phase 2B parallel-group trial sponsored by Pfizer. It enrolled 306 healthy nonpregnant women 18 through 49 years of age. The recorded start date is 2022-08-12, and the primary completion date is 2023-04-27.
The arms were GBS6 with Tdap, GBS6 with placebo, and placebo with Tdap. The listed primary outcome was the percentage of participants reporting local reactions from Day 1 through Day 7, with a 95% confidence interval specified as the dispersion measure. The registry record does not report numerical results for that primary outcome or its group comparisons.
Serious adverse event counts were 0 of 102, 0 of 99, and 1 of 103 across the three arms, and deaths were 0 in each arm. The record also reports aggregate other-adverse-event counts and lists bile duct stone at 1.0% among top serious terms. Full statistical methods, missing-data handling, and multiplicity procedures are not reported in the registry record.