Shigella Flexneri 2a Invaplex 50 Vaccine Dose Finding and Assessment of Protection: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Shigella Flexneri 2a Invaplex 50 Vaccine Dose Finding and Assessment of Protection (NCT00485134), a randomized phase 1, phase 2 study. The primary outcome is Post-challenge Diarrhea, Fever, and Blood in Stool Adverse Events by Study Group.

2. Clinical Question

3. Trial Design

NCT00485134 is recorded as a completed, randomized, double-blind, parallel phase 1 and phase 2 study. Enrollment is listed as 113. The recorded study start is 2007-05 and primary completion is 2009-07. The sponsor is the U.S. Army Medical Research and Development Command.

The study had a dose-finding stage followed by a challenge stage. Stage 1 included Dolphin-administered vaccine groups at 240 µg, 480 µg, and 690 µg, and a 240 µg electronic-pipette group identified for lot bridging. The arm descriptions mention sterile saline placebo for the Dolphin groups. They describe the investigational vaccine as a liquid in a glass vial; additional preparation details in some arm descriptions are incomplete in the registry record.

Stage 2 was to administer the selected dose using the Dolphin and the Stage 1 vaccination schedule. The immunized group and non-immunized controls were to receive the Shigella challenge strain approximately 42 days after their last vaccination. The challenge followed a 90-minute fast. The registry states that participants progressing to challenge had no limiting adverse events and a positive immune response, and that they received 800 colony forming units (CFU) of Shigella flexneri 2a strain 2457T.

These design details describe the planned sequence and the registry's recorded conduct, but they do not establish how many participants completed each stage or were included in an outcome analysis. Those analysis-population details 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.

Only the first 6 of 8 groups are shown.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: The decision criteria to progress to challenge with the Shigella challenge strain were no limiting adverse events (AEs) and positive immune response. These individuals were challenged with 800 colony forming units (CFU) of Shigella challenge strain, Shigella flexneri 2a strain 2457T.

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.

Only the first 6 of 9 groups are shown.

6. Primary Endpoint

The registry lists the primary outcome as Post-challenge Diarrhea, Fever, and Blood in Stool Adverse Events by Study Group, measured over 7 days after challenge.

The primary outcome is titled “Post-challenge Diarrhea, Fever, and Blood in Stool Adverse Events by Study Group.” It is measured in participants, and the registry classifies it as a number of participants with an event. The stated timeframe is 7 days after challenge. The outcome therefore concerns clinical events after the challenge, rather than an immune-response measure alone.

As a general statistical point, clinically meaningful outcomes can help describe how participants fare in terms that are relevant to the study question. When an outcome includes several event categories, however, the categories need to be interpreted on their own terms unless a clearly specified composite definition and analysis are reported. The registry title names diarrhea, fever, and blood in stool, but does not give a single effect estimate or a breakdown that permits an outcome comparison.

The record also says fecal samples were collected through day 77 or until discharge, with all stools collected for weighing and grading, a maximum of 3 stools per day for culture, and a rectal swab obtained if no stool was provided. This collection description is distinct from the stated primary-outcome timeframe of 7 days after challenge. The registry does not explain the relationship between the collection schedule and the primary endpoint window.

The registry record does not report primary-outcome counts, a treatment contrast, or an inferential analysis. It is therefore not possible to describe the posted record as establishing the direction or precision of a difference in this endpoint.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry records randomization, double masking, and a parallel design. Those labels describe broad design features, but do not supply the details needed to reproduce an analysis. The allocation procedure, masking procedures, analysis populations, and statistical methods for the primary outcome are not reported in the registry record.

What does randomization contribute?

As a general statistical principle, random allocation is intended to make assigned groups comparable in expectation with respect to characteristics that could affect outcomes. It does not guarantee that groups are identical in a particular study, and it does not by itself show that the final outcome groups remained comparable. Interpreting a randomized comparison also requires knowing who was assigned, who received the assigned intervention, who was followed, and who was included in each analysis. Those details are not reported for the primary outcome in this registry record.

What does masking mean for interpretation?

As a general concept, masking can reduce the influence of knowledge about assignment on care, outcome assessment, and reporting. The record labels the study double-blind, but does not identify which people were masked or describe how masking was maintained. The design label therefore cannot answer whether outcome assessment was vulnerable to knowledge of group assignment.

Why does the outcome definition matter?

The primary outcome title names diarrhea, fever, and blood in stool, while the registry classifies the measure as a number of participants with an event. For a multi-category outcome, an analysis might report each category separately or use a defined composite, but these approaches answer different questions. A composite requires clear rules about which events count and how the endpoint is summarized. The registry does not report those rules or a single primary-outcome comparison. Its stated reason for not reducing the endpoint to one comparison is that it has several categories or time points.

As a general statistical principle, the outcome window and observation process also matter. The registry states a primary-outcome timeframe of 7 days after challenge, while its specimen-collection description extends through day 77 or until discharge. Collection procedures include weighing and grading all stools, culture of a maximum of 3 stools per day, and a rectal swab if no stool was provided. The record does not explain how these collection details feed into the primary endpoint or how observations outside the stated timeframe were handled.

How should estimates and uncertainty be read?

As a general explanation, an effect estimate describes the size and direction of a comparison, while a confidence interval communicates the uncertainty around that estimate under the analysis assumptions. A p-value addresses how compatible the data are with a specified null model; it does not measure the size or clinical importance of an effect. These quantities must be interpreted alongside the outcome definition, analysis population, and design. The registry record reports no primary-outcome estimate, confidence interval, or p-value, so none can be interpreted for this study from the record.

Counts alone also need context. A count affected is informative only alongside its denominator and the definition of the event. Even when both are listed, differences between groups can arise by chance, and formal comparison requires a specified method. The posted serious and other adverse-event counts are descriptive; the registry does not report inferential testing for them.

What do the planning assumptions mean?

The registry describes sample-size illustrations as planning calculations based on the observed effect, with 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. For time-to-event calculations, the illustrations concern total events; other illustrations concern participants per arm. These are planning assumptions, not evidence about the trial's observed power. The numerical illustration results and the observed effect used for them are not reported in the registry record. The record also does not report the trial's own sample-size justification, so the planning assumptions should not be mistaken for that justification.

As a general principle, a calculation based on an observed effect is conditional on that estimate and its uncertainty. It does not demonstrate that a study was adequately powered, nor does it establish a treatment effect. Because the primary-outcome results and effect estimate are absent, the planning illustrations cannot be evaluated against a reported primary result.

10. Multiplicity, Missing Data and Analysis Populations

The study lists a primary outcome with multiple clinical categories and several secondary outcomes. Those secondary outcomes include loose stool occurrences, volumes, and durations; non-diarrheal clinical outcomes related to S. Flexneri 2a; and immune responses to Invaplex 50 and/or LPS. Evaluating multiple outcomes can raise multiplicity concerns because testing more hypotheses can increase the chance of at least one apparently notable result. As a general statistical principle, interpretation depends on prespecified outcome priorities and any planned adjustment or other strategy for handling multiple comparisons.

The registry record does not report a multiplicity strategy, a detailed statistical analysis plan, or the results of analyses for the listed outcomes. It also does not report how missing outcome data were handled, whether participants were analyzed according to assignment or another population definition, or how withdrawals and incomplete follow-up affected each analysis. These omissions prevent assessment of the primary analysis population and the robustness of any outcome estimate.

The primary outcome is not reduced to a single comparison in the registry record because it has several categories or time points. Without category-specific counts and a declared analysis approach, it is not possible to determine whether an apparent result would reflect a prespecified endpoint, one of multiple examined outcomes, or a particular time point. The record does not report whether interim analyses occurred or how any such analyses would have been addressed; that information is not reported in the registry record.

These are reporting gaps, not evidence that a particular analysis was or was not conducted. They mean the registry entry alone does not permit a full assessment of multiplicity, missing data, or analysis-population choices.

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: pregnancy (3.6%).

The registry lists serious adverse events, other adverse events, and deaths for Stage 1: Group A, Dolphin 240 µg; Stage 1: Group B, Dolphin 480 µg; Stage 1: Group C, Dolphin 690 µg; Stage 2: Immunized; and Stage 2: Controls. Serious adverse events are recorded as 0 among 12 at risk in each listed Stage 1 Dolphin group, 0 among 41 at risk in Stage 2 immunized participants, and 1 among 28 at risk in Stage 2 controls. Deaths are recorded as 0 for each listed group.

For other adverse events, the registry records 10 affected among 12 at risk for Stage 1: Group A, 11 among 12 for Stage 1: Group B, 11 among 12 for Stage 1: Group C, 41 among 41 for Stage 2: Immunized, and 28 among 28 for Stage 2: Controls. These are counts, not a severity scale or a comparison of risk. The record does not give a detailed definition of “other” adverse events or an inferential analysis.

The Stage 1 pipette group is not included in the listed adverse-event figures. The record also lists Pregnancy as the top serious-event term at 3.6%, without enough accompanying context to establish attribution or interpret its denominator. These descriptive entries should not be treated as a complete comparative safety analysis.

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

The registry record establishes that NCT00485134 was a completed, randomized, double-blind, parallel study with a dose-finding stage and a planned Shigella challenge stage. It records an enrollment of 113 and describes the dose groups, challenge conditions, progression criteria, and several outcomes of interest. It also gives adverse-event counts for selected groups.

Those details do not establish whether the vaccine reduced post-challenge diarrhea, fever, or blood in stool. The primary outcome is named, but its group-level results and statistical analysis are not reported. The record also does not report results for the listed secondary outcomes. Consequently, the magnitude, direction, and precision of a clinical outcome comparison cannot be determined from this registry record.

The adverse-event counts should be interpreted as descriptive entries for the groups and event categories listed. They do not establish that groups differed in safety, and they do not show whether a particular event was caused by vaccination or challenge. The serious-event entry for Pregnancy is especially limited because the record does not provide enough context to assess attribution or the applicable denominator.

As a general statistical point, absence of a reported result is not the same as evidence of no effect. Likewise, a completed study record and an enrollment figure do not reveal how many participants contributed data to a particular endpoint. Neither efficacy nor lack of efficacy should be inferred from the missing primary-outcome analysis. The registry record is informative about design and selected safety counts, but does not answer the clinical protection question quantitatively.

13. Why This Trial Matters Statistically

This study is statistically notable for combining dose selection with a later challenge stage. The registry describes multiple vaccine doses and administration approaches in Stage 1, then a selected dose and non-immunized controls in Stage 2. That sequence makes the distinction between dose-finding evidence and challenge-outcome evidence important: a dose-selection aim and a clinical protection aim are not interchangeable questions.

The eligibility rule for proceeding to challenge also matters for interpretation. The registry states that progression required no limiting adverse events and a positive immune response. As a general principle, results among participants meeting a post-vaccination criterion describe that selected group; they do not automatically represent everyone enrolled or randomized. The record does not report the numbers or analysis populations needed to assess this issue for the primary outcome.

The endpoint itself comprises multiple clinical categories, and the record does not reduce it to a single comparison. This illustrates why clear endpoint definitions and a prespecified strategy for multiple outcomes are important: different summaries can answer different questions, and a registry title alone is not enough to identify the actual analysis.

Finally, the study demonstrates the distinction between descriptive safety counts and comparative evidence. Counts at risk and counts affected provide useful context, but without the primary clinical results, analysis methods, and uncertainty estimates, they cannot establish a protection effect or a comparative safety conclusion. The registry entry identifies the research question and design, while leaving the central outcome comparison unresolved in the reported record.

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 principal trial-specific limitation of the registry record is the absence of the primary-outcome results. The endpoint is named and its timeframe is stated, but group-level counts, effect estimates, confidence intervals, and p-values are not reported. The record also does not report analyses for the secondary outcomes it lists. It therefore does not permit assessment of clinical outcome differences or their precision.

The outcome description combines diarrhea, fever, and blood in stool, and the registry indicates that the outcome has several categories or time points and is not reduced to one comparison. Category definitions, any composite rule, and the analysis approach are not reported. The record gives a primary timeframe of 7 days after challenge alongside a stool-collection description extending through day 77 or until discharge, without explaining the relationship between these timeframes.

The entry gives an enrollment figure but does not report participant flow or endpoint-specific analysis populations. It does not describe missing-data handling, withdrawals, adherence, or the statistical analysis plan. Although randomization and double masking are recorded, procedural details are not reported. The adverse-event figures omit the Stage 1 pipette group, and the Pregnancy term is listed without sufficient context for attribution or denominator interpretation.

The registry record does not report the trial's sample-size justification or whether interim analyses occurred. The listed planning assumptions are described as illustrations based on an observed effect, but the numerical results and underlying effect estimate are not reported. These gaps limit both reproducibility and the strength of conclusions that can be drawn from the registry entry alone.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Shigella Flexneri 2a Invaplex 50 Vaccine Dose Finding and Assessment of Protection — NCT00485134.
• Registry record last updated 2017-09-11; status last verified 2017-08.
• Results first posted 2017-09-11.
• 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

NCT00485134 is recorded as a completed randomized, double-blind, parallel phase 1 and phase 2 study sponsored by the U.S. Army Medical Research and Development Command. Enrollment is listed as 113, with study start 2007-05 and primary completion 2009-07.

The study aimed to select a safe, immunogenic dose of intranasal Shigella flexneri 2a Invaplex 50 vaccine and assess protection after challenge. Its primary outcome concerns post-challenge diarrhea, fever, and blood in stool adverse events. The registry describes challenge progression criteria and records serious, other, and fatal adverse-event counts for selected groups.

Primary and secondary outcome analyses are not reported in the registry record. The listed safety counts are descriptive and do not provide a comparative efficacy analysis or a full safety interpretation.