Human Challenge Model Refinement With Enterotoxigenic Escherichia Coli Strain B7A: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Human Challenge Model Refinement With Enterotoxigenic Escherichia Coli Strain B7A (NCT02773446), a randomized na study. The primary outcome is Number of Participants With Safety- Solicited Symptoms Related to Challenge Administration.

2. Clinical Question

The registered aim was to refine a human challenge model for the B7A strain, which expresses CS6. The central comparison was therefore about challenge conditions, rather than a comparison of a treatment with a placebo or usual care. The design varied the stated dose and fasting duration in Cohort 1, then described a later cohort using a regimen determined through analysis after Cohort 1. The record does not report the exact decision rule used to select that regimen.

3. Trial Design

The registry classifies the study as randomized, single masked, and parallel. It lists the phase as NA and the condition as Healthy Volunteer. The sponsor was Johns Hopkins Bloomberg School of Public Health. The study is marked completed, with a start date of 2016-04 and primary completion in 2016-12.

Cohort 1 had four groups. Group A received 8 logs of B7A after an overnight fast; Group B received 9 logs after a 90 minute fast; Group C received 9 logs after an overnight fast; and Group D received 10 logs after a 90 minute fast. These assignments create variation in dose and fasting duration. The registry does not describe the randomization sequence, allocation concealment, or how participants were assigned among those groups.

Cohort 2 was described as following analysis of Cohort 1. Group A comprised subjects from Cohort 1 who met the primary endpoint, and Group B comprised naive subjects. Both groups were to receive the optimal regimen as determined after Cohort 1. The record does not identify that regimen or specify the operational definition of “optimal.” Because Cohort 2 includes both participants from the earlier cohort and naive subjects, its group descriptions should be read as a staged design rather than assumed to represent entirely separate populations.

The record lists enrollment of 47. That figure should not be used to infer group totals or the number of unique participants across cohorts, since Cohort 2 Group A draws from Cohort 1. No protocol details beyond the registered descriptions are available in the registry record. In particular, reasons for the chosen dose levels, the order of assignments, masking procedures, and any changes to the design 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: Cohort 2 group A consists of 11 subjects from Cohort 1. These subjects were re admitted for a second admission. The total number of subjects enrolled was 47.

First Admission

Reasons for not completing this period:

Second Admission

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: Cohort 2 group A consists of 11 subjects from Cohort 1. These subjects were re admitted for a second admission. The total number of subjects enrolled was 47.

Only the first 6 of 7 groups are shown.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants With Safety- Solicited Symptoms Related to Challenge Administration, measured over 6 days post-challenge.

The primary endpoint is the number of participants with safety-solicited symptoms related to challenge administration, assessed over 6 days post-challenge. The symptoms listed are vomiting, abdominal pain, bloating, lightheadedness, anorexia, generalized myalgia, arthralgias, abdominal cramping, constipation, nausea, malaise, headache, and flatulence.

This is a participant-count endpoint: it concerns how many participants experienced at least a solicited symptom, not the total number of symptom episodes. A person with more than one listed symptom would still contribute as a participant with symptoms if the outcome is counted in the registered manner. The registry does not give further operational details, such as how symptom severity was graded, how a symptom was judged related to challenge, or whether an episode had to meet a threshold to count.

Solicited symptom monitoring is used in safety assessment because it asks about specified symptoms within a defined observation window, rather than relying only on spontaneous reports. For a challenge study, that structure can make symptom ascertainment more systematic across dosing groups. The registry record identifies this endpoint as primary, but does not report its group results. It is therefore not possible to describe the observed primary endpoint pattern or make a statistical comparison from the posted record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a participant count describe?

The primary endpoint is a count of participants with at least one solicited symptom related to challenge administration during 6 days post-challenge. A participant-level binary outcome places each person in one of two categories for this endpoint: symptoms recorded or no symptoms recorded under the endpoint definition. The group count is the number in the first category. It is not the number of symptom episodes, and it does not indicate how many different symptom types a participant experienced.

For a binary endpoint, a group’s count is usually interpreted alongside the number of participants assessed. That denominator defines the observed proportion, while a comparison between groups can describe a difference or ratio in event frequency. Any such calculation depends on matching the endpoint definition, observation period, and analysis population across groups. Here, the registry record does not report the primary endpoint counts or denominator details for that outcome, so no primary-endpoint proportion or contrast can be stated.

Why do confidence intervals matter?

In general, a confidence interval expresses uncertainty around an estimated effect. It gives a range of values compatible with the data and the method used, rather than presenting the estimate as exact. A narrow interval indicates greater precision under the assumptions of that method; a wide interval indicates less precision. An interval can be useful even when an estimate is not statistically distinguishable from no difference, because it shows which effect sizes remain compatible with the data.

No confidence interval for the primary endpoint is reported in the registry record. The adverse-event counts are presented descriptively, without intervals or a stated model. The observed counts therefore should not be read as precise estimates of the rates that would occur in a broader population or under other challenge conditions.

What can randomization and masking establish?

In general, random allocation is intended to balance measured and unmeasured participant characteristics across assigned groups, on average. Its value for interpretation depends on the actual allocation process, adherence, outcome ascertainment, and analysis. The record classifies allocation as randomized, but does not describe the sequence generation, concealment, or allocation procedures. It is not possible to evaluate those implementation details from the registry record.

The registry labels the study single masked. Masking can reduce the influence of expectations on behavior or outcome assessment, depending on who is masked and how outcomes are collected. The record does not state which parties were masked or how masking was maintained. Thus, the label alone does not establish that participants, staff, or assessors were all unaware of group assignment.

How should the staged cohorts be understood?

The four Cohort 1 groups vary in the stated dose and fasting duration. Cohort 2 is described as receiving the regimen chosen after Cohort 1 analysis. That sequence is important: Cohort 2 is not simply another set of concurrent groups assigned among the original four conditions. One Cohort 2 group consists of people from Cohort 1 who met the primary endpoint, while the other consists of naive subjects. Comparisons involving these groups may reflect differences in cohort history and selection as well as the challenge regimen. The record does not report an analysis that separates these influences.

In general, a study that selects a regimen after examining earlier cohort results needs clearly stated selection criteria and analysis methods for interpreting later data. Those details are not reported in the registry record. The phrase “optimal regimen” is used in the group descriptions, but the record does not define the optimization target or show the analysis that selected it.

What does the sample-size planning assumption mean?

In general, a sample-size illustration is a planning calculation, not a finding about participants who were observed. The record describes planning assumptions using a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and an effect defined as a stated fraction of the observed effect. Such calculations depend on the chosen effect size and assumptions. They do not establish that the trial was designed with those assumptions, nor do they measure the chance that an observed result is true.

The record explicitly characterizes this calculation as a planning illustration based on the observed effect, not the trial’s own design. No sample-size justification for the actual study is reported in the registry record. Because the primary endpoint has several categories or time points and is not reduced to one comparison, the registered record does not provide a single primary comparison to which a planning illustration can straightforwardly be tied. Any planning figure should therefore be treated as conditional on its stated assumptions, not as evidence of observed power or proof that a study was adequately sized.

Why do outcome definitions and analysis populations matter?

A statistical result is interpretable only when the outcome definition and analysis population are clear. For a symptom endpoint, relevant details include how symptoms were elicited, what counted as related to challenge, how missing assessments were handled, and which participants were included. The registry lists the solicited symptoms and observation window, but does not report those additional rules or the primary-endpoint analysis population.

The record lists 6 outcome measures and 0 analyses. This does not mean that no data were collected; it means the registry record does not contain posted analyses to interpret. The absence of a reported primary-endpoint estimate prevents assessment of its direction, magnitude, and precision. It also prevents a check of how the recorded endpoint relates to the separate adverse-event summaries.

10. Multiplicity, Missing Data and Analysis Populations

The record lists 6 outcome measures, including secondary outcomes concerning immune response to challenge and serology. It also describes multiple challenge regimens and a later cohort, while the primary endpoint itself involves several symptoms. Testing or comparing multiple outcomes, groups, or time points can increase the chance of finding an apparently notable result by chance if no multiplicity strategy is specified. The registry does not report a multiplicity adjustment, testing hierarchy, or other plan for handling multiple comparisons. The number of analyses listed is 0.

The registry does not report how missing symptom assessments or other missing outcome data were handled. It does not state whether participants with incomplete follow-up were excluded, counted as having no symptoms, or treated by another method. Missingness can affect interpretation when the chance of an unrecorded outcome differs across participants or groups. Without the number of assessed participants for the primary endpoint and the missing-data rules, the posted record does not establish how complete the endpoint information was.

Analysis populations are also not specified. The record does not identify whether the primary analysis included all randomized participants, only participants who received challenge, or a different population. It does not give details on adherence to the assigned regimen or exclusions after allocation. These omissions prevent evaluation of whether the analysis preserved the randomized comparison.

The staged design adds another interpretive issue: Cohort 2 included participants selected from Cohort 1 based on meeting the primary endpoint, along with naive subjects. The registry does not describe an analysis plan for this selection or how the immune-response and rechallenge outcomes were to be compared. No inferential analyses for these outcomes are reported in the registry record.

11. Safety Results

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

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: pulmonary embolism (5.3%).

The serious adverse-event summary records affected participants and participants at risk by group. It lists 0 serious-event participants among 7 at risk in each Cohort 1 group, 0 among 11 in Cohort 2 Group A, and 1 among 19 in Cohort 2 Group B. The record lists pulmonary embolism as a top serious term at 5.3%. It does not specify in the posted summary which group the term arose in, nor does it report whether the event was considered related to challenge.

The other-adverse-event counts are separate: 5 among 7 in Cohort 1 Group A, 3 among 7 in Group B, 7 among 7 in Group C, 3 among 7 in Group D, 4 among 11 in Cohort 2 Group A, and 11 among 19 in Cohort 2 Group B. These figures are descriptive group counts, not a test of whether one regimen is safer than another. They should not be substituted for the primary solicited-symptom outcome, whose results are not reported in the registry record.

The registry record does not provide event-level descriptions, timing, severity details beyond the serious-event classification, or causality assessments for these group summaries. Death fields are not reported as counts. The available figures therefore characterize the posted adverse-event summary but do not establish the cause of an event or provide a complete safety profile.

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

The results that can be described are limited to the registry’s adverse-event counts and the registered study design. The record shows that investigators planned a randomized challenge study that varied dose and fasting duration in Cohort 1, then used a regimen selected after Cohort 1 analysis in Cohort 2. It also records serious and other adverse-event counts by group. Those observations are descriptive and do not amount to an inferential comparison of regimens.

The primary endpoint was intended to measure participants with solicited symptoms related to challenge over 6 days post-challenge. Its results are not reported in the registry record. No effect estimate, uncertainty interval, or statistical test is posted for that endpoint. Therefore, the available record cannot establish the observed frequency of primary endpoint symptoms in each group, the size or direction of any difference, or the precision of a comparison.

The record also does not report the quantitative immune-response findings or an analysis of homologous protection after rechallenge. The stated study aims should not be mistaken for posted results. Similarly, the label “optimal regimen” does not reveal which regimen was selected or demonstrate what criterion was used to select it.

In general, a randomized design can support causal comparisons when allocation and analysis are appropriate, outcomes are measured consistently, and missing data and post-allocation events are handled transparently. The registry record does not include enough methodological or outcome information to assess those conditions fully. The event counts alone do not establish that a regimen caused or prevented symptoms or serious events. They also should not be generalized beyond the studied challenge setting without supporting evidence.

13. Why This Trial Matters Statistically

This study is statistically notable for its staged challenge design. Cohort 1 varied both the stated B7A dose and fasting duration, and Cohort 2 was described as using a regimen selected after analysis of the earlier cohort. That structure raises distinct questions about how a regimen is chosen, how the later cohort is interpreted, and whether selection based on earlier outcomes affects subsequent comparisons.

The primary endpoint is a participant count for a specified set of solicited symptoms within a defined observation period. Such an endpoint is more interpretable when the symptom list, attribution rules, assessment schedule, denominator, and missing-data handling are all reported. The registry supplies the symptom list and timeframe, but does not report primary endpoint results or those additional analysis details. This illustrates why a clearly named endpoint is not enough on its own to evaluate a study’s findings.

The adverse-event summary demonstrates the difference between descriptive safety counts and a comparative analysis. Counts by group can indicate what the registry recorded, but without uncertainty estimates, a stated analysis population, and an inferential method, they do not quantify the reliability of between-group differences. The serious-event term is also reported without a group attribution in the posted summary.

Finally, the record separates study aims from posted evidence: it describes planned immune-response and rechallenge assessments, while listing no analyses. For clinical researchers, that distinction is important. A registry can document a question and a design without supplying the results needed to evaluate the answer. Transparent reporting of endpoint results, analysis choices, and cohort selection would be needed for statistical interpretation beyond the descriptive 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 central limitation is that the registry record does not report results for the primary endpoint. Although it defines the solicited symptoms and observation window, it does not provide group counts, analysis populations, estimates, or uncertainty intervals for that endpoint. The posted adverse-event summaries cannot fill this gap because the record does not establish that those fields correspond to the primary endpoint definition.

The registry lists 0 analyses and does not report the statistical analysis plan. It is therefore unclear how the intended analyses handled multiple symptoms, multiple outcome measures, or the staged cohorts. No multiplicity strategy, missing-data method, or primary analysis population is reported. The randomization and single-masking labels are present, but procedural details about allocation and masking are not reported in the registry record.

Cohort 2 introduces further uncertainty because one group was drawn from Cohort 1 participants who met the primary endpoint, while the other was naive. The record does not specify the selected regimen, define “optimal,” or provide a detailed comparison method for these populations. The group summaries should not be interpreted as a simple direct comparison without those details.

For safety, the record provides affected-participant counts but little context on event timing, severity beyond serious classification, attribution, or completeness of follow-up. The pulmonary embolism term is listed at 5.3%, but its group attribution and relationship to challenge are not stated in the summary. The record does not report whether every participant completed the relevant assessments.

The enrollment figure and group-specific at-risk counts are not sufficient to reconstruct the analysis population for every outcome. In particular, Cohort 2 Group A draws from Cohort 1, so group counts should not be combined to infer unique participants. These reporting limitations constrain both safety interpretation and conclusions about dose or fasting effects.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Human Challenge Model Refinement With Enterotoxigenic Escherichia Coli Strain B7A — NCT02773446.
• Registry record last updated 2018-07-06; status last verified 2018-04.
• Results first posted 2018-07-06.
• 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

NCT02773446 is a completed randomized, single-masked, parallel human challenge study of ETEC strain B7A in healthy volunteers. The registry lists enrollment of 47, a start date of 2016-04, and primary completion in 2016-12. The study aimed to identify a safe and optimal dose and fasting regimen and to assess immune response and homologous protection after rechallenge.

Cohort 1 included four groups varying B7A dose and fasting duration. Cohort 2 included participants from Cohort 1 who met the primary endpoint and naive subjects receiving a regimen selected after analysis of Cohort 1. The primary endpoint was participants with safety-solicited symptoms related to challenge during 6 days post-challenge.

The registry record lists 6 outcome measures and 0 analyses. It reports group-level serious and other adverse-event counts, but does not report primary endpoint results, immune-response analyses, or the regimen selected for Cohort 2. Statistical interpretation is therefore limited to the design descriptions and descriptive adverse-event figures recorded.