Evaluation of a New Ebola Vaccine Using a Short-interval Prime-boost Vaccination: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Evaluation of a New Ebola Vaccine Using a Short-interval Prime-boost Vaccination (NCT02485912), a randomized phase 1 study. The primary outcome is Safety and Tolerability of Administration of ChAd3-EBO Z and MVA-EBO Z 7 Days Later. This Will be Done by Recording the Number of Participants Who Experience Adverse Events and the Severity of Any Adverse Events..

2. Clinical Question

3. Trial Design

This was a randomized, parallel-group phase 1 trial with no masking reported. Its official title describes it as a phase Ib safety and immunogenicity trial. The registry lists an enrollment of 40, with 20 participants in Group 1 and 20 in Group 2 in the posted results. Randomization means the planned comparison assigns participants to groups by chance, although the method used to generate or conceal the allocation is not reported in the registry record.

The vaccination sequence was common to both groups: ChAd3-EBO Z followed by MVA-EBO Z 7 days later. Group 1 received both vaccinations in the same arm; Group 2 received the booster in the opposite arm. Thus, the randomized contrast addresses the administration-site arrangement for the booster within this prime-boost regimen. It does not compare the vaccine regimen with no vaccination, nor does it isolate the effects of either vaccine from the other.

The registry gives a start date of 2015-07 and a primary completion date of 2016-01, and lists the study as completed. The primary safety timeframe is 26 weeks. The safety description includes solicited local and systemic reactogenicity signs and symptoms for 7 days following vaccination, unsolicited adverse events for 28 days, change from baseline in safety laboratory measures, and serious adverse events during the whole study duration. The registry also describes immune responses measured through tests on blood samples, but posted secondary outcome results are not included in the record summarized by these results.

This is a small early-phase study in healthy volunteers. It is suited to descriptive evaluation of safety and immune responses and to comparing the two specified administration arrangements in this setting. The registry record does not report a sample-size justification, the randomization procedure, or a detailed statistical analysis plan. Those omissions limit what can be inferred about the intended design properties beyond the listed groups, outcomes, and posted results.

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.

Overall Study

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 Safety and Tolerability of Administration of ChAd3-EBO Z and MVA-EBO Z 7 Days Later. This Will be Done by Recording the Number of Participants Who Experience Adverse Events and the Severity of Any Adverse Events., measured over 26 weeks.

The primary endpoint is safety and tolerability after ChAd3-EBO Z followed by MVA-EBO Z 7 days later. The registry defines it through the number of participants experiencing adverse events and the severity of any adverse events, with a 26 weeks timeframe. Its more detailed safety description covers solicited local and systemic reactogenicity, unsolicited adverse events, laboratory changes from baseline, and serious adverse events. The specified collection windows differ across these safety components.

A participant count is useful because it expresses how many people in each group experienced the endpoint, rather than only reporting a group average. This makes an event-based outcome interpretable in a small safety study. The comparison reported for the primary outcome is a risk difference, which expresses the difference between the proportions experiencing the outcome in the two groups. The registry reports 20 participants for each group and a risk difference of 0.0.

The registry record does not clarify which adverse-event categories contributed to the primary count of 20 participants in each group, or how severity was combined into that count. In the separate safety summary, other adverse events are reported for 17 participants in Group 1 and 18 in Group 2, while serious adverse events are reported for no participants in either group. Those separate counts should not be assumed to define or fully explain the primary endpoint count. The endpoint addresses recorded safety observations during follow-up; it does not measure whether the regimen prevents Ebola infection or disease.

7. Results Posted in the Registry

• Group 2: 100.0% (20/20) versus 100.0% (20/20) on control; risk difference 0.0 percentage points (95% CI 0.0 to 0.0), risk ratio 1.00 (1.00–1.00); unadjusted two-proportion z-test p = n/a.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary comparison is reported as a risk difference for a binary outcome: whether a participant is counted as experiencing the primary safety outcome. For a binary outcome, each participant contributes to one of two states, event or no event. A group proportion summarizes the share of participants with the event. The registry reports proportions of 1.0 in both groups, and a risk difference of 0.0. This is a direct comparison on the absolute-proportion scale.

What does the risk difference express?

General statistical explanation: a risk difference subtracts one group's event proportion from the other group's event proportion. A value of zero means the displayed proportions are equal. Unlike a relative measure, it is expressed on the scale of the proportion itself. The sign depends on which group is subtracted from which; the record identifies Group 2 as the comparison against the listed control group. The reported result is 0.0, so the direction does not alter the displayed conclusion of equal proportions. No additional arithmetic is needed to interpret the posted estimate.

What does the p-value tell the reader?

General statistical explanation: a p-value describes how compatible the observed data are with a specified null model, under the assumptions of the test used. It is not the probability that the null hypothesis is true, nor a measure of clinical importance. The registry reports p = 1.0 for the comparison. The statistical test and its assumptions are not reported in the registry record. Because the event proportions are displayed as equal, the p-value indicates no statistical evidence of a difference in the posted comparison; it does not establish that the underlying risks are identical.

How should the interval be understood?

General statistical explanation: a confidence interval gives a range of values compatible with the data under a particular method and its assumptions. It conveys uncertainty in an estimate, rather than identifying a guaranteed range for the true effect. The registry reports a risk-difference interval from 0.0 to 0.0. This displayed interval has no width, which makes it impossible to see uncertainty from the interval itself. The interval method and the reason for its displayed endpoints are not reported in the registry record. It should therefore be presented as the posted interval, not treated as strong evidence that the true difference must be exactly zero.

What can randomization contribute?

General statistical explanation: random allocation is intended to reduce systematic differences between groups at the start of a trial, so that a comparison can be interpreted as a contrast between assigned strategies. The registry identifies the study as randomized and parallel, but does not report the allocation sequence method or concealment process. Randomization cannot make the groups identical in a small study, and it does not eliminate uncertainty in the estimated effect. In this trial the assigned strategies differed in whether the booster was administered in the same or opposite arm, while both groups received the same vaccine sequence and timing.

What does the design imply for analysis?

General statistical explanation: a parallel-group comparison analyzes outcomes according to the separate assigned groups, with each participant contributing observations from their allocated strategy. The registry reports 20 participants in each group for the primary outcome, but does not state how missing observations, protocol deviations, or analysis populations were handled. It also does not report an adjusted model or a prespecified analysis method. The primary outcome is a count of participants, so the posted estimate is a simple binary comparison rather than a mean difference for a continuous measurement.

The record describes a sample-size planning illustration based on a two-sided significance level of 0.05, equal allocation, and a normal approximation, with the effect to detect expressed as a fraction of the observed effect. It notes that time-to-event illustrations use a total event count based on the Schoenfeld approach and that other illustrations are expressed as participants per arm. These are planning conventions, not the trial's own documented design justification. No numerical planning output or sample-size justification is reported in the registry record, and such illustrations are not observed power.

The registry lists no analysis records. It reports a primary outcome and a secondary immunogenicity outcome, but does not state how multiplicity was handled or provide posted secondary outcome estimates in the record described by these results. General statistical explanation: when several outcomes or comparisons are considered, the chance of at least one apparently unusual result can increase; a prespecified approach to multiplicity can help readers interpret a set of tests. Whether any such approach was planned or used is not reported. Similarly, methods for missing data and for selecting participants into each analysis population are not reported. These gaps mean that the posted summary can be described, but the analysis cannot be fully reconstructed from the registry record.

10. Confidence Intervals and Precision

The primary result is reported as a risk difference of 0.0 with an interval from 0.0 to 0.0. The two group proportions are each reported as 1.0, and the p-value is 1.0. These are the registry's displayed values. The interval is degenerate at the precision shown: its lower and upper endpoints are the same. Although a confidence interval ordinarily communicates uncertainty around an estimate, this displayed interval does not show a range of plausible differences.

That lack of displayed width should be treated cautiously. The registry does not report the interval method, the calculation details, or an explanation for why the endpoints coincide. It is not appropriate to interpret the interval as demonstrating certainty or exact equivalence in the underlying populations. Rather, readers should distinguish the reported estimate from the uncertainty that would ordinarily be expected when comparing groups of this size. The posted result is a summary, and the record does not give enough methodological detail to assess how its interval was produced.

The group sizes are 20 participants each. Even with equal displayed counts, a small study can provide limited information about the range of possible underlying risks, especially for outcomes that occur infrequently. The separate serious adverse-event summary lists zero participants with a serious event in each group, but the registry does not provide an interval for those separate counts. Zero observed events do not show that a serious event is impossible; they mean that none was recorded among these participants during the reported study observation.

For other adverse events, the record reports 17 participants in Group 1 and 18 in Group 2. No confidence interval or inferential comparison for those counts is reported in the registry record. The equal primary proportions and the separate safety counts answer different descriptive questions and should not be conflated. Overall, the primary estimate is clear as posted, but the displayed interval gives little usable information about precision, and the record does not report enough detail to replace it with another estimate.

11. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary safety outcome and one posted secondary outcome description concerning immunogenicity. It reports two outcome measures in total and one secondary outcome posted, but the record described by these results contains no posted secondary outcome estimates. It also lists no analysis records. The registry record does not report whether the trial used a prespecified hierarchy of outcomes, adjusted statistical tests for multiple comparisons, or designated any analyses as exploratory.

Multiplicity matters when several outcomes, time points, or comparisons are examined. General statistical explanation: more testing creates more opportunities for a result to appear unusual by chance, so prespecified testing strategies can affect how p-values are interpreted. The available record does not establish whether multiple tests were planned or conducted beyond the outcome descriptions. It is therefore not possible to assess multiplicity control or to infer that a particular adjustment was used. The primary p-value should be read as the posted comparison, without assuming a broader family of tests or an unreported correction.

Missing-data handling is not reported in the registry record. The primary endpoint has a 26 weeks timeframe, but the record does not state whether all enrolled participants completed that follow-up, how incomplete observations were treated, or whether any imputation was used. It also does not identify a modified intention-to-treat, per-protocol, or other analysis population. The posted group counts are 20 in each arm; the record does not explain whether those counts reflect all randomized participants or a defined subset for each outcome.

These omissions are important for interpreting a small trial. If outcome availability differed by group, the observed counts could depend on who contributed data. Without completion and missingness details, readers cannot evaluate that possibility. The record also does not report whether deviations from assigned vaccination schedules occurred or how they would affect analysis. No particular missing-data method or analysis population should be assumed from the posted summary alone.

12. 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 non-serious adverse event terms

The serious adverse-event summary lists 20 participants at risk in each group, with zero serious adverse events in Group 1 and zero in Group 2. It also lists zero deaths in both groups. These figures mean that no serious adverse events or deaths were recorded among the listed participants in the registry summary. They do not mean that serious events cannot occur, or that the regimen has been shown to have no serious risks.

Other adverse events are reported for 17 participants in Group 1 and 18 participants in Group 2, each among 20 participants at risk. The record does not provide the specific event terms in the serious-event list, and none are listed. These other-event counts should not be equated with serious adverse events: they are reported in a separate category. Nor should they be treated as a complete account of the primary endpoint, because the primary outcome count is 20 in each group and its event composition is not reported in the registry record.

The study's safety description includes active and passive collection, solicited local and systemic symptoms, unsolicited adverse events, laboratory changes, and serious adverse events. The registry specifies different observation windows for some categories. The displayed summary does not provide participant-level event timing, severity distributions, or detailed attribution. Accordingly, it supports a limited descriptive reading of recorded events in this study, rather than a full assessment of all possible harms or a comparison adjusted for other factors.

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

The results describe a randomized comparison of two ways to administer the MVA-EBO Z booster after ChAd3-EBO Z priming. The primary safety outcome is reported for 20 participants in each group, with equal displayed proportions and a risk difference of 0.0. The registry reports p = 1.0 and an interval from 0.0 to 0.0. Separately, no serious adverse events or deaths are listed in either group; other adverse events are listed for 17 participants in Group 1 and 18 in Group 2.

These findings are descriptive of the enrolled healthy Senegalese volunteers and of the recorded follow-up. They do not establish that the administration arrangements are equivalent. A lack of a detected difference is not the same as evidence that any difference is small enough to be unimportant. The small group sizes and the interval reported with no displayed width limit the strength of conclusions about the underlying difference in risk. The registry does not report a noninferiority or equivalence margin, and no such objective should be inferred.

The study also does not compare vaccination with no vaccination. Both groups received the same vaccine sequence, and the randomized contrast was the arm used for the booster. The results cannot isolate the effect of either vaccine from the full regimen, nor do they establish prevention of Ebola Virus Disease. Although immunogenicity was a stated study aim, immune-response results are not included in the posted record.

No serious adverse events among these participants is a useful safety observation, but it cannot rule out uncommon events or characterize risks in broader populations. Likewise, the other-event counts do not by themselves establish a meaningful difference between groups. The primary outcome's exact event composition, detailed severity information, missing-data handling, and analysis methods are not reported. The most defensible conclusion is therefore bounded: the registry records equal primary-outcome proportions and no serious adverse events in either small group, while leaving substantial uncertainty about broader safety, precision, and immune response.

14. Why This Trial Matters Statistically

Statistically, this trial illustrates the difference between an equal observed result and a precise estimate of equality. The posted primary comparison has identical displayed proportions, a risk difference of 0.0, and p = 1.0. Yet the interval is reported as 0.0 to 0.0, with no displayed width, and the method producing that interval is not reported. Readers must avoid treating a point estimate or a p-value as a substitute for transparent uncertainty information.

The study also shows why outcome definitions matter in safety reporting. The primary count is 20 participants in each group, while the separate summary lists other adverse events for 17 and 18 participants and no serious adverse events in either group. Without a clear account of which event types and severity levels are represented in the primary count, different summaries cannot safely be treated as interchangeable. Careful separation of endpoint definitions helps prevent an apparently simple count from being overinterpreted.

The randomized design gives the comparison a clear target: same-arm versus opposite-arm booster administration after a shared prime-boost schedule. That contrast is more specific than a general claim about vaccine safety. Its small scale is appropriate to recognize as an early-phase feature, but it also means that event counts provide limited information about rare outcomes or modest differences. Equal group sizes do not remove this uncertainty.

Finally, the record demonstrates the value of complete reporting. Details about randomization, analysis populations, missing observations, interval construction, multiplicity, and sample-size justification are not reported in the registry record. Their absence does not change the values posted, but it constrains independent assessment of how those values were derived and how broadly they should be interpreted. Clear reporting of both estimates and methods is essential for readers to distinguish the observed results from conclusions the design cannot support.

15. 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.

This study enrolled 40 healthy Senegalese adults aged 18-50 years, with 20 participants in each group in the posted primary results. That population and size constrain the range of people and outcomes to which the findings can be applied. The trial does not directly describe safety in people with Ebola Virus Disease or establish effects in populations outside the enrolled volunteers.

The comparison changes only the arm used for the MVA-EBO Z booster. Both groups received ChAd3-EBO Z followed by MVA-EBO Z 7 days later. Consequently, the results do not compare the vaccine regimen with no vaccination, and they do not estimate the separate contribution of either vaccine. The trial's stated interest in immune responses is not accompanied by posted immunogenicity estimates in the record.

The primary outcome count is 20 in each group, but the registry does not report the specific event composition behind those counts. It also does not report the statistical method for the risk-difference interval, the method of random sequence generation or allocation concealment, the analysis population, or the handling of missing data. The record gives no detailed severity distribution or event timing beyond the safety collection windows described in the outcome definition.

Although no serious adverse events or deaths are listed, the small number of participants limits what can be learned about uncommon events. The registry does not report a sample-size justification or a detailed analysis plan. It also does not report whether multiplicity was addressed. These limits mean that the displayed equality should be read as a result from this small comparison, not as proof of identical safety profiles or as a definitive estimate for other settings.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Evaluation of a New Ebola Vaccine Using a Short-interval Prime-boost Vaccination — NCT02485912.
• Registry record last updated 2019-02-06; status last verified 2016-02.
• Results first posted 2019-02-06.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

19. Record Summary

The registry identifies a completed randomized, parallel-group phase 1 study sponsored by the University of Oxford. It enrolled 40 healthy Senegalese adult volunteers aged 18-50 years. Both groups received ChAd3-EBO Z followed by MVA-EBO Z 7 days later; the booster was administered in the same arm in Group 1 and the opposite arm in Group 2. Masking is listed as none.

The primary outcome concerns safety and tolerability over 26 weeks. The posted primary counts are 20 participants in each group, with a risk difference of 0.0, an interval from 0.0 to 0.0, and p = 1.0. The separate safety summary lists no serious adverse events and no deaths in either group, and other adverse events for 17 participants in Group 1 and 18 in Group 2.

The record describes immunogenicity as a secondary outcome but includes no posted secondary result in the record described by these statistics. It does not report the detailed statistical plan, analysis populations, missing-data methods, interval method, or sample-size justification. These omissions limit assessment of precision and reproducibility.