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 SARS-CoV-2 Human Challenge Characterisation Study (NCT04865237), a non-randomized na study. The primary outcome is Number of Unsolicited Adverse Events in Healthy Participants Challenged With Wild Type SARS-CoV-2.
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT04865237 |
| Phase | NA |
| Condition | Covid19, SARS-CoV Infection, Corona Virus Infection |
| Allocation | Non-Randomized |
| Design model | Sequential |
| Masking | None (open label) |
| Enrollment | 36 |
| Arms in the primary outcome | n/a |
| Lead sponsor | Imperial College London |
| Results posted | Yes |
2. Clinical Question
Population
Healthy adults aged 18–30 were the intended population, according to the study summary. The registry records enrollment of 36 participants. The adverse-event summary lists 36 participants at risk in the Healthy Volunteers Dose Level 1 group; it does not provide a full account of the participant counts assigned to each dose level.
Intervention
The study used experimental intranasal SARS-CoV-2 challenge, with dose levels of 10^1, 10^2, and 10^3 TCID50. The registry also lists remdesivir as an intervention, but does not describe its role in the study arms or the primary endpoint.
Comparator
This was a non-randomized, sequential study, not a randomized comparison with a like-for-like control group. The dose levels are part of the challenge design, but the record does not provide a comparator suitable for interpreting a treatment effect.
Primary question
The primary endpoint asks how many unsolicited adverse events occurred in healthy participants challenged with wild-type SARS-CoV-2, over the stated Day 0 to 28 timeframe. The study summary describes a broader aim: optimizing a dose that would produce PCR-confirmed upper respiratory infection in the majority of challenged individuals with minimal or no illness, to inform later studies of treatments, vaccines, and immune protection.
The central statistical question is therefore descriptive: what unsolicited adverse-event burden was recorded during follow-up in this experimentally challenged group? The registry identifies safety as the purpose of the primary endpoint, but does not report a numerical primary-endpoint result or an analysis comparing dose levels. Its adverse-event summary reports serious and other adverse-event figures, which are not necessarily equivalent to the total count of unsolicited events specified by the endpoint.
3. Trial Design
This completed study is registered as NCT04865237, with phase listed as NA and enrollment of 36. The registry lists a start date of 2021-03-06 and primary completion on 2022-07-11. The sponsor is Imperial College London. Its official title describes a dose-finding human experimental infection study in healthy subjects using a GMP-produced SARS-CoV-2 wild-type strain.
The design is described as sequential, non-randomized, and unmasked. Participants were challenged intranasally at dose levels of 10^1, 10^2, or 10^3 TCID50, as described in the arm entries. A sequential design may move through dose levels in an ordered way, but the registry record does not explain the sequence, decision rules, or how participants were assigned to each level. Those details should not be inferred from the arm names alone.
The listed interventions include the three viral dose levels and remdesivir. However, the record does not explain how remdesivir fits into the design, whether or when it was used, or which participants received it. It also does not specify a concurrent unchallenged control group. Given the non-randomized allocation, comparisons across dose levels would not automatically have the interpretation of randomized treatment comparisons.
The brief summary states that the project sought to induce PCR-confirmed upper respiratory infection in most challenged participants while causing minimal or no illness. It also describes the study as groundwork for later evaluations of treatment and vaccine candidates and immune protection. These are stated aims, not reported findings. The registry gives 19 outcome measures, including infection confirmation, symptoms, illness, and viral dynamics, but records 0 analyses. The dose-specific participant flow, actual challenge exposure, and detailed statistical analysis plan 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: This study met the primary outcome of a 50% infection rate in the first, lowest dose group. Therefore, there was no further enrolment into the higher dose level groups (Dose Level 2 or 3).
Overall Study
| Stage | Healthy Volunteers Dose Level 1 | Healthy Volunteers Dose Level 2 | Healthy Volunteers Dose Level 3 |
|---|---|---|---|
| STARTED | 36 | 0 | 0 |
| COMPLETED | 36 | 0 | 0 |
| NOT COMPLETED | 0 | 0 | 0 |
Analysis population for the primary outcome, as defined in the registry: All subjects
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: Healthy Volunteers Dose Level 1
| Characteristic | Unit | Healthy Volunteers Dose Level 1 |
|---|---|---|
| Participants analysed | 36 | |
| Age, Categorical: <=18 years | Participants | 0 |
| Age, Categorical: Between 18 and 65 years | Participants | 36 |
| Age, Categorical: >=65 years | Participants | 0 |
| Age, Continuous | years | 21.8 (SD 2.89) |
| Sex: Female, Male: Female | Participants | 10 |
| Sex: Female, Male: Male | Participants | 26 |
| Ethnicity (NIH/OMB): Hispanic or Latino | Participants | 2 |
| Ethnicity (NIH/OMB): Not Hispanic or Latino | Participants | 34 |
| Ethnicity (NIH/OMB): Unknown or Not Reported | Participants | 0 |
| Race (NIH/OMB): American Indian or Alaska Native | Participants | 0 |
| Race (NIH/OMB): Asian | Participants | 0 |
| Race (NIH/OMB): Native Hawaiian or Other Pacific Islander | Participants | 0 |
| Race (NIH/OMB): Black or African American | Participants | 0 |
| Race (NIH/OMB): White | Participants | 33 |
| Race (NIH/OMB): More than one race | Participants | 0 |
| Race (NIH/OMB): Unknown or Not Reported | Participants | 3 |
| Region of Enrollment: United Kingdom | participants | 36 |
6. Primary Endpoint
The registry lists the primary outcome as Number of Unsolicited Adverse Events in Healthy Participants Challenged With Wild Type SARS-CoV-2, measured over Day 0 to 28 (28 days).
| Element | Registry entry |
|---|---|
| Result type | Number |
| Measure of spread | not applicable |
| Unit | Unsolicited Adverse Events |
| Endpoint classification | not reduced to a single comparison |
The primary endpoint is the number of unsolicited adverse events in healthy participants challenged with wild-type SARS-CoV-2. The stated timeframe is Day 0 to 28 (28 days). The endpoint description says it evaluates safety by assessing unsolicited adverse events within 30 days post-viral challenge, from Day 0 up to Day 28 follow-up. Those timeframe descriptions are not fully aligned: the endpoint label specifies Day 0 to 28, while its description uses both “within 30 days” and “up to Day 28.” The intended operational window is not clarified further in the registry record.
A count endpoint captures how many events were observed in the defined population and follow-up window. It is useful for describing event burden, but interpreting a count requires clarity about what qualifies as an event, how repeat events in the same participant are handled, and whether the reported value is a total number of events or a count of participants with events. Those endpoint details are not reported in the registry record.
The adverse-event summary lists participants at risk and participants affected by serious or other adverse events. These summaries can inform safety description, but they do not, by themselves, establish the total number of unsolicited events required by the primary endpoint. The registry record does not report a numerical result or analysis for that endpoint.
7. Why No Re-analysis Is Shown
8. Statistical Interpretation
The registry record establishes the study’s design and stated safety objective, but it does not provide a numerical primary-endpoint result or a statistical analysis of that endpoint. The adverse-event summary is therefore the main numerical safety information available in the record, and it should be read as a limited description rather than a complete analysis of the primary outcome.
For the group labelled Healthy Volunteers Dose Level 1, the adverse-event summary lists 36 participants at risk, 1 affected by a serious adverse event, 24 affected by other adverse events, and 0 deaths. These are counts in the registry’s adverse-event categories. The record does not give participant-level details, event timing, attribution to challenge, severity for the other events, or an account of whether individuals could appear in more than one category. The figures should not be treated as a complete count of all unsolicited adverse events unless that relationship is established in the record.
The top serious adverse-event term is Left Side Renal Pain, listed as 2.8%. The record does not explain in this summary how that term relates to the serious-event count, whether it represents an event or a participant, or what coding and attribution rules were applied. It also does not provide a dose-level comparison or a comparator group that would support a causal contrast.
There are 19 outcome measures listed and 0 analyses recorded. This difference matters: an outcome may be listed as planned or collected without a numerical result or inferential analysis being posted. The named secondary outcomes concern laboratory-confirmed infection, symptomatic infection, symptom scores, illness, and viral dynamics, but the registry record does not give their numerical findings. Consequently, neither the intended dose optimization nor the described infection and illness objectives can be evaluated from the registered results information.
The study was non-randomized and sequential. If outcomes differed across dose levels, those differences would need interpretation in light of the design, timing, participant selection, and any decisions made during progression between levels. The registry does not report enough detail to assess those issues or to treat dose groups as exchangeable. Nor is a statistical model or uncertainty estimate for the primary endpoint reported.
The sample-size assumptions describe a planning illustration based on an observed effect, using a two-sided significance level of 0.05, equal allocation, and a normal approximation. The effect to detect is described as a stated fraction of the observed effect. Time-to-event calculations are described as total event counts, while other calculations are participants per arm. The record does not include an observed primary-endpoint effect or the resulting planning values in the registry record for interpretation. Such planning illustrations are not observed power and cannot substitute for a result analysis.
Overall, the registry supports a description of the study’s intended safety question and a small set of adverse-event summary counts. It does not establish the overall frequency of unsolicited events, a dose-response relationship, or a comparative safety effect. Any broader conclusion would require results and methods that are not reported in the registry record.
9. Statistical Methods Explained
Statistical interpretation starts with identifying what was measured, in whom, and over what period. Here, the primary endpoint is a count of unsolicited adverse events in challenged healthy participants, with the endpoint label giving Day 0 to 28 (28 days). The description also refers to events within 30 days post-challenge and follow-up up to Day 28. This discrepancy in wording makes the exact endpoint window less clear than a single, consistently defined interval would be. The registry does not explain the operational rule used to resolve it.
What does a count endpoint describe?
A count endpoint records the number of events meeting a stated definition. It is distinct from a binary endpoint, which records whether each participant had at least one event, and from a rate, which relates events to person-time. Counts can include repeated events in one participant unless the endpoint definition explicitly restricts counting. The registry labels the primary endpoint as a number of unsolicited adverse events, but does not report whether repeat events were counted separately, how unsolicited events were defined, or how related and unrelated events were handled.
The adverse-event summary lists participants affected by serious and other adverse events. Counts of affected participants are not interchangeable with counts of all events: a participant may have multiple events, and a single event may have multiple classifications. The record does not provide enough detail to reconcile the participant-level summaries with the total unsolicited-event endpoint. A reader should therefore keep the reported categories separate rather than treating them as equivalent measures.
Why can over-dispersion matter?
General statistical explanation: count data are often analyzed with models that describe both their average and their variability. A simple count model may assume that the variability follows a particular relationship to the average. Over-dispersion means the observed variability is greater than that model expects. It can arise when event counts vary substantially between participants or when a small number of participants contribute many events. If over-dispersion is ignored, standard errors and confidence intervals may be too optimistic. The registry record does not report an analysis model, a dispersion assessment, or a statistical result for the primary count endpoint, so it is not possible to determine whether over-dispersion was assessed or how it was handled.
What can a confidence interval tell a reader?
General statistical explanation: a confidence interval expresses uncertainty around an estimated quantity under the assumptions of the analysis. A narrow interval indicates greater precision than a wide interval, but neither width nor location alone addresses all sources of bias. A confidence interval for an event count, event rate, or difference would answer different questions, so the estimand and method matter. No confidence interval for the primary endpoint is reported in the registry record.
How does the design affect comparisons?
Allocation is listed as non-randomized, masking as none, and the model as sequential. Randomization, when used, supports comparability of groups on average by allocating participants through a chance mechanism. Without random allocation, dose groups can differ for reasons other than dose. Sequential progression can also mean that later levels are studied at a different point in the process, after information from earlier levels has accrued. These are general design considerations, not claims about what occurred in this study. The registry does not describe the allocation process, progression criteria, or adjustment strategy.
Unmasked study conduct can affect reporting or assessment of outcomes, depending on how outcomes are collected. Adverse events are especially dependent on definitions, collection procedures, and ascertainment. The record does not describe the procedures used to solicit or classify events, so the extent to which outcome assessment could be influenced cannot be assessed.
What do the planning assumptions mean?
The registry’s planning description specifies a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. It defines the target effect as a stated fraction of the observed effect. These are assumptions for planning illustrations, not a description of observed trial findings. For time-to-event calculations, the planning description uses total event counts; for other calculations, it uses participants per arm. A significance level is a design convention for controlling the chance of a false-positive result under specified assumptions; it does not measure the probability that a result is true.
Sample-size planning based on an observed effect is sensitive to the effect estimate used. General explanation: a planning calculation asks how much information a future or hypothetical study might need to detect a chosen effect under assumed conditions. It does not show that the study had a particular probability of detecting an effect after the data were collected. This is why planning illustrations are not “observed power.” The record does not report the observed effect needed to interpret these calculations or a sample-size justification for the trial’s actual sequential design.
No analysis population, missing-data approach, covariate adjustment, interim-analysis plan, multiplicity procedure, or model for the primary count is reported in the registry record. The listed assumptions should not be extended into claims about the study’s actual analysis plan.
10. Multiplicity, Missing Data and Analysis Populations
The registry lists 19 outcome measures, including the primary adverse-event endpoint and secondary measures of infection, symptoms, illness, and viral dynamics. It records 0 analyses. The presence of multiple outcomes raises a general multiplicity issue: when many hypotheses are tested, the chance of at least one apparently notable result can increase unless the analysis plan accounts for the number and ordering of comparisons. The registry record does not report a multiplicity adjustment, a testing hierarchy, or a distinction between confirmatory and exploratory outcomes. It is therefore not possible to determine how multiplicity was addressed.
Missing-data handling is not reported in the registry record. For a follow-up endpoint, interpretation can depend on whether all participants completed the stated observation period, how incomplete follow-up was handled, and whether missing event information was treated as absence of an event or as unknown. The registry does not provide participant flow, completeness of follow-up, or sensitivity analyses for missing observations.
The primary endpoint population is described as “All subjects.” The adverse-event summary lists 36 participants at risk in the group named Healthy Volunteers Dose Level 1, while overall enrollment is 36. The record does not explain whether those figures describe the same analytic population, how participants were distributed by dose level, or whether any participants were excluded from an analysis. Analysis-population definitions such as intention-to-treat or per-protocol are not reported.
No interim analyses, stopping rules, or adjustments to inference for sequential decision-making are reported in the registry record. Their absence from the record does not establish that none were used; it means their use and details cannot be assessed from the posted information.
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 |
|---|---|---|---|
| Healthy Volunteers Dose Level 1 | 1 | 36 | 2.8% |
Deaths
| Arm | Deaths (participants affected of at risk) |
|---|---|
| Healthy Volunteers Dose Level 1 | 0 of 36 |
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 |
|---|---|
| Healthy Volunteers Dose Level 1 | 24 of 36 |
Most frequent serious adverse event terms
| Event term | Organ system | Healthy Volunteers Dose Level 1 |
|---|---|---|
| Left Side Renal Pain | Renal and urinary disorders | 1 of 36 |
Most frequent non-serious adverse event terms
| Event term | Organ system | Healthy Volunteers Dose Level 1 |
|---|---|---|
| Fibrin D dimer increased | Investigations | 6 of 36 |
| Medical device site rash | General disorders | 5 of 36 |
| Dry skin | Skin and subcutaneous tissue disorders | 4 of 36 |
| Anosmia | Nervous system disorders | 4 of 36 |
| Neutrophil count decreased | Investigations | 3 of 36 |
| Alanine aminotransferase increased | Investigations | 2 of 36 |
| COVID-19 | Infections and infestations | 2 of 36 |
| Lymphocyte count decreased | Investigations | 1 of 36 |
| Platelet count decreased | Investigations | 1 of 36 |
| Troponin increased | Investigations | 1 of 36 |
Serious events with the highest arm-level rate: left side renal pain (2.8%).
The adverse-event summary lists 1 participant affected by a serious adverse event among 36 at risk in the Healthy Volunteers Dose Level 1 group. It lists 24 affected by other adverse events and 0 deaths. The top serious adverse-event term is Left Side Renal Pain, listed as 2.8%. These are registry summary figures; the record does not provide event narratives, timing, attribution, or enough information to determine how the term-level listing maps onto the participant-level summary.
Seriousness and severity are different concepts in safety reporting. General explanation: seriousness usually reflects regulatory outcome criteria, while severity describes the intensity of an event. A serious event is not necessarily the most severe event, and a severe event is not necessarily serious. The registry’s brief summary does not provide severity grades for the other adverse events.
The primary endpoint concerns unsolicited adverse events, but the displayed safety figures distinguish serious from other adverse events and do not state that they represent the full count of unsolicited events. They also do not show whether a participant could experience more than one event or appear in more than one category. No causal relationship to viral challenge is established by these counts alone, and the record does not report a randomized control group for comparison. The figures are descriptive and should not be treated as a comparative safety estimate.
12. What the Result Does, and Does Not, Mean
The record describes a human challenge study designed to characterize infection and optimize a challenge dose in healthy adults aged 18–30. Its stated aim was to produce PCR-confirmed upper respiratory infection in the majority of challenged individuals with minimal or no illness, while collecting information on COVID-19 course and immune response. It also describes the study as preparation for later studies of treatments, vaccines, and immune protection.
Those aims do not establish that the desired infection rate, illness profile, or dose optimization was achieved. The registry lists outcomes related to laboratory-confirmed infection, symptoms, illness, and viral dynamics, but records 0 analyses and does not report numerical results for those outcomes. The primary endpoint is specified, but its numerical result is also not reported in the registry record.
The adverse-event summary offers limited descriptive safety information: 1 serious adverse event, 24 other adverse events, and 0 deaths among 36 participants at risk in the listed group. The record does not provide the context needed to determine event attribution, timing, recurrence, or completeness. These figures alone do not establish the overall number of unsolicited events or support comparisons across dose levels.
The study was non-randomized, unmasked, and sequential, with no like-for-like control comparison described. It is therefore not appropriate to interpret the registry summary as a randomized estimate of a treatment effect or as evidence that challenge caused or did not cause a particular outcome. The available information supports understanding the study’s purpose, design labels, and some reported safety categories. It does not support a conclusion about the effectiveness of a treatment or vaccine, the degree of protection from prior infection, or the dose level that best met the study’s objective.
13. Why This Trial Matters Statistically
This study is statistically notable because it uses experimental infection and sequential dose levels to pursue a dose-finding objective. Such a design raises questions about how each dose is selected, how evidence from earlier participants informs later decisions, and how uncertainty is represented when observations accumulate in sequence. The registry identifies the model as sequential but does not report the decision rules or analysis methods needed to evaluate those questions.
It also distinguishes an explicitly stated safety endpoint from the broader biological and clinical questions in the study summary. A count of unsolicited adverse events, infection confirmation, symptom burden, and viral dynamics are different outcomes, with different definitions and statistical interpretations. A study can list many measures while still providing no numerical results or analysis for them in its registry entry.
The record illustrates why design labels and outcome names are not substitutes for a full statistical plan. Non-randomized allocation limits straightforward group comparisons; unmasked conduct can matter for outcome ascertainment; and a count endpoint needs a clear event definition and method for repeated events. The absence of reported analyses means readers cannot evaluate estimates, precision, or model assumptions for the primary endpoint.
Finally, dose-finding studies often inform later research rather than answer every downstream clinical question themselves. The registry’s stated purpose is to establish an optimized challenge dose and study design for follow-on trials. That purpose is distinct from demonstrating treatment or vaccine efficacy. Understanding this distinction helps keep the interpretation aligned with what the record actually reports.
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 registry entry does not include numerical results for the primary endpoint, despite describing the endpoint and follow-up period. It also records 0 analyses despite listing 19 outcome measures. This leaves the principal safety question and the infection, symptom, and viral-dynamics objectives without reported statistical estimates in the record.
The endpoint timeframe is described as Day 0 to 28 (28 days), while the endpoint description refers to events within 30 days post-challenge and follow-up up to Day 28. The record does not resolve this wording difference. The primary endpoint is a count of unsolicited adverse events, but the event definition, handling of repeat events, attribution rules, and relation to the participant-based adverse-event summary are not reported.
The design is non-randomized, unmasked, and sequential. The record does not explain dose allocation, the order and criteria for progression, the number of participants exposed at each level, or how any dose comparisons would be adjusted. Remdesivir is listed as an intervention, but its role is not described in the arm information. No like-for-like control comparison is reported.
Details about participant flow, analysis populations, missing data, multiplicity, interim analyses, stopping rules, and the primary endpoint model are not reported in the registry record. These omissions limit assessment of precision, bias, and whether the listed adverse-event counts capture all relevant events. The stated planning assumptions do not provide a sample-size justification for the actual sequential study, and no observed effect is reported to interpret planning illustrations. Accordingly, the record supports only a cautious description of design and posted safety summaries, not a complete evaluation of the study’s objectives.
15. Related Tutorials
16. Related Statistical Calculators
17. Sources
- ClinicalTrials.gov: SARS-CoV-2 Human Challenge Characterisation Study — NCT04865237.
- Registry record last updated 2025-05-16; status last verified 2025-04.
- Results first posted 2025-05-16.
- 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
NCT04865237 is a completed, non-randomized, unmasked, sequential human challenge study sponsored by Imperial College London. It enrolled 36 participants, with dates listed as 2021-03-06 for study start and 2022-07-11 for primary completion. The study summary describes healthy adults aged 18–30 challenged intranasally at SARS-CoV-2 dose levels of 10^1, 10^2, or 10^3 TCID50.
The primary endpoint is the number of unsolicited adverse events over the stated Day 0 to 28 timeframe. The record lists 19 outcome measures and 0 analyses. Its adverse-event summary lists 1 serious adverse event, 24 other adverse events, and 0 deaths among 36 participants at risk in the named group; Left Side Renal Pain is the top serious term at 2.8%. Numerical primary-endpoint findings, detailed analysis methods, dose-specific results, and missing-data procedures are not reported in the registry record.