Study in Blood Stage Malaria Infection After DVI of Cryopreserved P. Falciparum (NF54 Strain) Sporozoites: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Study in Blood Stage Malaria Infection After DVI of Cryopreserved P. Falciparum (NF54 Strain) Sporozoites (NCT04310085), a na phase 1 study. The primary outcome is Incidence and Severity of Observed or Self-reported Adverse Events (AEs) Considered PfSPZ-DVI Challenge Inoculum-related..

2. Clinical Question

The study’s broader purpose was to assess whether intravenous administration of approximately 3200 P. falciparum sporozoites could be used to achieve blood-stage parasitaemia with a kinetics and PCR profile suitable for future characterization of antimalarial blood-stage activity. That objective is distinct from the stated primary endpoint, which concerns adverse events considered related to the inoculum.

The registry describes a maximum of two cohorts and sequential enrollment. The protocol’s planned parasitaemia targets were 5000 parasites/mL blood in Cohort 1 and 10000 parasites/mL blood in Cohort 2. Based on observed parasitaemia levels in Cohort 1, the target treatment threshold in Cohort 2 was maintained at 5000 p/mL rather than 10000 p/mL. This is a change in the planned threshold, not evidence by itself about the amount of parasitaemia achieved or the safety outcome.

The question is therefore primarily descriptive: what adverse-event experience was recorded in this challenge study? Because there was no concurrent control group, the results cannot establish how the observed events compare with events under a different intervention or without challenge.

3. Trial Design

This was a single-centre, open-label, pilot Phase Ib study with a single-group design. The registry records enrollment of 16 healthy, malaria-naïve male and female volunteers aged 18-55 years. Enrollment proceeded sequentially in two cohorts, with 8 participants in each; a participant could be enrolled in one cohort only. The registry does not report random allocation, and masking was none.

Participants received a direct venous inoculation challenge with cryopreserved P. falciparum NF54 strain sporozoites. The study summary describes the intended inoculum as approximately 3200 sporozoites. Antimalarial treatment was then administered using a three-day regimen, with monitoring until parasite clearance. Safety and tolerability were monitored over the study duration.

The cohorts were not randomized treatment arms. They were planned around parasitaemia thresholds: 5000 parasites/mL blood for Cohort 1 and 10000 parasites/mL blood for Cohort 2 in the protocol. The registry summary states that, based on observed parasitaemia in Cohort 1, the treatment threshold for Cohort 2 was maintained at 5000 p/mL rather than the protocol’s 10000 p/mL. The record does not give the actual parasitaemia values or explain further how the threshold decision was made.

The primary endpoint covers the incidence and severity of observed or self-reported adverse events considered related to the PfSPZ-DVI challenge inoculum, from screening until the end of study, day 28. Events were assigned to the phase in which they started—screening, challenge, or rescue—and each event was to be reported in only one phase.

A single-group design can describe events in the enrolled participants, but it does not supply a concurrent counterfactual. Differences between sequential cohorts should not be interpreted as randomized treatment effects: the cohorts were not like-for-like controls, and the registry does not report a statistical comparison between them.

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 Incidence and Severity of Observed or Self-reported Adverse Events (AEs) Considered PfSPZ-DVI Challenge Inoculum-related., measured over Screening until end of study, day 28..

The primary endpoint is the incidence and severity of observed or self-reported adverse events considered related to the PfSPZ-DVI challenge inoculum. Its timeframe runs from screening until the end of study, day 28. The registry states that each adverse event is assigned to the phase in which it began—screening, challenge, or rescue—so that an event is reported in only one phase.

This endpoint is used to describe safety and tolerability in the setting of an experimental malaria challenge. Counting or characterizing events can help convey what was observed during the study and when events began. The endpoint’s focus on events considered inoculum-related is narrower than a count of every event, and the record does not describe the criteria or process used to judge relatedness.

The registry’s endpoint definition includes both observed and self-reported events, but the posted results do not give a detailed breakdown by event type, severity, phase, or relatedness assessment. The available adverse-event figures distinguish serious events from other adverse events, but they do not provide the full endpoint detail needed to reconstruct the incidence and severity profile as defined.

In a small, uncontrolled challenge study, this endpoint is descriptive. It does not, on its own, establish that an event was caused by the inoculum or quantify how event rates would differ under another intervention. The registry record also does not report a separate primary efficacy endpoint or a numerical parasitaemia result in the information available for this analysis.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Interpreting this study begins with its design. It was a single-group, open-label study, not a randomized comparison. Participants were enrolled sequentially into two cohorts, each with 8 participants. This design can describe what happened among those enrolled, but it does not create an untreated or alternative-treatment group for estimating a comparative effect. The cohorts’ different planned parasitaemia thresholds do not make them control and treatment groups.

What does a count measure?

A participant count records how many people in a defined group met a stated condition, such as being affected by a serious adverse event. The denominator describes the group at risk for that outcome. In this record, the serious-event figures are 0 affected among 8 at risk in each cohort, while the other-adverse-event figures are 7 affected among 8 at risk in Cohort 1 and 8 affected among 8 at risk in Cohort 2. These counts do not indicate how many separate events occurred, because one participant may have experienced more than one event and the posted figures do not provide event totals.

Counts are useful for transparent description, especially when the groups are small. They should not be mistaken for precise estimates of the underlying frequency in a wider population. A small group can produce a count that is highly sensitive to the experience of individual participants. The registry record does not report confidence intervals or another uncertainty measure for these adverse-event counts.

Why does the missing control group matter?

A comparative effect requires a meaningful contrast between outcomes under different conditions. Random allocation is one way to make groups more comparable, but this study had no randomized allocation and no concurrent control arm. Consequently, the observed counts cannot show how many events would have occurred without the challenge or under another regimen. Even if cohort counts differ, that difference cannot be attributed to the cohort threshold or another cause from these data alone.

Open-label status means masking was none. In general, awareness of the intervention can affect reporting or assessment, particularly for subjective outcomes. The registry includes observed or self-reported events, but it does not report how events were elicited, assessed, or adjudicated. This does not establish that reporting was biased; it means the methods needed to evaluate that possibility are not reported in the registry record.

How should the endpoint be read?

The primary endpoint combines incidence and severity of observed or self-reported adverse events considered related to the inoculum. Its definition also specifies a timeframe and phase assignment: each event is allocated to the phase in which it started—screening, challenge, or rescue—and reported only once. Phase assignment helps organize event timing, but the registry does not provide phase-specific results in the available record. The posted safety figures therefore cannot be used to reconstruct the endpoint’s full timing or severity profile.

Relatedness is an additional judgment, separate from whether an event occurred. The endpoint is limited to events considered related to the challenge inoculum, but the record does not report the assessment criteria or the number of events judged related. The broader adverse-event counts should not be assumed to equal the count for that primary endpoint.

What can be concluded from the cohort structure?

The protocol described targets of 5000 parasites/mL blood in Cohort 1 and 10000 parasites/mL blood in Cohort 2. The registry summary says the Cohort 2 treatment threshold was maintained at 5000 p/mL based on observed parasitaemia in Cohort 1. This is important design context: the sequence and threshold decision mean the cohorts were not independent, fixed-threshold comparison groups. The record does not report the decision rule, actual parasitaemia measurements, or a statistical model for analyzing cohort outcomes.

There are no reported analyses in the record. It does not state a statistical test, a model for estimating the endpoint, an adjustment strategy, or a method for handling repeated events within participants. Statistical methods such as regression or time-to-event analysis cannot be attributed to the study unless reported. In general, accounting for repeated events can matter because several events from one participant are not statistically equivalent to events from separate participants; whether and how this was handled is not reported in the registry record.

How should planning calculations be interpreted?

A planning illustration based on an observed effect is not a measure of power achieved by this completed study. Sample-size calculations describe planning under stated assumptions; they do not establish that a study had a particular probability of detecting a result after data collection. The record’s planning assumptions refer to a two-sided significance level of 0.05, equal allocation, and a normal approximation, with the target effect expressed as a fraction of an observed effect. Those assumptions do not turn this single-group study into a randomized two-arm comparison. Any calculated sample sizes are planning illustrations, not observed power, and do not substitute for the absent comparative design or missing endpoint detail.

Overall, the available methods information supports a descriptive reading of the recorded counts and a cautious account of the study’s intended purpose. It does not support a reconstructed inferential analysis, an estimate of a causal contrast, or a judgment about precision beyond what the posted counts directly show.

10. Multiplicity, Missing Data and Analysis Populations

The registry record does not report a statistical plan for multiplicity, including whether any adjustment was planned for multiple outcomes, time points, or subgroup analyses. The study lists 9 outcome measures, but the available record does not describe a hierarchy of hypotheses or how the primary endpoint was protected from interpretation alongside other outcomes. Without a reported plan and analysis, it is not possible to determine whether multiplicity was addressed.

Missing-data handling is also not reported in the registry record. The record does not describe whether any participants withdrew, whether all enrolled participants contributed to each outcome, how incomplete adverse-event follow-up was handled, or how missing observations were treated. The at-risk figures in the adverse-event results do not, by themselves, establish completeness of follow-up for every endpoint or timeframe.

The analysis populations are not described in the available registry information. In particular, it does not report whether the safety summaries include all enrolled participants, participants who received the challenge, or another defined group. The registry says each participant could be enrolled in one cohort only, but that does not specify an analysis-population rule.

No statistical analyses are reported in the record. Thus there are no reported p-values, confidence intervals, adjusted estimates, or methods for handling multiple events within a participant. These absences limit assessment of statistical precision and the risk of selective interpretation. They should not be filled in by assuming a conventional approach; the analysis methods and missing-data procedures are not reported in the registry record.

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 non-serious adverse event terms

The registry reports 0 serious adverse events among 8 at-risk participants in Cohort 1 and 0 among 8 at-risk participants in Cohort 2. It also reports 0 deaths in each cohort. These figures describe the recorded experience during this study; they do not establish that the risk of serious events or death is absent.

For other adverse events, the registry reports 7 affected participants among 8 at risk in Cohort 1 and 8 affected among 8 at risk in Cohort 2. The record does not provide the number of individual events, their severity distribution, or a detailed event-by-event account. Nor does it show how many of these other events were considered related to the challenge inoculum, the focus of the primary endpoint.

The registry lists no top serious adverse-event terms. That should be read as an absence of listed serious-event terms in the posted record, alongside the reported zero serious events—not as a general guarantee about safety. Small, uncontrolled cohorts can describe what was recorded in the study but cannot establish comparative safety or precisely characterize uncommon outcomes.

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

The results establish a limited descriptive point: the registry reports no serious adverse events and no deaths in either cohort, while reporting other adverse events affecting 7 participants in Cohort 1 and 8 in Cohort 2. These are the posted counts for the groups at risk. They do not provide event-level detail, a complete severity profile, or the primary endpoint’s full breakdown by timing and inoculum relatedness.

The study was designed to assess whether direct venous inoculation could produce blood-stage parasitaemia with a profile that might support future characterization of antimalarial blood-stage activity. The registry also describes treatment with a three-day antimalarial regimen and monitoring until parasite clearance. However, the numerical parasitaemia and kinetics results are not reported in the information available for this analysis. The adverse-event counts therefore do not establish whether the broader challenge-model objective was achieved.

These data do not show that the challenge caused every recorded event, that the intervention is safe in a broader population, or that it is safer or less safe than an alternative. There was no concurrent comparator and no random allocation. The sequential cohorts were not designed as a like-for-like control comparison, and the treatment threshold in Cohort 2 was maintained at 5000 p/mL after observations in Cohort 1, rather than following the protocol’s 10000 p/mL threshold.

The posted results also do not quantify statistical uncertainty for the adverse-event counts. No inferential analysis or confidence intervals are reported in the available record. In general, absence of a reported event in a small study cannot rule out its occurrence in a larger or different population. The results are best understood as a description of recorded events in this pilot study, not as a definitive causal or comparative conclusion.

Finally, the record does not report the criteria for judging whether an event was related to the inoculum, the detailed missing-data procedures, or the analysis populations. These limits matter when interpreting the primary endpoint and when considering how closely the posted counts represent the endpoint as defined.

13. Why This Trial Matters Statistically

This trial is statistically notable because it combines an experimental challenge model with a small, single-group design and a primary endpoint focused on adverse events. The distinction between the study’s broader purpose and its stated primary endpoint is important: the intended future application concerns characterization of antimalarial blood-stage activity, while the primary endpoint measures events considered related to the challenge inoculum.

The sequential cohort structure also affects interpretation. Cohort 1 and Cohort 2 were associated with planned parasitaemia thresholds, and the registry says the treatment threshold in Cohort 2 was maintained at 5000 p/mL based on observed Cohort 1 parasitaemia. Because there was no randomization or concurrent control, the cohorts cannot provide a clean comparison of different thresholds. The study can report what was observed in each cohort, but the design does not isolate a causal effect of threshold or treatment.

Statistically, the safety counts demonstrate why denominators and endpoint definitions matter. Affected-participant counts are not the same as total event counts, and broader adverse-event figures are not necessarily identical to events judged related to the inoculum. The registry provides limited event detail, so the reported figures cannot fully answer questions about severity, timing, or relatedness.

The record also illustrates the value of distinguishing descriptive results from inferential claims. Without a reported analysis plan, uncertainty estimates, missing-data procedures, and analysis-population definitions, readers should avoid interpreting the counts as precise population estimates or comparative effects. The trial’s contribution is most appropriately framed as a pilot description within its specific challenge-study context; the available record does not support broader statistical conclusions.

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.

Several limitations are specific to this study’s design and registry information. It is a single-centre, open-label, single-group study without a concurrent control. The two cohorts were sequential and tied to parasitaemia thresholds, with the Cohort 2 treatment threshold maintained at 5000 p/mL after Cohort 1 observations. This means cohort outcomes do not constitute a randomized comparison of thresholds or treatments.

The enrolled population was healthy, malaria-naïve adults aged 18-55 years. The registry does not establish how well findings would apply to other populations. The small cohort sizes also limit the ability of the observed counts to characterize less common events precisely; no uncertainty intervals are reported in the available record.

The primary endpoint specifies adverse events considered related to the inoculum, with phase assignment and a timeframe through day 28. The posted adverse-event figures do not provide the detailed severity, event type, phase, or relatedness breakdown needed to fully assess that endpoint. The method for judging relatedness is not reported in the registry record.

Numerical parasitaemia and kinetics findings are not reported in the available information, despite their importance to the study’s stated broader purpose. The registry record also does not report the statistical analysis plan, missing-data methods, analysis populations, or a multiplicity strategy. These gaps limit both endpoint interpretation and assessment of the robustness of the reported results.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Study in Blood Stage Malaria Infection After DVI of Cryopreserved P. Falciparum (NF54 Strain) Sporozoites — NCT04310085.
• Registry record last updated 2021-09-05; status last verified 2021-09.
• Results first posted 2021-09-05.
• 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

The registry identifies NCT04310085 as a completed, single-centre, open-label pilot Phase Ib study of direct venous inoculation with cryopreserved P. falciparum NF54 strain sporozoites in healthy, malaria-naïve adults. The record reports enrollment of 16 participants in two sequential cohorts, with 8 participants in each. Participants received the challenge and artemether-lumefantrine; the summary describes a three-day treatment regimen with monitoring until parasite clearance.

The primary endpoint concerns incidence and severity of observed or self-reported adverse events considered related to the challenge inoculum from screening until the end of study, day 28. The registry reports 0 serious adverse events and 0 deaths in each cohort. Other adverse events affected 7 participants in Cohort 1 and 8 in Cohort 2. The record does not report detailed event severity, timing, or relatedness results, nor numerical parasitaemia findings or inferential analyses. Given the single-group design and absent control arm, the posted safety counts are descriptive rather than comparative.