Optimisation of Controlled Human Malaria Infection Using Sporozoites Administered by Needle and Syringe: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Optimisation of Controlled Human Malaria Infection Using Sporozoites Administered by Needle and Syringe (NCT01465048), a non-randomized na study. The primary outcome is Number of Participants Infected.

2. Clinical Question

In statistical terms, the central question is whether the observed infection outcomes differed across the administration regimens. A meaningful answer would require the number assessed and the number meeting the infection definition in each group, together with a clear account of how infection was determined when microscopy and PCR results might not agree. Those outcome details are not reported in the registry record. The record also does not describe a prespecified rule for combining the diagnostic methods, or say whether both tests had to be positive.

This is a pilot study focused on optimizing controlled human malaria infection procedures, rather than a randomized comparison intended to isolate a causal effect of one regimen against another. The study summary says route and dose would vary to identify a regimen with the greatest infection rate. Because allocation was non-randomized, any observed contrast would need cautious interpretation. The posted record describes the question and regimens, but does not provide the primary outcome results needed to evaluate an infection-rate contrast.

3. Trial Design

This completed study is identified as NCT01465048 and is titled “Optimisation of Controlled Human Malaria Infection Using Sporozoites Administered by Needle and Syringe.” The official title describes it as a pilot study using Plasmodium falciparum sporozoites administered by needle and syringe. The sponsor is the University of Oxford. The registry lists the study start as 2011-10 and primary completion as 2012-02.

The design is listed as parallel, non-randomized, and unmasked. The study summary describes an open-label human pilot study in which volunteers were inoculated with PfSPZ Challenge and the route and dose varied across regimens. The three listed regimens were 2,500 sporozoites intradermally at two sites, 2,500 sporozoites intramuscularly at one site, and 25,000 sporozoites intramuscularly at one site. The registry lists enrollment of 18, and the safety record lists six participants at risk in each regimen group.

Non-randomized allocation means the registry does not describe assignment by chance. The groups are not a like-for-like control comparison, so differences in their outcomes cannot be attributed to dose or route alone from the design description. The absence of masking also means the study is not described as blinded. The registry lists the phase as NA.

The primary outcome was the number of participants infected at 21 days after administration. Secondary outcomes concerned the frequency, incidence, and nature of adverse events and serious adverse events, and the dynamics of parasite growth following the different regimens. The record does not describe a statistical analysis plan, the method used to allocate participants, or a justification for the planned enrollment. It does not report an interim analysis plan. These details are not reported in the registry record and should not be inferred from the pilot designation or the study's stated aim.

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 Number of Participants Infected, measured over 21 days post administration of PfSPZ Challenge.

The primary endpoint is the number of participants infected by 21 days after PfSPZ Challenge administration. The registry description says infectivity was to be determined using thick film microscopy and highly sensitive PCR for Plasmodium falciparum DNA. This endpoint addresses whether the challenge regimen produced detectable infection within the stated follow-up window; it is a count of participants with an event, not a measure of parasite growth over time.

In general, a binary endpoint classifies each participant as meeting or not meeting a specified event definition during a specified period. Counting participants who meet the definition makes the outcome directly interpretable as an event count for each regimen. The result is only as clear as the outcome rule: readers need to know how the diagnostic tests were applied and how discordant test results were handled. Those details are not reported in the registry record.

The registry record gives the endpoint name, timeframe, and diagnostic methods, but does not report the number infected in any regimen. It also does not report the number with evaluable test results or clarify whether all enrolled participants contributed to the primary endpoint. Accordingly, the endpoint's definition can be described, but its observed results and precision cannot be assessed from the record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes the study as a parallel, non-randomized, open-label pilot. Its primary outcome is a count of participants infected by 21 days after administration. Statistical interpretation depends on distinguishing the outcome definition, the way participants entered the groups, the amount of outcome information, and the analysis used. Several of those details are not reported in the registry record.

What does a participant count represent?

A count of participants with an event is a binary-outcome summary: for each person, the outcome is classified as infection or no infection under the study's definition. Such counts can be presented separately for each regimen. In general, event counts are most informative when accompanied by the number assessed, the operational definition of the event, and an account of participants whose status could not be determined. The registry names thick film microscopy and highly sensitive PCR, but does not explain the rule for interpreting the two tests together or report how many participants had evaluable results.

The timeframe matters because the endpoint concerns infection by 21 days after challenge, not infection at any later point. Results from a different observation period would answer a different question. The registry also lists parasite-growth dynamics as a secondary outcome, which is conceptually distinct: dynamics concern change over time, whereas the primary outcome is whether a participant was classified as infected within the stated period.

Why does allocation matter?

In general, randomization assigns participants to groups by chance, helping make treatment groups comparable on average. This record says allocation was non-randomized. That does not prevent a descriptive summary of each regimen, but it limits how confidently a difference can be attributed to dose or route. For example, if groups differ in participant characteristics or other aspects of study conduct, those differences may also relate to the outcome. The registry does not report the allocation procedure or characteristics by group, so the possibility and extent of imbalance cannot be assessed.

The parallel design indicates separate groups receiving the different regimens rather than a sequence of regimens for every participant. The safety record lists six participants at risk in each regimen group. That does not establish that all participants had complete primary endpoint results; completeness for the infection endpoint is not reported. The record lists enrollment of 18, but the analyzed population and exclusions, if any, are not described.

How should uncertainty be understood?

In general, a confidence interval describes the range of values compatible with the observed data under the chosen statistical method and its assumptions. For a binary outcome, uncertainty is affected by the number of participants and the number with the event. When groups are small, estimates can be imprecise, and a single event can have a substantial influence on a group summary. The primary infection counts are not reported, and no confidence intervals or other primary outcome analyses are listed, so no specific estimate or precision statement is available.

The registry lists 0 analyses. It does not report a model, a test for comparing regimens, adjustment for covariates, or a prespecified statistical plan. It would therefore be inappropriate to attribute a particular inferential method to the study. In general, an unadjusted comparison describes group differences without accounting for other variables; an adjusted analysis attempts to account for selected measured factors, but depends on the model and data. Neither approach is documented for the primary endpoint in the registry record.

The record also includes general planning assumptions for illustrative sample-size calculations: a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation, with the effect to detect defined as a stated fraction of the observed effect. Those assumptions describe planning illustrations, not the original trial design. The record does not report the trial's own sample-size justification. No numerical planning estimates are included in the registry record, and an illustration based on an observed effect should not be confused with evidence that the study had a particular probability of detecting an effect.

What limits a comparison?

The regimens vary in both dose and route across the listed groups. The two 2,500-sporozoite regimens differ by route and injection sites, while the two intramuscular regimens differ by dose. In general, changing more than one feature across groups makes it difficult to isolate the contribution of a single feature. The record does not describe a factorial design or an analysis designed to separate dose and route effects. Because allocation was non-randomized, the groups are also not described as like-for-like controls.

Finally, the study is open label, and the record does not report masking of participants or assessors. For an endpoint based on laboratory tests, the relevance of masking depends on how samples were collected, processed, and interpreted; those procedures are not detailed. The registry does not report missing-data handling, analysis populations, or sensitivity analyses. These omissions leave important aspects of the analysis and its robustness unknown.

10. Exact Estimates for Each Arm

No two-arm comparison is shown for this trial, but each arm's result can still be summarized with exact interval estimates computed from the posted counts. The Clopper-Pearson interval is exact and conservative; the Wilson interval is a common closer-to-nominal alternative; the Bayesian credible interval uses a uniform Beta(1, 1) prior. For small groups the three can differ noticeably.

R code and output

arm    <- c("PfSPZ Challenge 2,500 ID", "PfSPZ Challenge 2,500 IM", "PfSPZ Challenge 25,000 IM")
n      <- c(6, 6, 6)
events <- c(5, 3, 6)
for (i in seq_along(n)) {
  ci <- binom.test(events[i], n[i])$conf.int          # Clopper-Pearson exact interval
  cr <- qbeta(c(0.025, 0.975), 1 + events[i], 1 + n[i] - events[i])   # Bayesian, uniform Beta(1,1) prior
  wi <- (function(x, n) { p <- x / n; z <- qnorm(0.975); d <- 1 + z^2 / n
    c((p + z^2 / (2 * n) - z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d, (p + z^2 / (2 * n) + z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d) })(events[i], n[i])
  cat(sprintf("ARM|%s|%d|%d|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f\n", arm[i], n[i], events[i], 100 * events[i] / n[i],
              100 * ci[1], 100 * ci[2], 100 * wi[1], 100 * wi[2], 100 * cr[1], 100 * cr[2]))
}

ARM  PfSPZ Challenge 2,500 ID  6  5  83.3  35.9  99.6  43.6  97.0  42.1  96.3
ARM  PfSPZ Challenge 2,500 IM  6  3  50.0  11.8  88.2  18.8  81.2  18.4  81.6
ARM  PfSPZ Challenge 25,000 IM  6  6  100.0  54.1  100.0  61.0  100.0  59.0  99.6

11. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary outcome, the number of participants infected by 21 days, and two secondary outcome areas: adverse events and serious adverse events, and parasite-growth dynamics. It does not report a statistical analysis plan or analyses for these outcomes. No approach to multiplicity is reported in the registry record.

In general, multiplicity arises when several outcomes, groups, timepoints, or statistical comparisons are examined. Looking at many results can increase the chance of finding an apparently notable result by chance, so an analysis plan may define which outcomes are primary and how multiple comparisons will be handled. The listed primary and secondary outcomes establish a hierarchy in the registry description, but do not establish how multiple tests or comparisons were handled.

Missing-data handling is also not reported. The record does not state whether all enrolled participants had an evaluable infection outcome, how participants with incomplete follow-up or unavailable test results were classified, or whether any imputation or sensitivity analysis was used. It also does not report the analysis population for the primary endpoint. Enrollment and the safety populations at risk are recorded, but these do not show whether the primary endpoint analysis included the same participants.

These gaps matter because missing outcomes can affect event counts, and because conclusions can differ depending on how unavailable results are treated. The record gives no basis for reconstructing those decisions. No primary statistical analyses are listed, so there is no reported inferential result to assess for multiplicity or missing-data sensitivity.

12. 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 non-serious adverse event terms

The safety record lists three regimen groups. Each group has six participants at risk for serious adverse events, with zero affected by a serious adverse event. The record lists other adverse events affecting two participants in the 2,500 intradermal group, two in the 2,500 intramuscular group, and three in the 25,000 intramuscular group. The serious adverse event term list is empty. Death counts are not reported in the registry record.

These are counts of participants recorded as affected, not a complete description of the nature, timing, severity, or duration of events. The registry lists the secondary safety outcome as the frequency, incidence, and nature of adverse events and serious adverse events, but the posted safety information does not supply detailed event terms. The record also does not describe the observation period for these safety counts.

In general, zero recorded serious events in a small group does not prove that the risk is absent. A lack of recorded events can be compatible with uncertainty, particularly when the number observed is limited. The counts should be read descriptively and not as a randomized comparison of safety between regimens. Non-randomized allocation and the absence of a like-for-like control group further limit comparative interpretation. The record does not provide a statistical analysis of safety differences.

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

The study record documents an effort to optimize controlled human malaria infection using Plasmodium falciparum sporozoites administered by needle and syringe. It identifies three regimens that vary in dose, route, and injection sites, and defines the primary outcome as the number of participants infected within 21 days. It also records selected safety counts and lists parasite-growth dynamics as a secondary outcome.

Those details establish what the study intended to examine, but they do not establish which regimen had the greatest infection rate. The primary infection counts are not reported in the registry record, and 0 statistical analyses are listed. There is therefore no posted estimate of infection frequency by regimen, no statistical comparison to interpret, and no reported confidence interval to describe its precision. The absence of a reported result should not be interpreted as a finding of no difference.

The safety counts show zero participants affected by a serious adverse event in each group, with other adverse events recorded for the stated numbers of participants. These are descriptive entries for the listed groups. They do not establish that serious events cannot occur, nor do they support a definitive safety ranking among regimens. Death counts are not reported in the registry record.

Even if outcome counts were available, the design would require cautious interpretation. Allocation was non-randomized, the study was open label, and the groups are not described as like-for-like control groups. Differences in outcomes could not automatically be attributed to a single regimen feature. The registry record also does not report the allocation method, analysis populations, handling of missing outcomes, or statistical analysis plan.

The appropriate statistical reading is therefore limited: this record describes the pilot's question and methods at a broad level, but does not provide the primary result needed to answer the question about comparative infectivity. It offers some safety counts without enough event detail for a comprehensive safety assessment. No treatment recommendation or conclusion about clinical effectiveness follows from these registry entries.

14. Why This Trial Matters Statistically

This study raises a useful statistical design issue: when a pilot varies dose and route across small, non-randomized groups, its results are primarily descriptive unless the design and analysis support stronger comparisons. The stated objective was to identify a regimen with the greatest infection rate, and the primary outcome was defined as infection by 21 days. That makes the outcome clinically and methodologically specific, while also emphasizing the need for explicit diagnostic rules and complete outcome reporting.

The regimens vary across several features. One uses 2,500 sporozoites intradermally at two sites, another uses the same dose intramuscularly at one site, and a third uses 25,000 sporozoites intramuscularly at one site. In general, when regimen features change together, a contrast between groups cannot isolate which feature accounts for an outcome difference. The non-randomized allocation adds the possibility that groups differ in other ways as well.

The record is also a reminder that an outcome definition is not the same as an outcome result. Naming microscopy and PCR identifies how infection was to be assessed, but readers also need the observed counts, denominators, handling of test disagreement, and uncertainty estimates to interpret infectivity. Those primary results are not reported in the registry record.

Finally, safety counts require context. A zero count of serious adverse events in groups with six participants at risk is descriptive, not a guarantee of no risk. The sparse group sizes and lack of comparative analysis limit what can be learned about differences in safety. This record is statistically informative about the intended question and design constraints, but its missing primary results prevent assessment of the central infectivity objective.

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.

The primary limitation for interpretation is that the registry record does not report the number of participants infected in any regimen. The primary endpoint's timeframe and diagnostic methods are described, but no outcome counts, estimates, or confidence intervals are listed. As a result, the central question about infectivity cannot be answered from the posted results.

The design is non-randomized and unmasked. The registry explicitly cautions that the arms are not a like-for-like control comparison. It does not explain how participants were allocated, whether group characteristics were comparable, or whether any adjustment was planned. The three regimens also vary by dose, route, and injection sites, limiting attribution of a potential difference to a single factor.

Enrollment is listed as 18, and the safety record lists six participants at risk in each regimen group. The record does not report the primary endpoint analysis population, the number with complete follow-up, or whether any participants were excluded from the infection analysis. Missing-data procedures and sensitivity analyses are not reported. The diagnostic rule for combining microscopy and PCR findings is also not specified.

Although safety counts are listed, detailed adverse event terms, timing, severity, and duration are not reported in the registry record; the serious adverse event term list is empty. Death counts are not reported. The record does not provide detailed results for parasite-growth dynamics, despite listing that outcome as secondary. It also does not report an analysis plan, multiplicity procedures, interim analyses, or a sample-size justification. These limitations reflect the scope of the registry record and do not establish what may have been documented elsewhere.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Optimisation of Controlled Human Malaria Infection Using Sporozoites Administered by Needle and Syringe — NCT01465048.
• Registry record last updated 2013-06-24; status last verified 2013-06.
• Results first posted 2013-05-28.
• 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

NCT01465048 is a completed, open-label, non-randomized parallel pilot study sponsored by the University of Oxford. The registry lists enrollment of 18 and describes three PfSPZ Challenge regimens: 2,500 sporozoites intradermally at two sites, 2,500 sporozoites intramuscularly at one site, and 25,000 sporozoites intramuscularly at one site. The primary outcome was the number infected by 21 days, assessed using thick film microscopy and highly sensitive PCR.

The registry record does not report primary infection counts or statistical analyses. Safety entries list six participants at risk for serious adverse events in each group, with zero affected in each group; other adverse events affected two, two, and three participants, respectively. Detailed event terms and death counts are not reported. Because allocation was non-randomized and the arms are not like-for-like controls, comparative conclusions require caution.