Human Lung Responses to Respiratory Pathogens: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Human Lung Responses to Respiratory Pathogens (NCT01967628), a randomized phase 1, phase 2 study. The primary outcome is Antimicrobial Activity by Airway Surface Liquid (ASL) as Measured by Relative Light Units (RLU).

2. Clinical Question

The scientific motivation is that seasonal changes in bacterial infection incidence and severity may reflect, in part, variation in vitamin D, an immune modulator with seasonal periodicity. The hypothesis is that vitamin D levels are an important determinant of innate defense against inhaled bacteria. This rationale motivates measuring a laboratory response in airway surface liquid, but it does not establish that the measured response captures all aspects of respiratory infection or clinical illness. The primary outcome is a surrogate laboratory measure, and the registry record does not report clinical infection outcomes as secondary results.

3. Trial Design

The registry lists NCT01967628, titled “Human Lung Responses to Respiratory Pathogens,” as a completed study sponsored by the University of Iowa. It lists Phase 1 and Phase 2, randomized allocation, triple masking, and a single-group model. Two interventions are described: Vitamin D3 and a sugar capsule placebo. Because the record simultaneously identifies randomized allocation and a single-group model, its design labels do not provide a fully consistent description of the study structure. The precise allocation process and the roles of the masked parties are not reported in the registry record.

The enrollment was 98. The registry lists a start of 2007-06 and primary completion of 2010-07. Those dates describe the study timeline, not necessarily the timing of every participant’s treatment or outcome assessment. The primary outcome timeframe is 3 months. The posted outcome estimates have 21 participants in the Vitamin D3 arm and 19 in the Sugar Capsule arm. The record does not explain how those outcome counts relate to total enrollment, including whether participants did not contribute an outcome, were excluded from a particular analysis, or had missing measurements.

The intervention is described as Vitamin D3 (1000 international units) daily for 3 months. The placebo is described as a sugar capsule. The primary outcome is antimicrobial activity of airway surface liquid, assessed using a bioluminescent bacterial challenge and relative light units. The record reports arm-level means with 95% confidence intervals, but no analysis comparing the arms. A protocol, detailed statistical analysis plan, sample-size justification, and interim analysis information are not reported in the registry record. The listed study phase and design features do not fill in those missing details.

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: Subjects were included if they were 18-60 years.Subjects were excluded if taking a multivitamin or vitamin D supplement within the previous three months, pregnant or breastfeeding,had a vaccination within one month, significant comorbid conditions, respiratory infection, history of positive tuberculin skin test, or use of antibiotics recently.

Overall Study

Reasons for not completing this period:

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 Antimicrobial Activity by Airway Surface Liquid (ASL) as Measured by Relative Light Units (RLU), measured over 3 months.

The primary endpoint is titled “Antimicrobial Activity by Airway Surface Liquid (ASL) as Measured by Relative Light Units (RLU).” The study description says that airway surface liquid samples were challenged with bioluminescent bacteria and that live bacteria were measured in relative light units after 2 minutes. The measure is used as a laboratory surrogate for antimicrobial activity: the description interprets a reduction in live bacteria, reflected by lower relative light units after the challenge, as increased antimicrobial activity.

This endpoint focuses on an innate airway defense response rather than a clinical outcome such as diagnosed infection. It provides a way to examine a biological response in airway surface liquid under the challenge conditions described. A surrogate measure can help investigate a proposed biological mechanism, but its meaning is limited to what the assay measures. The registry does not report whether the assay has been validated as a predictor of respiratory infection outcomes, nor does it report results for clinical infections.

The outcome is reported as a mean in relative light units with a 95% confidence interval and a 3-month timeframe. The registry description does not clarify whether the negative arm estimates represent changes from baseline, transformed measurements, or another quantity derived from the assay. It also does not explain the reference value against which these estimates should be interpreted. This limits interpretation of the arm-level values beyond their reported scale and intervals.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous and reported as a mean with a 95% confidence interval. A continuous outcome can take values across a measurement scale, rather than being limited to a category such as yes or no. A mean describes the arithmetic center of a set of measurements. It does not show how widely individual measurements varied. The record does not post a measure of spread for these arm-level outcomes.

What do the confidence intervals describe?

In general, a confidence interval represents uncertainty around an estimate under the statistical method used. A narrower interval indicates greater precision under that method; a wider interval indicates less precision. The reported intervals are -1122.0 to -40.0 for Vitamin D3 and -1170.0 to 199.0 for Sugar Capsule. The registry identifies them as 95% confidence intervals but does not report the method used to construct them. Their width should be considered alongside the limited outcome counts of 21 and 19 participants.

Confidence intervals for separate group means do not directly provide the uncertainty around a difference between groups. A comparative analysis would need to estimate the between-arm contrast and its uncertainty, with a method appropriate to the design and outcome. No such estimate or test is posted. The registry specifically notes that values were missing or not estimable for a two-arm comparison of this continuous endpoint. Therefore, statistical significance of a group difference cannot be inferred from the arm-level intervals.

Why does the outcome definition matter?

The endpoint description connects lower live-bacteria readings after challenge with increased antimicrobial activity. The listed estimates, however, are negative, and the record does not explain whether these are raw measurements, changes from a reference, or another derived quantity. In general, the estimand—the quantity an analysis is intended to estimate—must be clearly defined to interpret the direction and meaning of a result. Here, the exact estimand and its reference are not reported in the registry record. Accordingly, the reported means should not be treated as raw bacterial counts or as a defined treatment effect.

Randomization, when implemented as intended, can support a fair comparison by making treatment assignment independent of participant characteristics on average. The registry lists randomized allocation, but its model is described as single group even though Vitamin D3 and Sugar Capsule arms are listed. The detailed allocation process and any analysis accounting for it are not reported. The record’s design labels alone cannot resolve that ambiguity or establish how the posted results were analyzed.

What can be concluded from the analysis information?

The registry reports zero posted analyses and does not include a two-arm result for this continuous endpoint. It does not report the analysis population definition, covariate adjustment, handling of repeated measurements, missing-data method, or model assumptions. The choice among statistical methods can matter: for example, general methods that assume a particular distribution may be less suitable when the data are strongly skewed or include influential observations. This is general statistical context, not a description of the analysis used in this trial; the analysis method is not reported in the registry record.

The record also does not report whether the endpoint was assessed at baseline, whether the outcome was analyzed as a final value or change, or whether additional measurements were collected. These details affect how an estimate should be interpreted and how a comparative analysis might be specified. They cannot be inferred from the 3-month timeframe alone.

Sample-size calculations are planning tools, not measures of what a completed trial actually achieved. The registry includes general assumptions for planning illustrations based on an observed effect, but no specific planning results are posted in the registry record for this study. Any such illustration would not be observed power and would not replace a reported treatment contrast with a confidence interval. The trial’s own sample-size justification is not reported in the registry record.

Multiplicity refers generally to the increased chance of a false-positive finding when multiple hypotheses are tested. The record reports one primary outcome measure and no posted secondary outcome results, but does not provide a statistical plan describing how multiple measurements, time points, or analyses were handled. It would therefore be inappropriate to assume a particular multiplicity adjustment or to claim that none was needed. The number of analyses shown as posted is zero, which means the registry record does not supply a comparative analysis to interpret.

10. Multiplicity, Missing Data and Analysis Populations

The registry identifies one primary outcome measure and reports no secondary outcome results. That does not establish that only one outcome or analysis was planned or performed; the protocol and statistical analysis plan are not reported in the registry record. The record also does not describe any multiplicity strategy. In general, testing many outcomes, subgroups, time points, or analysis choices can increase the chance of a chance-positive result. Statistical plans may address that issue by defining a primary comparison and specifying adjustments, but no such plan is reported.

The posted outcome counts are 21 for Vitamin D3 and 19 for Sugar Capsule, while enrollment was 98. The record does not explain how the analyzed participants were selected or whether unobserved outcomes account for the difference. It does not report missing-data counts, reasons for missingness, imputation, sensitivity analyses, or an analysis population such as intention-to-treat. It is therefore not possible to determine from the registry record whether the outcome estimates include all randomized participants or how incomplete outcomes were handled.

The record also does not report whether measurements were made at baseline or whether the endpoint is a final measurement or a change from baseline. These omissions matter because analysis populations, missing-data handling, and outcome definitions shape the interpretation of results. No assumptions or methods should be attributed to the investigators without a report of them.

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 5% reporting threshold.

For the Vitamin D Airway Surface Liquid group, the registry reports 21 participants at risk for serious adverse events, 0 affected, 21 at risk for other adverse events, 0 affected, and 0 deaths. For the Placebo Airway Surface Liquid group, it reports 19 participants at risk for serious adverse events, 0 affected, 19 at risk for other adverse events, 0 affected, and 0 deaths. No serious adverse-event terms are listed.

These figures are counts in the posted safety record. A zero reported count means that no affected participants are recorded for that category in these groups; it is not proof that an event is impossible or that the intervention has no risk. The record does not provide event definitions, observation duration for safety reporting, ascertainment procedures, or information about events outside these categories. The counts should not be interpreted as a comparative safety analysis, especially because no statistical comparison is reported. They also should not be generalized beyond the participants and reporting captured in the registry.

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

The posted results describe arm-level estimates for a laboratory endpoint: mean -581.0 relative light units for Vitamin D3 and mean -485.0 relative light units for Sugar Capsule, with the respective 95% confidence intervals and participant counts. They do not report an estimate of the difference between the groups. Thus, the record does not quantify how much the groups differed, how precise that difference would be, or whether a statistical test found evidence of a difference.

The biological rationale concerns vitamin D and innate lung defenses. The endpoint uses a bioluminescent bacterial challenge of airway surface liquid and measures live bacteria after 2 minutes. This can inform a laboratory question about antimicrobial activity under the assay conditions. It is not itself a count of respiratory infections, a measure of pneumonia or bronchitis, or a direct report of clinical benefit. The registry does not post secondary clinical outcomes that could connect the assay findings to those outcomes.

The interpretation is further limited because the record does not clarify what the negative arm means represent or define their reference. Although its endpoint description interprets a reduction in live bacteria as increased antimicrobial activity, it does not say whether the posted means are raw values, changes, or another summary. The estimates should therefore not be given a stronger biological interpretation than the record supports.

In general, a mechanistic or surrogate measure can be useful for studying a proposed pathway, but its relevance to patient outcomes depends on evidence connecting the measure to those outcomes. Such validation is not reported in the registry record. The posted safety counts record no affected participants in the listed categories, but they do not establish absence of risk. Overall, the registry supports a description of the reported arm-level assay estimates and safety counts, not a conclusion about comparative clinical effects or the effectiveness of vitamin D3.

13. Why This Trial Matters Statistically

This study illustrates an important distinction in clinical trial reporting: arm-level summaries are not the same as a comparative treatment estimate. The registry reports means and confidence intervals within each arm, but no contrast between the Vitamin D3 and Sugar Capsule groups. Without a contrast and its uncertainty, readers cannot directly assess the estimated difference between assigned groups. The fact that the intervals are reported for both arms does not resolve that gap.

It also demonstrates why outcome definitions are essential. The endpoint is described as a surrogate measure based on live bacteria in a challenged airway surface liquid sample, yet the registry does not clarify whether the posted negative means are raw values or a derived quantity. Clear definitions of the analysis target, scale, reference, and timing are central to interpreting an estimate, especially when its numerical sign is not self-explanatory.

The record provides another useful lesson about denominators. Enrollment was 98, while the posted arm-level outcome counts are 21 and 19. Without an account of participant flow and missing outcomes, readers cannot establish how the analyzed participants relate to the full enrolled population. Transparent reporting of outcome availability and analysis populations helps readers judge the reach and limitations of an estimate.

Finally, the registry labels allocation as randomized and masking as triple, while describing the model as single group and listing two intervention arms. Reporting design features consistently, alongside the analysis plan and results, is important for understanding what comparison was intended and how it was evaluated. Here, the absence of a posted two-arm analysis leaves the principal comparative question unanswered by the registry results.

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 most important limitation of the registry record for interpretation is the absence of a comparative result for the primary endpoint. Although it reports arm-level means, intervals, and counts, it does not report the between-arm estimate or its uncertainty. The record states that values were missing or not estimable for a two-arm comparison. As a result, the primary comparative question cannot be answered from the posted outcome results.

The endpoint’s numerical meaning is also incompletely specified. The description explains the assay and its intended interpretation, but does not clarify why the arm-level means are negative or whether they represent raw readings, changes, or another derived measure. It does not identify the reference value for those estimates. This prevents a confident interpretation of the estimates’ direction in biological terms.

The registry reports enrollment of 98, but outcome counts of 21 and 19 in the arms. Participant flow, outcome availability, reasons for missing data, and the analysis population are not reported in the registry record. Nor are methods for handling missing outcomes, baseline measurements, covariate adjustment, or statistical model assumptions. The effect of these unknowns on the posted estimates cannot be assessed.

The design description lists randomized allocation and triple masking, while also labeling the model single group and listing two intervention arms. The precise design structure and masking procedures are not reported. The record also lacks a reported protocol, sample-size justification, interim analysis information, and statistical analysis plan. Finally, the primary outcome is a laboratory surrogate; clinical infection outcomes and its validation as a predictor of clinical outcomes are not reported. These limitations restrict both statistical and clinical interpretation.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Human Lung Responses to Respiratory Pathogens — NCT01967628.
• Registry record last updated 2018-03-29; status last verified 2018-02.
• Results first posted 2018-03-29.
• 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

NCT01967628 is a completed study titled “Human Lung Responses to Respiratory Pathogens,” sponsored by the University of Iowa. The registry lists Phase 1 and Phase 2, randomized allocation, triple masking, and a single-group model. Enrollment was 98, with a listed start of 2007-06 and primary completion of 2010-07. Vitamin D3 (1000 international units) daily for 3 months was compared with a sugar capsule placebo.

The primary outcome was airway surface liquid antimicrobial activity assessed by a bioluminescent bacterial challenge and reported in relative light units at 3 months. The registry reports means and 95% confidence intervals for 21 Vitamin D3 participants and 19 Sugar Capsule participants. It does not report a two-arm comparison, and it does not clarify the interpretation of the negative means. No secondary outcome results are posted. Safety listings record zero serious adverse events, zero other adverse events, and zero deaths in each listed group.