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 Tick-borne Illness and Clothing Study (NCT01454414), a randomized na study. The primary outcome is Work Related Tick Bites.
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT01454414 |
| Phase | NA |
| Condition | Tick Bites, Tick-borne Illness |
| Allocation | Randomized |
| Design model | Parallel |
| Masking | Double |
| Enrollment | 159 |
| Arms in the primary outcome | 2 |
| Lead sponsor | University of North Carolina, Chapel Hill |
| Results posted | Yes |
2. Clinical Question
Population
North Carolina outdoor workers at risk of tick exposure are the population described in the registry record. The record notes that outdoor occupations can involve frequent exposure to tick-infested habitats. Specific eligibility criteria and participant characteristics are not reported in the registry record.
Intervention
Workers were assigned uniforms treated with long-lasting permethrin. The described uniform set included pants, shorts, shirts, socks, and hats. The registry record identifies the intervention as permethrin-impregnated uniforms.
Comparator
The comparator was placebo uniforms sent to Insect Shield, washed, and refolded, with no permethrin applied. This provides a comparison with uniforms processed in a similar way, as described in the record.
Primary question
The primary question was whether the intervention group had a different rate of work-related tick bites than the placebo group. A tick bite was defined as a tick attached to or embedded in the skin, and bites were recorded weekly for two years.
The registry record frames the study around the concern that outdoor workers may sustain repeated tick bites despite preventive strategies. The clinical question is therefore about the number of reported work-related bites, not directly about whether participants developed a tick-borne illness. Seroconversion against a tick-borne illness was listed as a secondary outcome, but its result is not included in the posted outcome figures available for interpretation.
The comparison is randomized and parallel, according to the record. Random allocation is intended to make groups comparable on average, but the registry record does not describe the sequence-generation procedure, allocation concealment, or participant-level baseline characteristics. The result should consequently be read as a comparison of the groups as analyzed in the registry record, with those details of implementation left unresolved.
3. Trial Design
The study is recorded as randomized, double masked, and parallel. Enrollment is listed as 159. The registry record identifies the sponsor as the University of North Carolina, Chapel Hill, and gives the study status as completed. The recorded phase is NA. The start is listed as 2010-10 and primary completion as 2012-10.
Participants were assigned to permethrin-impregnated uniforms or placebo uniforms. The intervention description includes pants, shorts, shirts, socks, and hats treated with long-lasting permethrin. The placebo uniforms were sent to Insect Shield, washed, and refolded without permethrin. The record does not give further information about the uniform distribution schedule, adherence monitoring, replacement garments, or the way masking was maintained.
The primary outcome was work-related tick bites, assessed weekly for two years. The posted group summaries list 66 participants in the permethrin group and 64 in the placebo group. They report 265 tick bites for the permethrin group and 780 for the placebo group. These are group-level count summaries; the record does not provide participant-level counts, the distribution of bites among individuals, or details about the amount of time each participant contributed to follow-up.
The registry lists two outcome measures and one reported analysis. A secondary outcome is seroconversion against a tick-borne illness. The record does not report further trial-protocol details such as the planned analysis population, primary analysis model specification, handling of missing observations, interim analyses, or sample-size justification. Those omissions limit what can be said about whether the posted analysis follows a prespecified statistical plan.
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
| Stage | Permethrin Impregnated Uniforms | Placebo |
|---|---|---|
| STARTED | 79 | 80 |
| Baseline: Subjects That Sent Uniforms | 67 | 66 |
| COMPLETED | 66 | 64 |
| NOT COMPLETED | 13 | 16 |
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.
| Characteristic | Unit | Permethrin Impregnated Uniforms | Placebo | Total |
|---|---|---|---|---|
| Participants analysed | 67 | 66 | 133 | |
| Age, Continuous | years | 39.1 (SD 9.2) | 38.8 (SD 9.3) | 39.0 (SD 9.2) |
| Sex/Gender, Customized: Female | participants | 14 | 11 | 25 |
| Sex/Gender, Customized: Male | participants | 52 | 55 | 107 |
| Race/Ethnicity, Customized: White | participants | 56 | 58 | 114 |
| Race/Ethnicity, Customized: Black | participants | 2 | 0 | 2 |
| Race/Ethnicity, Customized: Other | participants | 0 | 1 | 1 |
| Education, categorical: High school or less | participants | 5 | 10 | 15 |
| Education, categorical: Some college | participants | 20 | 14 | 34 |
| Education, categorical: Bachelor or graduate degree | participants | 40 | 42 | 82 |
| Work division, categorical: NC Forest service | participants | 21 | 27 | 48 |
| Work division, categorical: NC Division of Parks and Recreation | participants | 31 | 25 | 56 |
| Work division, categorical: NC Wildlife Resources Commission | participants | 9 | 10 | 19 |
| Work division, categorical: NC Local or County Parks and Recreation | participants | 5 | 4 | 9 |
| Years in current position, continuous | years | 7.9 (SD 6.7) | 8.2 (SD 6.8) | 8.0 (SD 6.7) |
| Tick bites in previous year, continuous | tick bites | 19.3 (SD 32.0) | 19.6 (SD 39.3) | 19.4 (SD 35.8) |
| Previous diagnosis with tick-borne illness: Lyme disease | participants | 4 | 4 | 8 |
| Previous diagnosis with tick-borne illness: Rocky Mountain spotted fever | participants | 6 | 7 | 13 |
| Previous diagnosis with tick-borne illness: Ehrlichiosis | participants | 2 | 3 | 5 |
| Previous diagnosis with tick-borne illness: Anaplasmosis | participants | 0 | 0 | 0 |
| Previous diagnosis with tick-borne illness: Babesiosis | participants | 0 | 0 | 0 |
| Use of tick bite prevention measures: Long pants | participants | 62 | 62 | 124 |
| Use of tick bite prevention measures: Long sleeves | participants | 48 | 52 | 100 |
| Use of tick bite prevention measures: Hat | participants | 43 | 38 | 81 |
| Use of tick bite prevention measures: Tucked or taped pant legs | participants | 5 | 7 | 12 |
| Use of tick bite prevention measures: Repellent on skin | participants | 22 | 22 | 44 |
| Use of tick bite prevention measures: Repellent on clothing | participants | 38 | 35 | 73 |
| Use of tick bite prevention measures: Tick checks after working outdoors | participants | 58 | 60 | 118 |
| Use of self-applied permethrin on clothing | participants | 30 | 34 | 64 |
6. Primary Endpoint
The registry lists the primary outcome as Work Related Tick Bites, measured over Weekly for two years.
| Element | Registry entry |
|---|---|
| Result type | Number |
| Measure of spread | not applicable |
| Unit | tick bites |
| Endpoint classification | not reduced to a single comparison |
The primary endpoint is the number of work-related tick bites. The registry defines a bite as a tick attached to or embedded in the skin and states that bites were assessed weekly for two years. The outcome is therefore a count of events, rather than a simple yes-or-no indicator of whether a participant ever had a bite.
Counts can retain information about repeated events. Two participants may both have at least one bite but differ in how many bites they experience; an ever-bitten measure would not distinguish those patterns. A count endpoint can therefore describe the burden of repeated events over follow-up. The reported arm totals are 265 tick bites for the permethrin group and 780 for the placebo group, alongside group sizes of 66 and 64, respectively.
As a general statistical point, a count comparison needs to account for how many participants contribute observations and how long they are observed. It may also need to account for variation in event counts between participants. The registry record identifies Poisson regression for the reported comparison, but does not give the model specification or participant-level follow-up information. The endpoint is tick bites, not a direct measure of tick-borne disease. Although seroconversion against a tick-borne illness is listed as a secondary outcome, its results are not reported in the registry record figures.
7. Why No Re-analysis Is Shown
8. Statistical Interpretation
The posted group summaries show 265 work-related tick bites in the permethrin-uniform group and 780 in the placebo-uniform group. The registry's reported analysis used Poisson regression and gives protective effectiveness of 0.646, with a 95% confidence interval from 0.288 to 0.824 and p = 0.004. The stated protective-effectiveness measure is defined as 1 minus the rate ratio.
On the reported scale, the point estimate indicates lower tick-bite rates in the permethrin group than in the placebo group. The confidence interval expresses uncertainty around that estimate: values across the interval remain compatible with a range of effect magnitudes. It does not identify the exact effect in the underlying population, and it should not be read as a range containing most individual participants' outcomes. The p-value indicates how unusual data at least as incompatible with the no-difference model would be under the assumptions of the analysis; it is not the probability that the null hypothesis is true.
The registry reports a nominal p-value of 0.004. The record does not describe adjustment for multiple outcome testing, a prespecified testing hierarchy, or other multiplicity procedures. As a result, the p-value should be understood in the context of the reported analysis rather than as evidence that all possible analyses or outcomes were accounted for.
The arm totals are useful descriptive context but do not, by themselves, give a rate comparison that accounts for individual follow-up. The model-based reported estimate is the main comparative result. The record does not report the distribution of bites among participants, the person-time at risk, or how repeated events and variable observation time were handled in the model. These omissions mean that the analysis cannot be independently reconstructed in full from the posted summaries.
The result addresses work-related tick bites as recorded over the stated follow-up period. It does not establish from the posted figures whether tick-borne illness was prevented, because the secondary seroconversion outcome has no posted result. Nor does the registry record support conclusions about other populations, occupations, garments, or settings not described in the trial. The interval and model result give an estimate of the bite-count comparison, not a guarantee that the same magnitude would occur in every setting.
9. Statistical Methods Explained
The registry reports Poisson regression for the primary comparison. The stated effect measure is protective effectiveness, defined as 1 minus the rate ratio. The estimated value is 0.646, with a 95% confidence interval from 0.288 to 0.824 and p = 0.004. The record does not give additional details about the model, such as covariates, offsets, robust variance estimation, or diagnostic checks.
What does Poisson regression do?
As a general statistical explanation, Poisson regression is commonly used to model counts of events. It relates an expected event count or event rate to a group or other explanatory variables. A rate ratio compares event rates between groups; the reported protective-effectiveness scale transforms that ratio by subtracting it from one. The registry gives the transformed estimate but does not post the underlying rate ratio as a separate result.
For a rate comparison, time at risk matters: a group observed for longer may accumulate more events even if its underlying event rate is similar. A model can account for differing exposure time when its specification includes an appropriate exposure-time term. The registry record does not state whether or how such a term was used. It also does not give individual observation times, so the rate construction cannot be independently checked from the group summaries alone.
Why does count variation matter?
As a general statistical point, a basic Poisson model assumes a particular relationship between the average count and its variance. Real event counts may vary more across participants than that model allows, a situation often called over-dispersion. When over-dispersion is present and not adequately addressed, standard errors and confidence intervals can be too optimistic. The registry record does not report a dispersion assessment or describe an adjustment for over-dispersion. This is not evidence that over-dispersion occurred; it means that the posted record does not show how this issue was evaluated.
Repeated events also create a structure in which observations from the same participant are related. Appropriate modeling decisions may depend on whether the analysis used individual participant counts, repeated weekly observations, or another aggregation. The registry record does not specify the data structure used in the Poisson regression, so the handling of within-participant dependence is not reported in the registry record.
How should the confidence interval be read?
As a general explanation, a confidence interval describes uncertainty in an estimate under the statistical model and its assumptions. The 95% confidence interval for the reported protective-effectiveness estimate extends from 0.288 to 0.824. The interval is not a statement that 95% of participants had an effect in this range, nor is it a guarantee about the true value. Its interpretation depends on the model being suitable and the analysis being carried out as intended. The registry does not provide enough model detail to assess all of those assumptions.
The p-value, 0.004, is another model-based summary. In general, it describes the compatibility of the observed data with a specified null model, assuming that model and the analysis assumptions are correct. It does not measure the probability that the result is due to chance, and it does not quantify the size or practical importance of the effect. The estimated effect and its confidence interval are more informative about magnitude and precision than a p-value alone.
What can the arm summaries support?
The registry lists 265 bites among 66 participants assigned to permethrin uniforms and 780 bites among 64 assigned to placebo uniforms. These are descriptive group summaries. They do not disclose whether the totals arose from a relatively even distribution of bites or from a small number of participants with many events. Nor do they show how missing weeks or unequal follow-up were handled. A recalculation from the posted summaries is not reported as estimable for this count endpoint, so the model-based analysis is the available comparative estimate.
The record does not report the analysis population, model covariates, missing-data method, protocol-based hypotheses, or sensitivity analyses. These omissions do not invalidate the posted estimate on their own, but they restrict independent evaluation of its robustness. They also prevent a precise assessment of whether the reported confidence interval fully reflects features such as participant-level clustering, variable follow-up, and count dispersion.
10. Analyses Reported by the Sponsor
| Method | Estimate | 95% CI | P-value |
|---|---|---|---|
| Poisson regression | Protective effectiveness= 1 - rate ratio 0.646 | 0.288 to 0.824 | 0.004 |
The registry's reported comparison is the Poisson regression analysis, with protective effectiveness of 0.646, a 95% confidence interval from 0.288 to 0.824, and p = 0.004. A separate unadjusted recalculation is not reported as estimable for this count endpoint from the posted summary statistics. The arm-level counts and group sizes do not include the participant-level event or follow-up information needed to reconstruct the model-based rate comparison.
Accordingly, there is no recalculated estimate to compare numerically with the sponsor's analysis. The registry record does not report a discrepancy between a sponsor analysis and a recalculated result. This does not independently verify the model specification or its assumptions; those details are not reported in the registry record. The sponsor's estimate remains the only posted inferential comparison for the primary outcome, and interpretation should account for the limits on reconstructing it from the available summaries.
The reported estimate concerns work-related tick bites. The registry record does not provide a posted result for the secondary seroconversion outcome, so the sponsor's primary analysis should not be treated as a substitute for an illness outcome analysis.
11. Multiplicity, Missing Data and Analysis Populations
The registry identifies one reported analysis and two outcome measures. The primary outcome is work-related tick bites; the listed secondary outcome is seroconversion against a tick-borne illness. The record does not describe a prespecified testing hierarchy, multiplicity adjustment, or how the primary and secondary outcomes were intended to be interpreted together. It is therefore not possible to determine from the registry record whether the reported p-value reflects a broader planned family of statistical tests.
Multiplicity is a general statistical concern when several hypotheses are tested. The chance of at least one apparently strong result can increase as more tests are considered, depending on how the tests are related and how results are selected. This does not establish that the reported p-value is misleading; it means the testing context is not sufficiently documented to assess adjustment or selective reporting.
The registry record does not report a missing-data method, the number of participants with complete outcome follow-up, or how missed weekly assessments were handled. It also does not describe an analysis population, such as whether all randomized participants were included or whether exclusions occurred. The overall enrollment is listed as 159, while the posted arm summaries list 66 and 64 participants. The record does not provide participant-flow details that reconcile enrollment with the analyzed group summaries, so no explanation for that difference can be established.
For a repeated count outcome, incomplete observation can affect both the event total and the time over which events could be observed. As a general statistical point, missing observations may be especially consequential if the chance of missing a report is related to bite occurrence or participant characteristics. The record does not report whether missingness was assessed or addressed. Without those details, the sensitivity of the result to missing follow-up cannot be evaluated from the registry entry.
Interim analyses, stopping rules, and the statistical analysis plan are not reported in the registry record. No assumptions about them can be made from the reported endpoint result alone.
12. 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 |
|---|---|---|---|
| Permethrin Impregnated Uniforms | 0 | 67 | 0.0% |
| Placebo | 0 | 66 | 0.0% |
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 |
|---|---|
| Permethrin Impregnated Uniforms | 0 of 67 |
| Placebo | 2 of 66 |
Most frequent non-serious adverse event terms
| Event term | Organ system | Permethrin Impregnated Uniforms | Placebo |
|---|---|---|---|
| Tick-borne illness (serologically confirmed) | Infections and infestations | 0 of 67 | 2 of 66 |
The adverse-event information lists 67 participants at risk in the permethrin group and 66 in the placebo group for the serious-event figures. The recorded serious-event counts are 0 in the permethrin group and 0 in the placebo group. The record lists no top serious-event terms. These figures describe what is posted in the registry; they do not establish that no serious events occurred outside the recorded reporting period or analysis.
For other adverse events, the record lists 67 participants at risk in the permethrin group and 66 in the placebo group. The counts are 0 and 2, respectively. Deaths are not reported in the registry record. These adverse-event denominators differ from the group sizes shown for the primary tick-bite outcome, and the record does not explain the distinction between those populations.
As a general statistical point, event counts should be read alongside the number of people assessed, the observation period, and the event definitions. Those details are not fully described for these safety figures. The counts are descriptive and do not provide a modeled comparison or a measure of uncertainty. They should not be taken as proof of equivalent safety or as a complete account of adverse events.
13. What the Result Does, and Does Not, Mean
The posted primary analysis estimates a lower rate of work-related tick bites in the permethrin-uniform group than in the placebo-uniform group. Its protective-effectiveness estimate is 0.646, with a 95% confidence interval from 0.288 to 0.824 and p = 0.004. The group summaries also report 265 bites in the intervention group and 780 in the placebo group. These results describe the trial's primary count outcome as analyzed in the registry.
The estimate does not directly answer whether participants avoided tick-borne illness. Seroconversion against a tick-borne illness is listed as a secondary outcome, but its result is not reported in the registry record. Tick bites and seroconversion are distinct outcomes, and evidence about one cannot be substituted for a result on the other.
The posted analysis also does not resolve how well the result applies outside the study population and setting. The registry describes North Carolina outdoor workers but does not provide detailed participant characteristics or the range of occupations represented. It does not report the model specification, missing-data handling, or participant-level event distribution. Those gaps affect how confidently readers can assess the precision and transportability of the estimate.
As a general statistical point, a randomized comparison is designed to support a causal contrast, but the strength of that interpretation depends on implementation, outcome ascertainment, follow-up, and analysis. The registry labels the trial randomized and double masked; it does not report the procedures used to generate or conceal allocation or to preserve masking. The posted information therefore permits description of the reported result, but not a full audit of the protections against bias.
The confidence interval communicates uncertainty about the model-based estimate, not certainty about future outcomes. The p-value is not a measure of clinical importance, and neither statistic establishes that the same estimate would apply to every worker or setting. The result should be confined to the trial's recorded comparison and endpoint.
14. Why This Trial Matters Statistically
This study illustrates how a clinical trial can use an event-count endpoint rather than a single occurrence indicator. Repeated tick bites are counted as separate events, preserving information that an ever-bitten measure would discard. That choice makes the outcome relevant to the stated aim of assessing the number of bites sustained by outdoor workers.
It also highlights why event-count analyses require careful attention to follow-up and variability. Group totals alone do not show how events are distributed among participants or how much observation time each person contributes. Poisson regression is the reported method, but the registry does not describe the model details or its checks. The example therefore makes visible the gap between a compact effect estimate and the information needed to reproduce or fully evaluate its analysis.
The record reports both an effect estimate and a confidence interval, allowing readers to consider magnitude and precision rather than relying only on a p-value. At the same time, the secondary illness outcome has no posted result in the available record, underscoring the importance of distinguishing a behavioral or exposure endpoint from a disease endpoint.
For biostatisticians and clinical researchers, the entry raises practical questions about analysis populations, repeated-event dependence, dispersion, missing weekly reports, and multiplicity. These are not claims that the trial handled those matters incorrectly. They are areas where the registry does not supply enough information for independent assessment. The study is a useful case in reading registry results proportionately: identify what the model estimates, retain uncertainty, and avoid extending an endpoint-specific result beyond the reported evidence.
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 registry record contains limited detail for independently checking the primary analysis. It identifies Poisson regression and gives the protective-effectiveness estimate, confidence interval, and p-value, but does not report model covariates, exposure-time handling, dispersion checks, or treatment of within-participant dependence. The person-level distribution of bite counts and individual follow-up time are also not reported.
Participant flow and analysis populations are unclear. Enrollment is listed as 159, while the primary-outcome arm summaries list 66 and 64 participants. The record does not explain how these figures relate, whether participants were excluded from the analysis, or whether all randomized participants contributed outcome data. The number of missed weekly reports and the method for addressing missing observations are not reported.
There is no posted detail about allocation concealment, the implementation of double masking, or outcome ascertainment procedures beyond the definition of a tick bite. The registry does not report baseline characteristics, adherence to assigned uniforms, or whether participants crossed between groups. These details could help assess comparability and the potential for measurement or implementation differences, but no conclusion about them can be drawn from their absence.
The record does not report a protocol-based sample-size justification, interim analyses, a statistical analysis plan, multiplicity procedures, or sensitivity analyses. It also lists seroconversion as a secondary outcome without posting its result in the available outcome figures. Finally, no deaths are reported in the registry record, and the safety summaries do not provide broader context on definitions or observation time. These limitations narrow interpretation to the reported primary comparison and descriptive safety counts.
16. Related Tutorials
17. Related Statistical Calculators
18. Sources
- ClinicalTrials.gov: Tick-borne Illness and Clothing Study — NCT01454414.
- Registry record last updated 2021-03-30; status last verified 2014-05.
- Results first posted 2014-06-30.
- 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.
Continue through the Clinical Biostats statistical pathway
Use this trial's endpoint as a starting point for deeper study of effect measures, confidence intervals, and sample-size planning.
19. Record Summary
NCT01454414, titled Tick-borne Illness and Clothing Study, is recorded as a completed randomized, double-masked, parallel study sponsored by the University of North Carolina, Chapel Hill. Enrollment is listed as 159; the recorded start is 2010-10 and primary completion is 2012-10. The intervention was long-lasting permethrin-treated uniforms, compared with placebo uniforms processed without permethrin.
The primary outcome was work-related tick bites, defined as ticks attached to or embedded in the skin and assessed weekly for two years. The registry reports a Poisson regression analysis with protective effectiveness of 0.646, a 95% confidence interval from 0.288 to 0.824, and p = 0.004. Seroconversion against a tick-borne illness is listed as a secondary outcome, but its result is not reported in the registry record. The record does not provide enough model, participant-flow, or missing-data detail to fully reconstruct or independently evaluate the primary analysis.