Pertussis Infection in Adolescents and Adults With Prolonged Cough: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Pertussis Infection in Adolescents and Adults With Prolonged Cough (NCT01597687), a non-randomized na study. The primary outcome is Serological Evidence of Pertussis Infection.

2. Clinical Question

The study’s stated aim was to assess the burden of pertussis infection among people with prolonged cough and to examine health-economic impact. Its primary outcome focused on serological evidence, defined using anti-pertussis toxin immunoglobulin G measured by ELISA. The registry distinguishes seropositivity from sero-confirmed infection and from two active-or-recent-infection thresholds. These are operational classifications based on antibody levels, not direct counts of all infections in the wider community.

The primary analysis population was the According-to-Protocol cohort, described as participants with valid laboratory results. The registry record does not report how many participants met each antibody threshold, so the prevalence or burden observed in the analyzed cohort cannot be described from these registry entries. Nor does the record support comparing countries as if participants had been assigned to comparable groups. The study can address its descriptive question only to the extent that valid laboratory results and the cohort reflect the intended population; details about recruitment and representativeness are not reported in the registry record.

3. Trial Design

The registry record identifies this as a completed, non-randomized, single-group study with no masking and no phase designation beyond NA. Enrollment was 337. The study began on 2012-06-20, and primary completion was recorded on 2013-05-02. GlaxoSmithKline is listed as the sponsor. These record entries describe the study’s registration and conduct status; they do not establish how many participants contributed valid primary-outcome data.

The brief summary describes an assessment of pertussis infection and health-economic impact among adolescents and adults with prolonged cough in Malaysia, the Philippines, Taiwan and Thailand. The recorded group descriptions, however, identify Malaysia, Taiwan and Thailand groups, each described as adults aged >19 years with cough of 2 weeks or more. A Philippines group and the eligibility details for adolescents are not reported in the registry record. This difference in the descriptions should be kept in mind when interpreting the intended geographic and age coverage.

Recorded study activities were blood collection and data collection. Blood sampling was the basis for the primary serological endpoint, assessed at Day 0. The registry also lists secondary outcomes covering clinical features and several dimensions of health-economic impact, including days absent from work due to cough, income loss related to missing work, health-resource visits, travel costs, and medical costs. The listed secondary outcomes are not a substitute for the primary outcome findings, and their actual results are not reported in the registry record.

Because there was no random allocation and no control arm, the study is descriptive rather than a randomized comparison of interventions. Country-group differences, if reported elsewhere, would not by themselves identify causal effects: participants were not assigned to country groups, and the registry record does not describe adjustment for differences between them. The record also does not report the recruitment procedures, sample-size justification, or statistical plan. The total enrollment should therefore not be treated as the number with a valid primary endpoint or as evidence that a particular precision target was achieved.

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: A total of 337 subjects were enrolled into this study, but only 312 had valid laboratory results. 2 subjects were excluded due to vulnerable conditions, 16 subjects were excluded due to protocol violation and 7 subjects were excluded due to vaccination with pertussis vaccines within 1 year before blood sample collection.

Overall Study

Analysis population for the primary outcome, as defined in the registry: The analysis was done on the According-to-Protocol (ATP) cohort which included all subjects with valid laboratory results.

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 Serological Evidence of Pertussis Infection, measured over At time of blood sampling (Day 0).

The primary endpoint is serological evidence of pertussis infection, measured as the number of participants meeting antibody-based definitions at blood sampling on Day 0. The assay measured anti-pertussis toxin immunoglobulin G. The registry defines seropositive as an anti-PT IgG ELISA level of at least 10 IU/ml, seronegative as a level below 10 IU/ml, and sero-confirmed infection as a level of at least 62.5 IU/ml. It also defines active infection 1 using a level of at least 100 IU/ml and active infection 2 using a level of at least 125 IU/ml, each described as indicative of active or recent infection.

These thresholds provide consistent rules for classifying laboratory results. They distinguish evidence at different antibody cutoffs; they should not be treated as interchangeable labels or as direct proof of the timing of infection beyond the definitions in the registry. The endpoint is useful for describing the frequency of threshold-defined serological evidence in the analyzed cohort. It does not, by itself, establish how many infections occurred in the broader population of people with cough.

The analysis population was the According-to-Protocol cohort, comprising participants with valid laboratory results. The number meeting any of the endpoint definitions is not reported in the registry record. Therefore, the record details do not permit a quantitative estimate of the observed burden under any threshold.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Statistical methods help distinguish the question a study can answer from the strength and scope of the evidence it reports. In this study, the primary endpoint was a count of participants meeting laboratory definitions, while the design was non-randomized and single-group. The registry record identifies the endpoint and analysis population, but does not report analysis results or a statistical analysis plan. The explanations below are general, and do not imply that any particular method was used in this study.

What does a threshold-defined count represent?

A count records how many observations fall into a category. Here, categories were defined by anti-PT IgG ELISA levels: seropositive at least 10 IU/ml, sero-confirmed at least 62.5 IU/ml, active infection 1 at least 100 IU/ml, and active infection 2 at least 125 IU/ml. Seronegative was defined as below 10 IU/ml. Each label depends on its stated cutoff. A count describes the participants classified under that rule; it is not automatically a measure of all infection in a source population.

For a frequency to be interpretable, the count needs a relevant denominator: the number of participants whose results could be classified. The record describes the According-to-Protocol cohort as all participants with valid laboratory results, but does not report the cohort size or the counts in each category. The enrollment figure of 337 describes study enrollment and should not be assumed to equal the number of valid laboratory results.

What would a confidence interval add?

In general, a confidence interval expresses statistical uncertainty around an estimate. For a proportion or rate, it can show the range of values reasonably compatible with the observed data under the method’s assumptions. A narrower interval generally indicates greater precision, while a wider interval indicates more uncertainty. A confidence interval does not correct biased recruitment, measurement problems, or an analysis population that differs from the population of interest. No confidence interval for the primary endpoint is reported in the registry record, so its precision cannot be evaluated.

Why does the study design matter?

Random allocation is one way to make comparison groups more similar, on average, with respect to both measured and unmeasured characteristics. This study was not randomized and is described as single-group. Its country group labels therefore do not create a randomized comparison. Any apparent difference between countries could reflect participant characteristics, recruitment, measurement, or other factors as well as differences in underlying infection frequency. A descriptive study can characterize results within its observed cohort, but causal interpretations require design and analysis support beyond a difference in counts.

Masking was recorded as none. Masking can reduce the influence of knowledge of group or treatment assignment on some aspects of study conduct or assessment. Its relevance depends on the endpoint and procedures. Here, the primary outcome was a laboratory classification, but the registry entries do not describe laboratory blinding or provide enough detail to assess whether knowledge could influence measurement. The recorded absence of masking should not be treated as proof that measurement was biased; the implications cannot be determined from the information recorded.

How should planning assumptions be read?

Sample-size planning uses assumptions about the effect to be detected, statistical significance, allocation, and analysis method to illustrate the number of participants that might be needed for a target level of power. The registered planning illustration describes a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and effects expressed as a fraction of the observed effect. These are planning assumptions for an illustration, not evidence that this single-group, non-randomized study was designed with two equal arms. The registry record does not report the study’s own sample-size justification. Planning sample sizes are not observed power and cannot substitute for outcome estimates or uncertainty intervals.

Power is a design concept describing the chance that a specified procedure will detect an effect of a specified size under its assumptions. It cannot be recovered from the enrollment number alone, and it does not measure the probability that a result is true. No trial-specific power calculation or detectable-effect target is reported in the registry record. With no reported primary outcome estimates, an effect-based planning illustration cannot be interpreted as evidence about the study’s findings.

What do analysis entries and outcome counts tell us?

The registry record lists 23 outcome measures and zero analysis entries. The measure count indicates that multiple outcomes were registered; it does not establish how many were reported or analyzed. The absence of analysis entries in the record means results and methods are not documented in those entries. It does not establish that investigators conducted no analyses. Similarly, the listed secondary outcomes identify topics such as work absence and costs but do not provide estimates.

When studies assess multiple outcomes or make many statistical comparisons, the chance of at least one apparently unusual result can increase. General approaches to multiplicity may include defining a primary endpoint in advance, adjusting interpretation or thresholds, or treating secondary analyses as exploratory. The registry entries do not report a multiplicity procedure, and no correction or unadjusted result can be inferred. The primary endpoint designation helps identify the main registered outcome, but it does not reveal the full analysis strategy.

Why can over-dispersion matter?

In general, over-dispersion means that the observed variability in count data is greater than expected under a particular model. If a model assumes less variability than the data contain, standard errors may be too small and uncertainty intervals too narrow. Analysts may use models that account for extra variation when the data and design call for them. This general issue cannot be assessed: the primary endpoint is described as a participant count, and the registry record does not report a model, dispersion estimate, or analysis result.

10. Multiplicity, Missing Data and Analysis Populations

The registry record lists 23 outcome measures, including the primary serological outcome and secondary outcomes concerning clinical features and health-economic impact. It records zero analysis entries. The outcome count indicates that multiple endpoints were registered, but does not establish which were analyzed, how many comparisons were made, or whether any multiplicity adjustment was planned. The registry record does not report a multiplicity strategy, interim analyses, or a statistical analysis plan. Their absence from the record should not be interpreted as proof that none occurred.

Multiplicity matters because examining many outcomes or comparisons can increase the chance of finding an apparently notable result by chance. General methods include identifying a primary question in advance, controlling error across selected comparisons, or treating secondary findings as exploratory. The registry entries do not support attributing any of these approaches to this study. Since outcome estimates and analysis entries are not reported, there are no documented results on which to assess multiplicity or distinguish confirmatory from exploratory findings.

The primary analysis population is the According-to-Protocol cohort, defined as participants with valid laboratory results. The number in that cohort, the number with missing or invalid laboratory results, and the reasons for any missing data are not reported in the registry record. It also does not report how missing data were handled, whether all enrolled participants were considered for the primary analysis, or whether any sensitivity analyses were conducted. Enrollment of 337 should therefore not be interpreted as the primary endpoint denominator.

These omissions limit assessment of selection due to missing laboratory results and of the robustness of the analysis population. They also prevent checking whether registered outcomes were reported consistently. The record’s completed status and its list of outcome measures do not resolve these uncertainties.

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.

The safety entries report serious adverse events and deaths by country group. For the Malaysia Group, 76 participants were at risk for serious adverse events; the number affected was 0, and deaths were 0. For the Taiwan Group, 89 participants were at risk; the number affected was 0, and deaths were 0. For the Thailand Group, 147 participants were at risk; the number affected was 0, and deaths were 0. The registry lists no participants at risk for other adverse events in these entries, and the corresponding affected counts are 0.

These figures are counts as recorded, not evidence that risk was absent. A zero affected count means no event was recorded in the stated group and safety category in these entries; it does not establish that an event could never occur or that the study could rule out uncommon events. The group-specific at-risk counts are not necessarily the same as the enrolled population or the primary analysis population. The registry record does not provide further detail on safety collection, follow-up, event definitions, or exposure time. No serious adverse event terms are listed. These limitations matter when interpreting zero counts, especially because the safety data do not describe all possible events or establish a comparative safety profile. The groups are not randomized comparison arms.

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

The record supports several clear descriptions: the study was completed, non-randomized, single-group and unmasked; enrollment was 337; and the primary outcome used antibody thresholds measured at blood sampling on Day 0. The primary analysis population was the According-to-Protocol cohort, described as participants with valid laboratory results. The registered aims included assessing pertussis burden and health-economic impact among people with prolonged cough.

Those design details do not provide the primary result. Threshold-specific counts, denominators, estimates of uncertainty, and statistical tests are not reported in the registry record. It is therefore not possible to state from these registry entries how common serological evidence was in the analyzed participants, how precise an estimate might be, or whether results varied across country groups. The same limitation applies to secondary outcomes about clinical features, work absence, income loss, healthcare use and costs.

The endpoint definitions also constrain what the study can mean. They classify antibody levels according to specified rules, including cutoffs described as seropositive, sero-confirmed, or indicative of active or recent infection. A classification under a laboratory cutoff is not by itself a population-wide infection rate, a measure of transmission, or proof of when an infection occurred. The record does not report enough information to extend the finding beyond the analyzed cohort or to assess its representativeness.

Because there was no randomized comparison, the study does not estimate a causal treatment effect. Country groups are not like-for-like control arms, and enrollment does not substitute for a randomized design. The safety entries report zero serious adverse events and deaths in the listed groups, but they do not establish zero risk or compare safety between randomized treatments.

Overall, the registry record documents a descriptive study question and its measurement definitions, but not the numerical outcome findings needed to quantify burden or economic impact. Conclusions about the size, precision, or broader meaning of the observed results are not supported by the record entries.

13. Why This Trial Matters Statistically

This study illustrates a common distinction in clinical research: a clear outcome definition does not guarantee that the outcome result is available for interpretation. The registry specifies the antibody assay and multiple thresholds, making the intended classification rules understandable. Yet without category counts, denominators, and uncertainty estimates, those definitions alone cannot quantify the burden observed in the cohort.

It also demonstrates why analysis populations matter. The primary endpoint applies to participants with valid laboratory results, whereas enrollment describes the people entered into the study. Those quantities need not match. A reader needs the analyzed denominator and information about missing or invalid measurements to judge how broadly the result may apply. The registry record does not report those details.

The design is also important. A single-group, non-randomized study can provide descriptive information, but country groups do not become causal controls simply because they are listed separately. This is particularly relevant for a study spanning multiple locations, where population composition and recruitment may differ. The record’s stated scope includes Malaysia, the Philippines, Taiwan and Thailand, while the listed group descriptions name Malaysia, Taiwan and Thailand; the record does not explain this difference.

Finally, the registered secondary outcomes show that the study aimed to examine more than laboratory classification, including economic consequences of cough. Such outcomes can be useful for understanding the burden of illness, but their value depends on transparent definitions, denominators, analysis methods and results. The record lists 23 outcome measures but no analysis entries. Statistically, that distinction between registered questions and reported findings is essential: it prevents an aim or endpoint definition from being mistaken for evidence about the result.

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 specific to this record restrict interpretation. First, the primary outcome counts and analyzed denominator are not reported. Although enrollment was 337, the primary analysis was based on the According-to-Protocol cohort with valid laboratory results, and its size is not given. The frequency of serological classifications and their precision therefore cannot be assessed.

Second, the record describes a non-randomized, single-group study with no masking. The country groups are not randomized controls, so comparisons between them would not establish causal effects. The registry record does not report an adjustment strategy or enough detail about recruitment to assess comparability or representativeness.

Third, the stated study scope and group descriptions do not fully align in geographic and age detail. The brief summary names Malaysia, the Philippines, Taiwan and Thailand and refers to adolescents and adults. The listed group descriptions name Malaysia, Taiwan and Thailand and specify adults aged >19 years with cough of 2 weeks or more. The record does not report a Philippines group or adolescent eligibility details, so coverage of those populations cannot be determined from the group entries.

Fourth, the endpoint is based on serological thresholds at blood sampling on Day 0. The record does not provide the threshold-specific counts, additional timing information, or details needed to assess how antibody classifications correspond to infection timing in this cohort. The stated categories should be read according to their registered definitions.

Finally, the record does not report a trial-specific sample-size justification, statistical analysis plan, missing-data handling, multiplicity approach, or interim analyses. It lists 23 outcome measures and zero analysis entries. The status of completed does not compensate for these reporting gaps, and the secondary health-economic aims cannot be evaluated without their results.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Pertussis Infection in Adolescents and Adults With Prolonged Cough — NCT01597687.
• Registry record last updated 2019-08-09; status last verified 2019-06.
• Results first posted 2019-08-09.
• 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

NCT01597687, titled Pertussis Infection in Adolescents and Adults With Prolonged Cough, is recorded as a completed, non-randomized, single-group study sponsored by GlaxoSmithKline. Enrollment was 337. The study start was 2012-06-20, and primary completion was 2013-05-02. The brief summary describes an assessment of pertussis burden and health-economic impact in Malaysia, the Philippines, Taiwan and Thailand.

The primary outcome was serological evidence of pertussis infection at blood sampling on Day 0, based on anti-PT IgG ELISA thresholds. The primary analysis population was the According-to-Protocol cohort of participants with valid laboratory results. The registry lists 23 outcome measures and zero analysis entries. It does not report the primary outcome counts, analysis denominator, statistical estimates, or results for the listed health-economic outcomes.

Safety entries list zero serious adverse events and zero deaths in each of the Malaysia, Taiwan and Thailand groups. These counts do not establish absence of risk. The registry record supports a description of study design and outcome definitions, but does not provide the results needed to quantify infection burden or economic impact.