Tdap Vaccine in Post-Partum Women: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Tdap Vaccine in Post-Partum Women (NCT01711645), a na phase 4 study. The primary outcome is Geometric Mean Fold Rise in Serum Immunoglobulin G (IgG) by ELISA at Week 2.

2. Clinical Question

3. Trial Design

This completed Phase IV study was open label and used a single-group design. The registry lists an enrollment of 55 and describes 55 postpartum subjects receiving Adacel® Tdap vaccine. The record does not describe a randomized allocation or a separate control arm, so it does not support a direct comparison between vaccinated participants and an untreated or differently treated group.

The intervention was one intramuscular 0.5 mL dose. Blood collection was planned before vaccination and at Week 2 for the primary immune-response endpoint. The brief summary describes monitoring immune response and longevity in serum and milk, with observation lasting over two years. The stated purpose was to evaluate immunogenicity and inform vaccination-schedule optimization for women who may have a subsequent child. The registry record does not report a comparison of alternative schedules.

The record lists the study start as 2012-10-26 and primary completion as 2015-08-28, and gives the status as completed. It describes the enrollment period as fifteen months. Those registry entries provide context about the study but do not establish when every participant was enrolled or how long each participant contributed data.

The primary analysis population is described as participants with specimens collected and data reported at both baseline and the post-vaccination timepoint. The record does not state how many participants met that definition for each antigen or endpoint. It also does not report the detailed statistical analysis plan, handling of incomplete measurements, or whether all planned follow-up measurements were obtained.

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

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: All participants with specimens collected and data reported for baseline and the post-vaccination timepoint were included in the analysis population.

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 Geometric Mean Fold Rise in Serum Immunoglobulin G (IgG) by ELISA at Week 2, measured over Prior to and 2 weeks following vaccination.

The primary endpoint is the geometric mean fold rise in serum immunoglobulin G (IgG), assessed by ELISA at Week 2 relative to the pre-vaccination baseline. The registry names four antigens: pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN) and fimbriae (FIM). Antibody concentrations were expressed in ELISA units per milliliter (EU/mL), and the endpoint description says the geometric mean of participants’ fold rises was calculated with a 95% confidence interval.

In general, a fold rise expresses a post-measurement concentration relative to a baseline concentration. A geometric mean is commonly used for ratio-like measurements that can be skewed, because it summarizes them on a multiplicative scale. The confidence interval conveys uncertainty around the estimated geometric mean; a narrower interval indicates greater precision under the analysis assumptions, while a wider interval indicates less precision.

These are measures of antibody response, not direct measures of clinical protection. The registry record gives the endpoint definition and measurement schedule, but not the numerical geometric mean fold rises or their confidence intervals. Consequently, the size and precision of the reported primary response cannot be evaluated from the registry information.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a geometric mean fold rise represent?

In general, a fold rise compares a later measurement with a participant’s baseline measurement. The study describes measuring serum IgG concentrations before vaccination and at Week 2, then calculating each participant’s fold rise and summarizing those rises with a geometric mean. This is different from taking an arithmetic average of antibody concentrations: the geometric mean is suited to positive ratio-like values and summarizes them on a multiplicative scale.

The endpoint is antigen-specific. The registry names PT, FHA, PRN and FIM, with antibody concentrations measured by ELISA and expressed as EU/mL. A fold-rise summary for one antigen does not substitute for the summary for another antigen. The registry record does not report the actual endpoint estimates, so the scale and variation of the observed responses cannot be assessed from the record.

How should the confidence interval be read?

In general, a confidence interval describes uncertainty around an estimated quantity under the statistical model and sampling process used. The primary endpoint specifies a 95% confidence interval around the geometric mean fold rise. Its width would help readers assess precision: a narrower interval would indicate a more precise estimate, and a wider interval would indicate more uncertainty. A confidence interval is not a range containing every participant’s response, nor does it describe the full spread of individual antibody measurements.

The interval’s interpretation depends on the analysis method and available observations. The registry identifies participants with both baseline and post-vaccination specimens and reported data as the analysis population, but it does not report how many participants contributed to each antigen-specific estimate or how missing measurements were handled. Numerical intervals are not reported in the registry record.

What does the single-group design imply?

In general, a within-participant before-and-after analysis uses each participant’s baseline as a reference. This can describe change over time in the measured group. Without a concurrent comparison group, it cannot distinguish the effect of vaccination from all other factors that could influence measurements over time. The registry records allocation as not applicable and the model as single group, consistent with no randomized treatment comparison.

Open-label status means the study was not described as masked. The registry record does not give further details about whether laboratory personnel were blinded to sample timing or other participant information. It would be inappropriate to assume masking procedures that are not reported. The endpoint is a laboratory immune-response measure, and the design does not itself establish a relationship between that measure and a clinical outcome.

What do the planning assumptions mean?

In general, sample-size calculations estimate how many participants a future design might require under specified assumptions about the effect, variability, allocation and statistical threshold. The record’s planning description uses a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and an effect defined as a fraction of the observed effect. Such calculations are planning illustrations, not observed power for this trial.

The actual study is a single-group trial, whereas the planning assumptions describe allocation between two arms. Those assumptions therefore should not be mistaken for the trial’s observed design or for evidence that a randomized comparison took place. The numerical observed effect and resulting planning estimates are not reported in the registry information. A planning illustration also cannot repair the absence of a reported endpoint estimate or establish that the study was adequately powered.

Why does outcome structure matter?

The registry lists 32 outcome measures, including serum and breast-milk immune-response measures at different timepoints. In general, examining many outcomes can raise multiplicity concerns: the chance of finding an apparently notable result can depend on how many analyses are performed and how results are selected or adjusted. The registry record does not report a multiplicity strategy or a complete statistical analysis plan. No assumptions about adjustment, hierarchy, or selective reporting should be made without that information.

Repeated measurements can also be correlated because multiple samples may come from the same participant. In general, an analysis of longitudinal measurements needs to account for that dependence if it models several timepoints together. The primary endpoint is described as a baseline-to-Week 2 fold rise; detailed modeling choices beyond that description are not reported in the registry record. The record also does not explain how variation across participants or assay measurements was handled.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 32 outcome measures and describes several immune-response outcomes at multiple timepoints. Among the named secondary outcomes are breast-milk IgA geometric mean concentrations to PT and FHA at baseline, Week 2, Week 6 and Month 6. The primary endpoint covers serum IgG responses to four antigens at Week 2. This creates a setting in which the interpretation of individual estimates can depend on which outcomes were prespecified as primary or secondary and how multiple comparisons were addressed.

In general, multiplicity refers to the increased chance of apparently noteworthy findings when many hypotheses or outcomes are examined. Researchers may address this through a prespecified hierarchy, adjustment procedures, or cautious interpretation of exploratory outcomes. The registry record does not report whether any such approach was planned or used. It does not provide a detailed statistical analysis plan or numerical outcome results sufficient to assess whether analyses followed a prespecified strategy.

Missing-data handling is also not described in detail. The primary analysis population is defined as participants with baseline and post-vaccination specimens and reported data. The record does not state how many participants contributed to each analysis, whether any measurements were missing, why data might be unavailable, or how missing observations were handled. It also does not report whether analyses used additional populations beyond the stated endpoint population.

These omissions limit assessment of potential selection effects and the robustness of any estimate. No conclusion about the direction or effect of missing data, or about multiplicity adjustment, can be drawn from the registry record.

11. 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 5% reporting threshold.

The posted adverse-event summary lists 0 serious adverse events among 55 at risk and 0 other adverse events among 55 at risk in the Adacel® Tdap Vaccine group. These figures describe the counts entered in the registry summary. They do not provide event narratives, event definitions, timing, duration, or a detailed account of the period during which participants were monitored for each category.

A count of zero in a registry summary means that no events in that category are listed in the reported data; it does not establish that the intervention has no possible risks. The record does not report deaths, so deaths should not be described as absent. It also does not provide a comparison group for evaluating relative safety. Without additional detail, these counts cannot show whether the study captured every event or how the safety experience compares with another schedule, treatment, or population.

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

The registry describes a completed, open-label, single-group Phase IV study of one intramuscular 0.5 mL dose of Adacel® Tdap vaccine in postpartum women. Its primary endpoint is the geometric mean fold rise in serum IgG, measured by ELISA from baseline to Week 2 for PT, FHA, PRN and FIM. The record defines the endpoint and analysis population, but does not report numerical primary results or confidence intervals.

Accordingly, the posted information does not establish the magnitude or precision of the immune response. It also cannot determine whether one vaccination schedule is preferable, because the record describes no schedule comparison. The study’s purpose of informing future vaccination schedules is not itself evidence that a particular schedule is optimal. Nor does a laboratory antibody endpoint alone establish a clinical benefit or protection from disease.

The secondary outcomes include breast-milk IgA measures at baseline, Week 2, Week 6 and Month 6 for PT and FHA. Their numerical results are not reported in the registry record. The record therefore does not permit assessment of persistence across those timepoints or comparison between serum and milk responses.

The safety summary lists 0 serious adverse events and 0 other adverse events among 55 at risk in the study group, while deaths are not reported. These entries are limited to the registry summary and do not provide a complete account of event ascertainment or comparative safety. Overall, the registry information supports a description of the study’s design, intended measurements and posted safety counts, but not a numerical conclusion about immunogenicity, durability, clinical outcomes, or comparative safety.

13. Why This Trial Matters Statistically

This trial illustrates why endpoint definition and endpoint results need to be distinguished. The registry explains what was measured, when serum was collected, which antigens were assessed, and how the primary quantity was summarized. Without the numerical geometric mean fold rises and confidence intervals, however, readers cannot judge the size or precision of the measured responses. A well-described endpoint is not a substitute for a reported estimate.

Its single-group design also clarifies the limits of before-and-after evidence. Comparing each participant’s post-vaccination measurement with baseline can describe change within the study group, but does not supply a randomized control comparison. That distinction is important when a study’s purpose is to inform a future schedule: a result from one schedule does not, by itself, rank that schedule against alternatives.

The record also highlights the statistical challenges created by multiple immune markers and timepoints. The registry lists 32 outcome measures, but does not report a detailed analysis plan or multiplicity approach. In general, clear prespecification helps readers distinguish confirmatory questions from exploratory ones and understand how uncertainty should be interpreted across outcomes.

Finally, the safety summary is useful only within its reporting limits. Counts can describe what was entered for the study group, but without timing, definitions and comparison data they do not establish the full safety profile. These distinctions are broadly useful when interpreting registry records: design, estimates, uncertainty, analysis choices and reporting completeness all shape what can be concluded.

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 consequential limitation of the registry information is that it describes the primary endpoint without reporting its numerical result. No geometric mean fold-rise estimate or 95% confidence interval is given for the named antigens. The record therefore does not allow evaluation of the response magnitude, precision, or differences across antigens.

The single-group, open-label design limits comparative interpretation. No randomized control group or alternative vaccination schedule is described, so the study cannot directly compare schedules. The within-participant baseline is a reference measurement, not a concurrent control. The registry also does not report procedures for laboratory masking, so none can be assumed.

The analysis population is defined by availability of baseline and post-vaccination specimens and reported data, but the record does not state the number analyzed for each endpoint or explain missing-data handling. Although the summary refers to observation over two years and lists milk measurements through Month 6, it does not provide numerical results for those secondary measures or detailed information about follow-up completeness.

The record lists 32 outcome measures but does not report a detailed analysis plan or multiplicity strategy. It also does not report clinical outcome results that would connect laboratory measurements to health outcomes. Adverse-event counts are presented without detailed event definitions or timing, and deaths are not reported in the registry record. These limitations restrict interpretation to the design and entries explicitly described.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Tdap Vaccine in Post-Partum Women — NCT01711645.
• Registry record last updated 2017-05-05; status last verified 2015-04-08.
• Results first posted 2017-05-05.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

18. Record Summary

NCT01711645 is a completed, open-label, single-group Phase IV study of Adacel® Tdap vaccination in postpartum women. The registry lists enrollment of 55 and describes a single intramuscular 0.5 mL dose. The primary endpoint is the geometric mean fold rise in serum IgG by ELISA from baseline to Week 2 for PT, FHA, PRN and FIM, with a 95% confidence interval.

The record also describes serum and breast-milk immune-response monitoring and lists 32 outcome measures. It does not report numerical primary endpoint results, detailed missing-data procedures, or a full statistical analysis plan. The safety summary lists 0 serious adverse events and 0 other adverse events among 55 at risk; deaths are not reported in the registry record. The single-group design provides no direct randomized comparison of vaccination schedules.