Immunogenicity & Reactogenicity of Boostrix 10 Years After Previous Booster Vaccination in Study NCT01267058: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Immunogenicity & Reactogenicity of Boostrix 10 Years After Previous Booster Vaccination in Study NCT01267058 (NCT00548171), a non-randomized phase 4 study. The primary outcome is Number of Subjects With Anti-diphtheria (Anti-DT) and Anti-tetanus Toxoid (Anti-TT) Antibody Concentrations Equal to or Above (≥) 0.1 International Units Per Milliliter (IU/mL).

2. Clinical Question

The clinical question is about antibody measurements after repeat vaccination, not a direct comparison of randomized treatment assignments. Although the study description refers to efficacy and safety, the primary endpoint recorded is an immunogenicity measure: whether antibody concentrations meet specified cutoffs. The registry lists additional antibody outcomes, including measurements for pertussis antigens and further diphtheria and tetanus assessments. Those outcomes do not change the definition of the primary endpoint.

The record does not report the primary endpoint count, the number assessed for that endpoint, or a statistical comparison. Consequently, it does not establish how many participants met the cutoffs or how precisely any such proportion was estimated. The study question can be described from the protocol information, but its primary result cannot be interpreted from this registry record.

3. Trial Design

This completed Phase 4 study evaluated a Boostrix booster in adults ten years after a previous booster vaccination in study NCT01267058. The registry lists an enrollment of 203. The study began on 2007-11-05, and primary completion was recorded as 2008-04-30. The sponsor was GlaxoSmithKline.

The design was parallel and non-randomized, with no masking. Group membership followed the vaccine participants had received in the earlier study: the Boostrix I Group had previously received Boostrix, while the Boostrix II Group had previously received Td vaccines. Participants in both groups received one dose of Boostrix in this study. The groups therefore represent different histories of prior vaccination, not randomized allocation to Boostrix versus a control in the current study.

The registry description says that only people who received a booster in the previous study were eligible and that no new recruitment would occur in this booster phase. This makes the study a follow-up of participants from the earlier study rather than a newly recruited comparison. The registry record does not report the number enrolled in each group or the number included in each outcome analysis.

The primary endpoint was assessed one month after the booster. The analysis was described for participants who had received Boostrix as their first booster in the earlier study. The immunogenicity cohort definition also refers to evaluable participants who received the booster and had immunogenicity results available for at least one study antigen. How those criteria were applied to the primary endpoint, and the number meeting them, are not reported in the registry record.

Because treatment history determined group membership, differences between groups could reflect prior vaccination history or other differences between participants, as well as any response to the current booster. The record itself cautions that the arms are not a like-for-like control comparison. It does not report a randomized treatment contrast, masking, or a statistical analysis 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

Analysis population for the primary outcome, as defined in the registry: The ATP cohort for immunogenicity included all evaluable subjects who had received the booster dose of Boostrix™ vaccine and for whom immunogenicity data were available. This included subjects for whom assay results were available for antibodies against at least one study vaccine antigen component at the post-vaccination blood-sampling time point.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Number of Subjects With Anti-diphtheria (Anti-DT) and Anti-tetanus Toxoid (Anti-TT) Antibody Concentrations Equal to or Above (≥) 0.1 International Units Per Milliliter (IU/mL), measured over One month after the booster vaccination [PI(M1)].

The primary endpoint counts participants whose anti-diphtheria (anti-DT) and anti-tetanus toxoid (anti-TT) antibody concentrations were at or above 0.1 IU/mL. The registry describes these as seroprotection cutoffs and states that concentrations were assessed by Enzyme-Linked Immunosorbent Assay (ELISA). The assessment time was one month after the booster vaccination.

This is a binary outcome at the participant level: for the endpoint, a participant either meets the stated antibody criterion or does not. Counting participants who meet a prespecified cutoff gives a clinically interpretable summary of the proportion crossing that laboratory threshold. It is distinct from reporting the antibody concentration itself, which is a continuous measurement, and distinct from measuring clinical disease or symptomatic infection.

The registry says the primary analysis included only participants who had received Boostrix as their first booster in NCT01267058. It also describes an immunogenicity cohort based on evaluability and available antibody data. The number assessed and the endpoint count are not reported in the registry record. No confidence interval or comparative estimate is reported either, so the degree of statistical precision cannot be assessed from the recorded primary outcome information.

The antibody cutoff is an operational definition used for this endpoint. Meeting it should be read as meeting the recorded laboratory criterion, not as evidence in itself about a participant’s clinical outcome. The endpoint does not establish protection against every relevant outcome, and the registry does not report a clinical efficacy result.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The endpoint is a count of participants meeting an antibody threshold. For each participant with an evaluable result, the laboratory measurement is classified against the stated cutoff. The resulting count is meaningful only alongside the number assessed; together, those values describe the observed frequency of meeting the criterion. Neither the count nor its denominator is reported for the primary endpoint in this registry record.

What does a confidence interval communicate?

In general, a confidence interval expresses the statistical uncertainty around an estimate under the assumptions of the analysis. For a participant frequency, it indicates a range of values compatible with the observed data and the chosen method. A narrow interval suggests greater precision than a wide interval, though precision does not remove design limitations or establish a causal effect. No confidence interval for the primary endpoint is reported in the registry record, so its precision cannot be evaluated.

Why does the non-randomized design matter?

Randomization is generally used to make treatment groups comparable, on average, with respect to factors known and unknown at assignment. Here, the groups were defined by earlier vaccination history, and both received Boostrix in the current study. Therefore, group differences could be related to that history or to other differences between participants. Statistical adjustment, if conducted, would not necessarily remove all such differences. The registry record does not report an adjusted comparison or a statistical plan, and it cautions that the arms are not like-for-like controls.

The primary analysis is further restricted to participants who received Boostrix as their first booster in the prior study. This is an eligibility criterion for the stated primary analysis, not a randomized allocation in the present study. It also means that the enrollment figure alone cannot identify the primary analysis population. The registry record does not give the endpoint denominator or explain exclusions and missing measurements for this analysis.

What can a binary endpoint show?

A binary endpoint reduces a laboratory concentration to whether it meets a specified cutoff. This can make the result straightforward to communicate, but it discards information about how far measurements lie above or below that cutoff. A separate analysis of antibody concentrations can preserve more of that information, and the registry lists antibody concentration outcomes among its secondary measures. Their numerical results are not reported in the registry record.

More generally, a threshold outcome depends on the chosen cutoff, the assay, and the timing of measurement. The record specifies the anti-DT and anti-TT threshold, ELISA assessment, and one-month time point. It does not provide further information about assay variability, measurement procedures, or the handling of values that could not be evaluated. Those details should not be inferred.

How should multiple outcomes be considered?

When a study assesses multiple endpoints, repeated statistical testing can increase the chance of at least one apparently notable result arising by chance. Statistical methods may address this through prespecified outcome hierarchies or multiplicity adjustments, but the registry record does not report whether such methods were used. It lists 20 outcome measures and no analyses. That information does not establish how many tests were performed, which outcomes were emphasized, or whether results were selected for reporting.

The record also does not report a statistical analysis plan, an interim analysis, or a sample-size justification. General planning calculations based on an observed effect, where available, are not the same as observed power and do not establish the precision or reliability of an outcome. No primary endpoint estimate is reported from which to evaluate such planning assumptions.

What is the role of the safety counts?

The serious adverse event summary reports 0 events among 203 at risk, while the other adverse event summary reports 163 participants affected among 203 at risk. These are separate safety categories. A count of participants with an adverse event is not automatically a count of event episodes, and a pooled summary does not show group-specific patterns. The record reports 0 deaths. General interpretation of these counts still depends on how events were defined, collected, and followed; the registry record does not provide the observation period or detailed event listings in the information recorded.

Statistical interpretation should therefore separate what is defined from what is observed and reported. The endpoint definition is explicit, but the primary outcome values are missing. The design and population restrictions are described, but the primary denominator and analysis methods are not. The safety summaries provide selected counts, but they do not answer the immunogenicity question.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 20 outcome measures, including antibody counts, antibody concentrations, seronegative status for anti-DT antibodies, and seropositive status for pertussis antibodies. It records no analyses. The multiplicity strategy, any hierarchy among outcomes, and any adjustment for multiple testing are not reported in the registry record. Listing several outcomes does not establish how many were analyzed or which results were considered primary in the final analysis.

The primary endpoint description specifies an immunogenicity population and restricts the primary analysis to participants who had received Boostrix as their first booster in the earlier study. The broader cohort description refers to evaluable participants who received the booster and had antibody data available for at least one antigen. The registry record does not report how many participants met the primary analysis criteria, how many had missing endpoint measurements, or how missing measurements were handled.

It also does not report whether the analysis was based on a particular analysis population beyond the stated eligibility description, whether any sensitivity analyses were conducted, or how protocol deviations were treated. These omissions limit assessment of possible selection effects and the robustness of the endpoint result. Since the primary endpoint result itself is not reported, the impact of missing data on its estimate cannot be assessed.

The registry record does not report a statistical analysis plan, interim analyses, or a sample-size justification. These items should not be inferred from the study’s enrollment or from any planning illustration. The absence of these details in the record does not establish that the procedures were or were not used; it means they are not reported in the registry record.

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 2.5% reporting threshold.

Most frequent non-serious adverse event terms

The pooled safety summary records 0 serious adverse events among 203 participants at risk, 163 participants with other adverse events among 203 at risk, and 0 deaths. These are aggregate counts for the Boostrix Pooled Group. They are not group-specific comparisons between the Boostrix I and Boostrix II groups.

The serious adverse event count is distinct from the count for other adverse events. The latter indicates participants reported in that category, not necessarily the number of individual event episodes. The record lists no serious adverse event terms. It does not provide details about the kinds of other adverse events, their severity, timing, relatedness assessment, or duration of safety follow-up.

The zero counts should be read as the counts recorded for this study summary, not as proof that an event could never occur or that risk is absent. The pooled summary also cannot establish comparative safety between prior-vaccination groups. As the registry record does not report the safety observation period or a detailed safety analysis, the precision and scope of these counts are limited to what is recorded.

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

The record establishes that this study assessed antibody responses one month after a Boostrix booster in adults who had taken part in an earlier booster study. Its primary measure was the number meeting anti-DT and anti-TT antibody cutoffs of 0.1 IU/mL or above by ELISA. The primary analysis was specified for those who had received Boostrix as their first booster in the earlier study.

However, the registry record does not report the primary endpoint count, analysis denominator, antibody concentrations, confidence interval, or statistical test. It therefore does not show how many participants met the threshold or how precisely that outcome was estimated. The endpoint definition alone is not an outcome result.

The study also does not provide a randomized comparison of Boostrix against a control in this booster phase. Participants in the two groups were distinguished by their earlier vaccination, and both received Boostrix in the current study. Any comparison between them would not, on this design alone, establish that the prior vaccine history caused an outcome difference.

The listed safety summary reports no serious adverse events and no deaths, alongside 163 participants with other adverse events among 203 at risk. These figures are aggregate, and the record lacks detailed event information and the safety observation period. They should not be extended beyond the summaries recorded.

Thus, the record describes the study’s question, design, endpoint, and selected safety counts, but does not report the primary immunogenicity result. It cannot support a conclusion about the magnitude or precision of the antibody response, a comparative effect, or a clinical efficacy outcome.

13. Why This Trial Matters Statistically

Statistically, this study illustrates why a clearly defined endpoint is not enough to interpret a trial result. The registry specifies the antibody cutoff, assay, and assessment time, and it states which earlier vaccination history defined the primary analysis. Those details make the intended outcome understandable. Yet without the observed count and denominator, readers cannot estimate the frequency of participants meeting the threshold or judge its precision.

The design also makes the distinction between description and comparison important. The groups arose from prior vaccination history rather than random assignment in this study, and both received Boostrix. A group contrast would therefore be vulnerable to differences that accompany those histories. The registry’s caution that the arms are not like-for-like controls is central to interpreting any comparative claim.

The record also highlights the need to report analysis populations and missing-data handling. The stated immunogenicity cohort depends on evaluability and available data, while the primary analysis has an additional prior-vaccine restriction. Without outcome denominators and missing-data information, it is not possible to understand who contributed to the primary estimate or assess the influence of unavailable measurements.

Finally, the registry lists many outcome measures but no analyses, and it does not report a statistical plan or multiplicity approach. Such information helps readers distinguish prespecified analyses from exploratory ones and evaluate the possibility of selective emphasis. The primary result is not reported, so none of those statistical assessments can be completed from this record.

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 main limitation for interpreting the primary question is that its result is not reported in the registry record. There is no primary outcome count or denominator, and no estimate, confidence interval, or test result. The registry’s enrollment figure cannot be treated as the number analyzed for the primary endpoint because eligibility and immunogenicity evaluability criteria apply, and the endpoint denominator is not reported.

The design is non-randomized, unmasked, and based on prior vaccination history. Both groups received Boostrix in the current study, so the record does not describe a randomized control comparison. Differences between groups, if assessed, could reflect participant characteristics or earlier treatment histories. The registry itself warns that the arms are not a like-for-like control comparison.

The primary endpoint uses a thresholded antibody outcome. This makes the endpoint interpretable as meeting or not meeting a laboratory cutoff, but it does not show the distribution of concentrations or clinical outcomes. The registry lists concentration measures as secondary outcomes, but their results are not reported in this record.

The record does not report the statistical analysis plan, multiplicity approach, handling of missing data, analysis denominator, or interim analyses. It also does not report the safety follow-up period or detailed descriptions of other adverse events. These gaps constrain assessment of analysis choices, outcome completeness, and the context of the pooled safety counts. The limitations are those of the information recorded; they do not establish which procedures may have occurred outside the registry entry.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Immunogenicity & Reactogenicity of Boostrix 10 Years After Previous Booster Vaccination in Study NCT01267058 — NCT00548171.
• Registry record last updated 2020-01-13; status last verified 2019-12.
• Results first posted 2019-01-15.
• 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

This completed Phase 4, non-randomized, unmasked, parallel study evaluated one Boostrix booster in adults ten years after earlier booster vaccination. Enrollment was 203. The groups were based on prior vaccination in NCT01267058, and both received Boostrix in the current study. No new recruitment was planned for this phase.

The primary endpoint was the number of participants with anti-DT and anti-TT concentrations at or above 0.1 IU/mL by ELISA one month after vaccination. The stated primary analysis was limited to participants who had received Boostrix as their first booster in the earlier study. The endpoint result and analysis denominator are not reported in the registry record.

The pooled safety summary records 0 serious adverse events, 163 participants with other adverse events, and 0 deaths. The registry lists 20 outcome measures and no analyses. The primary result, statistical plan, missing-data handling, and multiplicity approach are not reported in the registry record.