Study of Adacel® Vaccine Administered to Persons 10 Years of Age: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Study of Adacel® Vaccine Administered to Persons 10 Years of Age (NCT01311557), a non-randomized phase 4 study. The primary outcome is Summary of Geometric Mean Titers of Anti-Pertussis Titers Following a Single Dose of Adacel® Vaccine.

2. Clinical Question

In practical statistical terms, the central question was whether the measured antibody response after one dose differed between participants enrolled in the younger age band and those enrolled in the older age band. The recorded endpoint measures antibody titers against pertussis antigens, rather than clinical cases of pertussis or a direct measure of illness. The registry lists pertussis toxin, filamentous hemagglutinin, pertactin, and fimbriae types 2 and 3 as the antigens measured.

The record describes a Phase 4 study and states that its purpose was to assess safety and immunogenicity in persons 10 years of age. It does not report a numerical estimate for the primary comparison, a confidence interval, or a statistical test result. Therefore, the registry information describes the question and measurement approach but does not show the magnitude or precision of the age-group comparison.

3. Trial Design

This was a completed Phase 4 study with a parallel-group model and non-randomized allocation. The registry reports an enrollment of 1302 and lists Sanofi as the sponsor. The study began in 2011-03, reached primary completion in 2011-11, and is recorded as completed. These are registry characteristics, not evidence about the adequacy of recruitment or the completeness of follow-up.

The two groups were defined by age at enrollment: participants 10 to <11 years of age and participants 11 to <12 years of age. Both group descriptions name Adacel® as the intervention. This is therefore an age-group comparison among vaccinated participants, not a randomized comparison of vaccination against an untreated or placebo group. The record explicitly identifies the comparison as non-randomized and cautions that the arms are not a like-for-like control comparison.

Masking is listed as none. The record does not report further detail about who knew participants’ group assignments or how measurements and safety reports were collected. It also does not report the recruitment process, eligibility criteria beyond the age descriptions, or whether the groups differed in other characteristics at baseline. Those details matter when interpreting an age-based comparison because group differences may reflect factors other than age.

The primary endpoint was measured at Day 30 post-vaccination. Its analysis population is identified as the Per-protocol Analysis Set. The record describes six outcome measures and four analyses, but it does not give a complete statistical analysis plan or provide the numerical results of the primary comparison. The registry record does not report the protocol’s sample-size justification, any interim analyses, or detailed methods for handling departures from the protocol.

Although enrollment is stated in the record, that figure should not be taken as the number analyzed for every outcome. The endpoint description specifically names a per-protocol population, and the analysis-set size is not reported in the registry record. The age-group counts in the adverse-event listing are reported separately for safety outcomes and do not establish the size of the primary endpoint population.

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 1302 participants who met all inclusion criteria and none of the exclusion criteria were enrolled and vaccinated.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Geometric mean titers were assessed in the Per-protocol Analysis Set.

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 Summary of Geometric Mean Titers of Anti-Pertussis Titers Following a Single Dose of Adacel® Vaccine, measured over Day 30 post-vaccination.

The primary endpoint is the summary of geometric mean titers of anti-pertussis antibodies following a single dose of Adacel®. Titers were assessed at Day 30 post-vaccination by enzyme-linked immunosorbent assay (ELISA). The listed antigens were pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), and fimbriae types 2 and 3 (FIM). The record identifies the measure as titers in 1/dilutions and specifies a 95% confidence interval as the dispersion measure.

As a general statistical concept, a geometric mean is useful for measurements such as antibody titers that can vary multiplicatively and may be skewed on their original scale. It summarizes a central level while respecting proportional rather than purely additive changes. The registry record does not describe the precise transformation or calculation procedure used for these reported summaries.

Antibody titers are immunogenicity measurements: they describe a laboratory-measured immune response. They are not themselves counts of disease cases, and this primary endpoint does not directly measure clinical protection. Comparing the geometric mean titers between age groups can describe how measured antibody levels differ, but interpretation depends on the numerical estimates, their uncertainty, and the analysis methods.

The stated analysis population is the Per-protocol Analysis Set. The actual geometric mean titers, confidence intervals, and age-group comparison results are not reported in the registry record. Consequently, the endpoint definition is available, but the size and precision of the observed primary response cannot be assessed from the registry information.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary measure is a geometric mean titer (GMT). As a general statistical explanation, geometric means are often used when measurements are positive and differences are naturally understood in multiplicative terms. A geometric mean is based on averaging on a logarithmic scale and then returning to the original scale. This approach can make a skewed distribution of titers easier to summarize than an ordinary arithmetic mean. The registry record names a geometric mean but does not report its exact calculation procedure, any transformation details, or how values below assay limits were handled.

The endpoint description specifies a 95% confidence interval. In general, a confidence interval communicates uncertainty in an estimated quantity, rather than listing all observed values. It is interpreted in relation to the method and assumptions used to construct it. The width of an interval is informative about precision, while the estimate itself describes the central result. Neither the interval limits nor the numerical GMTs are reported in the registry record, so precision and the direction or size of the observed age-group comparison cannot be evaluated from those registry entries.

ELISA is the laboratory method named for measuring the anti-pertussis titers. The registry lists PT, FHA, PRN, and FIM as the antigens measured. The outcome is therefore a set of laboratory antibody measurements, not a count of clinical disease events. The stated time point is Day 30 post-vaccination. This definition matters because a result at that time point describes the measured response at that point; it does not, on its own, describe how long the response persists.

The record identifies the Per-protocol Analysis Set for the primary endpoint. In general, a per-protocol analysis focuses on participants meeting specified study requirements. The exact requirements and the number included are not reported in the registry record. Because this population can differ from all enrolled participants, the analysis-set definition and participant flow are important for interpreting the result. The registry’s enrollment figure does not tell readers how many participants contributed to each endpoint.

Allocation was non-randomized, and the groups were defined by age. In general, random allocation helps balance measured and unmeasured baseline factors on average, while a non-randomized comparison can retain differences between groups that affect outcomes. Here, the groups also represent different age bands, so any measured contrast is an association between age-group membership and the observed response in this study, rather than a result isolated from all other possible group differences. The record itself states that the groups are not a like-for-like control comparison.

The record specifies confidence intervals for the primary endpoint but does not report a hypothesis test, p-value, model, covariate adjustment, or a formal decision threshold for the primary comparison. Those methods are not reported in the registry record. It is therefore not possible to determine from the record whether the reported analysis used adjusted or unadjusted estimates, how comparisons across antigens were handled, or what inferential criteria were applied.

The registry describes multiple objectives and outcome measures. In general, when many outcomes or comparisons are examined, the chance of at least one apparently unusual result can increase if each is assessed separately. Methods for addressing multiplicity can include prespecified prioritization or adjustment, but the statistical plan for this study is not reported in the registry record. The existence of several objectives does not establish which measures were formally tested or whether any multiplicity approach was used.

A planning illustration based on an observed effect is not the same as observed power or an analysis of the trial’s results. The stated illustration assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation, with the target effect expressed as a fraction of the observed effect. The record describes the illustration as planning information, not the trial’s own design. Such calculations depend on assumptions and should not be read as evidence that the study was designed around a particular effect. No sample-size illustration values are reported in the registry information.

For continuous or laboratory outcomes, the distribution of measurements can affect the choice and behavior of statistical summaries. As a general concept, over-dispersion means that variability is greater than a model assumes; if not accommodated, uncertainty estimates can be too small. The registry does not report a distributional assessment or model for the titer data, so no conclusion can be drawn about whether over-dispersion was present or how it was handled.

Safety summaries require a different interpretation from antibody summaries. Counts of participants with events and counts of events are not interchangeable, and the registry’s adverse-event entries should be read according to the labels attached to each measure. The information reported does not include event timing, duration, relatedness assessments, or detailed definitions for the other-event measure. A numerical comparison based only on the posted counts would therefore omit important context.

10. Multiplicity, Missing Data and Analysis Populations

The study description lists several objectives: comparing pertussis antibody responses and booster responses between age groups, comparing tetanus and diphtheria booster responses, and comparing tetanus and diphtheria seroprotection rates. The registry also lists secondary outcomes involving tetanus and diphtheria seroprotection, anti-pertussis geometric mean titers before and after vaccination, and solicited injection-site or systemic reactions. The record reports six outcome measures and four analyses, but does not describe how the outcomes were prioritized statistically.

As a general statistical issue, multiple outcomes or comparisons can raise the probability of finding an apparently notable result by chance when each is tested independently. A prespecified primary endpoint can help distinguish the main question from supporting analyses, while an adjustment or other multiplicity strategy can address the broader set of comparisons. The detailed statistical plan, including any multiplicity method, is not reported in the registry record. The primary endpoint is identified, but its numerical results and associated test information are not available in the record.

Missing-data methods are also not described. The registry does not report how participants without a Day 30 measurement were handled, whether any values were excluded from the primary analysis, or whether sensitivity analyses were conducted. The primary population is named as the Per-protocol Analysis Set, but its detailed definition and size are not reported. These omissions limit assessment of how analysis-set selection or missing measurements may have affected the endpoint summary.

Accordingly, neither the degree of multiplicity control nor the influence of missing data can be evaluated from the registry information. These are unreported analysis details, not evidence that a particular method was or was not applied.

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 5.00% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: cerebrovascular accident (0.2%).

The registry lists serious adverse events separately by age group. For participants 10 to <11 years of age, the serious-event count is 0 among 651 at risk. For participants 11 to <12 years of age, the count is 1 among 651 at risk. The record identifies cerebrovascular accident as a top serious term and displays 0.2%. It does not report additional context for that percentage, such as its denominator or the way the term was assigned, so it should be read as the registry’s listed figure rather than recalculated or expanded.

The record also lists other adverse-event counts of 521 for the younger group and 522 for the older group, each with 651 at risk. These entries are distinct from the serious-event counts. They do not by themselves describe event severity, relatedness to vaccination, duration, or the number of separate events per participant. Further definitions and context for the other-event measure are not reported in the registry record.

Safety counts from non-randomized age groups are descriptive. They do not establish that a difference between the groups was caused by age or vaccination. The registry entries do not provide a full safety analysis or uncertainty estimates for these comparisons. The figures should therefore be read as the reported event summaries, not as a standalone causal or comparative safety conclusion.

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

This study addresses an immunogenicity question: how anti-pertussis antibody geometric mean titers at Day 30 after one dose of Adacel® compare between participants enrolled at 10 to <11 years and those enrolled at 11 to <12 years of age. The registry identifies the assay, antigens, endpoint summary, and analysis population. It also records safety outcomes and secondary objectives concerning tetanus, diphtheria, pertussis antibodies, and solicited reactions.

The numerical primary results are not reported in the registry record. That means the recorded information does not establish whether the age-group titers were higher in one group, how large any difference was, or how precise the estimates were. The description of a 95% confidence interval does not resolve those questions without the interval limits and corresponding estimates.

Even if numerical differences were available, this design would require cautious interpretation. The comparison was non-randomized, with age-defined groups, and the registry explicitly says the arms are not a like-for-like control comparison. An observed contrast would describe the groups as studied; it would not, by itself, isolate an effect caused by age or any other single factor.

The antibody endpoint is a laboratory measure of immunogenicity. It should not be equated with a direct count of illness or with proof of clinical protection. The listed secondary seroprotection outcomes are relevant to the study objectives, but their numerical results are not reported in the registry information. Likewise, the serious and other adverse-event counts are descriptive entries and do not supply a complete account of event characteristics or comparative uncertainty.

The appropriate conclusion from the registry record is therefore limited: the study compared antibody responses across two age groups after the same named vaccine, using a specified laboratory endpoint and a per-protocol analysis population. The record does not contain the numerical outcome results needed to characterize the observed immunogenicity contrast or to make a broader claim about clinical outcomes.

13. Why This Trial Matters Statistically

This study is statistically notable because its comparison is between two age groups receiving the same named vaccine, rather than between randomized treatment and control groups. That distinction changes the interpretation of any contrast. Non-random allocation can leave group differences that are not accounted for by the age categories, so the comparison is descriptive and cannot automatically be read as a causal effect.

The endpoint also illustrates the difference between a laboratory response and a clinical outcome. Geometric mean titers summarize antibody measurements against specified pertussis antigens. Such summaries can be useful for comparing immune responses, but they do not directly count disease events. The endpoint’s time point, Day 30 post-vaccination, defines when the measurement was made; it does not answer questions about longer-term persistence.

The registry identifies a confidence interval as the uncertainty summary for the primary endpoint. That is important because a point estimate alone would not show how precisely the response was estimated. In this record, however, neither the numerical estimates nor the intervals are reported, leaving the primary comparison’s magnitude and precision unassessable.

The study’s multiple objectives also make transparency about analysis choices important. The record lists several antibody, seroprotection, and safety outcomes, while the detailed analysis plan and multiplicity approach are not reported. Clear reporting of how the primary question is distinguished from secondary comparisons helps readers assess the evidence without overemphasizing isolated findings.

Finally, naming the Per-protocol Analysis Set identifies the population used for the primary endpoint, but its size and detailed definition are not reported. This highlights why participant flow and analysis-population descriptions matter: enrollment alone does not show how much information contributed to a particular estimate. Overall, the record makes the design and endpoint understandable while leaving essential result and analysis details unavailable.

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 registry record restrict interpretation. Most importantly, numerical results for the primary geometric mean titer endpoint are not reported. The record gives no age-group estimates, confidence-interval limits, or primary comparison test results. As a result, the direction, magnitude, and precision of the primary comparison cannot be assessed.

The non-randomized design is an additional limitation for causal interpretation. The age groups are not described as like-for-like controls, and the registry does not report baseline characteristics or adjusted analyses that could show how other group differences were handled. Masking is listed as none, while further details about outcome ascertainment are not reported.

The primary endpoint population is identified as the Per-protocol Analysis Set, but its size, detailed criteria, and relationship to the enrolled population are not reported in the registry record. The record also does not describe missing-data procedures, exclusions, sensitivity analyses, or the handling of assay values outside measurable limits.

Although several outcomes and objectives are listed, the detailed statistical analysis plan and any multiplicity approach are not reported. Numerical results for the listed secondary outcomes are also absent from the registry information. The adverse-event entries provide counts and a serious-event term, but do not give further details on timing, duration, attribution, or event-level context. These gaps limit evaluation of both the immunogenicity findings and the safety summaries.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Study of Adacel® Vaccine Administered to Persons 10 Years of Age — NCT01311557.
• Registry record last updated 2016-04-08; status last verified 2016-04.
• Results first posted 2016-04-08.
• 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

The registry record describes a completed Phase 4, non-randomized, parallel-group study sponsored by Sanofi. It lists an enrollment of 1302 and compares participants enrolled at 10 to <11 years of age with participants enrolled at 11 to <12 years of age. Both groups received a single dose of Adacel®.

The primary endpoint was anti-pertussis geometric mean titers measured by ELISA at Day 30 post-vaccination, with a 95% confidence interval specified and the Per-protocol Analysis Set named as the analysis population. The numerical primary results are not reported in the registry record. The detailed statistical plan, missing-data methods, multiplicity approach, and analysis-set size are also not reported.

The registry lists serious-event counts of 0 and 1 in the two age groups and other-event counts of 521 and 522. It identifies cerebrovascular accident as a top serious term and displays 0.2%. These entries are descriptive; the record does not provide further context sufficient for a broader comparative safety assessment.