Challenge Dose of Hepatitis B Vaccine in Subjects Who Previously Received Engerix™-B Vaccine: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Challenge Dose of Hepatitis B Vaccine in Subjects Who Previously Received Engerix™-B Vaccine (NCT00524576), a non-randomized phase 4 study. The primary outcome is Number of Participants With Immunological Response to Challenge Dose in Terms of Anti-hepatitis B Surface Antigen (Anti-HBs) Antibody Concentration.

2. Clinical Question

3. Trial Design

The study is registered as a Phase 4, completed, non-randomized, single-group study of Engerix™-B in people who had previously received primary vaccination with the same vaccine. The registry lists an enrollment of 144. Its stated purpose was to evaluate the response to an additional, or challenge, dose given approximately 72-78 months after the primary vaccination. The recorded study start was 2007-11-28, and primary completion was 2008-05-14.

The primary vaccination history defined two groups: people who had received 2 doses at Month 0 and 6, and people who had received 3 doses at Month 0, 1 and 6. Both groups received a single Engerix™-B challenge dose during the booster study. This is not a randomized comparison of challenge dose against no challenge dose. The groups also have different primary vaccination histories, so a difference between them could not be attributed to the challenge dose alone.

The primary outcome was assessed 30 days after the challenge dose. The primary analysis population was the According-to-Protocol cohort for immunogenicity. Data from the Australian center were not included in that analysis following data quality issues detected at the investigator site. The registry record does not report the number of participants retained in that cohort or provide details of the data quality issue.

The record lists additional immunogenicity and safety outcomes, including antibody concentrations, solicited symptoms, unsolicited adverse events and serious adverse events. It lists no analyses in its analysis record. The statistical analysis plan, sample-size justification, interim analyses, and handling of missing outcome data are not reported in the registry record. The overall enrollment figure should therefore not be treated as the denominator for every outcome.

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: Analysis was performed on the According-to-Protocol (ATP) cohort for analysis of immunogenicity. Data from the Australian center were not included following data quality issues detected at the investigator site.

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 Participants With Immunological Response to Challenge Dose in Terms of Anti-hepatitis B Surface Antigen (Anti-HBs) Antibody Concentration, measured over 30 days post-challenge dose.

The primary endpoint counts participants with an immunological response to the challenge dose, assessed by Anti-HBs antibody concentration 30 days afterward. The response rule depends on the participant’s initial serostatus. For participants initially seronegative, defined by an Anti-HBs concentration below 3.3 mIU/mL before vaccination, response means a post-booster concentration of at least 10 mIU/mL. For participants initially seropositive, response means a post-booster concentration at least four-fold the pre-vaccination concentration.

This endpoint turns antibody measurements into a participant-level response classification. The separate rules recognize that participants with detectable antibody before vaccination begin from a different baseline than those initially seronegative. A threshold-based outcome can make the proportion meeting a prespecified biological criterion easier to interpret than a concentration alone, while the continuous concentration outcome listed in the registry can describe antibody levels without reducing them to a yes-or-no classification.

The endpoint is an immunological measure, not a direct measure of clinical hepatitis B outcomes. The registry record does not report the number of responders, the denominator used for each baseline-serostatus category, or an uncertainty interval for the response estimate. It therefore establishes how response was defined, but not how many participants met that definition.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a response count represent?

A binary endpoint classifies each participant as meeting or not meeting a stated rule. Here, the rule is based on Anti-HBs antibody concentration after the challenge dose, with the criterion depending on initial serostatus. A response count would need a corresponding denominator to be interpretable as a rate. The registry record states the analysis population but does not report the primary response counts or denominators. No response proportion can therefore be recovered from the posted record.

The underlying antibody concentration is a continuous measurement, while the response classification is binary. Dichotomizing a continuous measure can make a criterion-based result easier to communicate, but it also leaves out how far a measurement lies above or below the threshold. The registry separately lists concentration of Anti-HBs antibodies as an outcome, but does not report its numerical results. These two kinds of outcomes would answer related but not identical questions.

Why does the analysis population matter?

An analysis population specifies which participants contribute to an outcome estimate. The primary outcome description identifies the According-to-Protocol cohort for immunogenicity and states that data from the Australian center were excluded following data quality issues. This is important because the enrollment figure is not necessarily the denominator for an endpoint analysis. The registry record does not state the cohort size, explain the data quality issue in detail, or give enough information to assess how the exclusion affected the result.

In general, excluding observations can affect interpretation if the excluded participants differ from those retained or if outcome availability is related to the outcome itself. That is a general consideration, not a claim about what happened in this study. The registry record does not report enough detail to assess the direction or magnitude of any such effect.

What can a non-randomized group contrast show?

Random allocation is one way to make comparison groups more similar, on average, with respect to factors that could affect an outcome. This study is described as non-randomized and single-group, with two groups defined by prior dose schedules; both received the challenge dose. Therefore, the groups are not a randomized treatment-versus-control contrast. If their outcome values were reported, a difference could reflect prior schedule, participant characteristics, or other differences, as well as any relationship to the challenge response. The registry record does not report an adjusted analysis that could address measured differences, and it lists no analyses.

Because no no-challenge group is described, the study does not directly estimate what would have happened to these participants at the same time without the challenge dose. A before-and-after antibody comparison might address a different descriptive question, but the primary outcome record does not provide the needed paired measurements or a reported analysis of change.

How should uncertainty be assessed?

A confidence interval, in general, gives a range of values compatible with the observed data under a specified statistical procedure. It communicates precision as well as the direction and magnitude of an estimate. A p-value, in general, summarizes how incompatible observed data are with a specified null model; it is not the probability that a hypothesis is true. The registry record does not report confidence intervals, p-values, or a primary outcome estimate, so neither precision nor statistical evidence for a group difference can be assessed.

For a binary endpoint, uncertainty depends on the observed count and denominator. For a continuous antibody concentration, the relevant summary and uncertainty method would depend on the distribution and analysis plan. The registry does not report the chosen summary method, transformations, model, or statistical test. It would be inappropriate to infer those choices from the endpoint alone.

What does over-dispersion mean in general?

For count outcomes, over-dispersion means that observed variability is greater than expected under a model’s assumptions. If present, it can make standard errors too small under an overly restrictive model and make uncertainty appear more precise than it is. This is a general statistical concept. The primary endpoint in this record is a participant count defined by a binary response, and no model or over-dispersion assessment is reported. There is no basis for saying that over-dispersion was assessed or affected this trial.

The registry also lists antibody concentration and several safety outcomes. Different outcome types can call for different summaries and methods. The record does not identify a statistical model for those outcomes, give adjusted estimates, or specify how multiplicity was addressed. The analysis record lists no analyses, so no particular modeling or inferential procedure should be attributed to the study.

10. Exact Estimates for Each Arm

No two-arm comparison is shown for this trial, but each arm's result can still be summarized with exact interval estimates computed from the posted counts. The Clopper-Pearson interval is exact and conservative; the Wilson interval is a common closer-to-nominal alternative; the Bayesian credible interval uses a uniform Beta(1, 1) prior. For small groups the three can differ noticeably.

R code and output

arm    <- c("Engerix 2 Doses + Challenge Dose", "Engerix 3 Doses + Challenge Dose")
n      <- c(53, 21)
events <- c(53, 21)
for (i in seq_along(n)) {
  ci <- binom.test(events[i], n[i])$conf.int          # Clopper-Pearson exact interval
  cr <- qbeta(c(0.025, 0.975), 1 + events[i], 1 + n[i] - events[i])   # Bayesian, uniform Beta(1,1) prior
  wi <- (function(x, n) { p <- x / n; z <- qnorm(0.975); d <- 1 + z^2 / n
    c((p + z^2 / (2 * n) - z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d, (p + z^2 / (2 * n) + z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d) })(events[i], n[i])
  cat(sprintf("ARM|%s|%d|%d|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f\n", arm[i], n[i], events[i], 100 * events[i] / n[i],
              100 * ci[1], 100 * ci[2], 100 * wi[1], 100 * wi[2], 100 * cr[1], 100 * cr[2]))
}

ARM  Engerix 2 Doses + Challenge Dose  53  53  100.0  93.3  100.0  93.2  100.0  93.4  100.0
ARM  Engerix 3 Doses + Challenge Dose  21  21  100.0  83.9  100.0  84.5  100.0  84.6  99.9

11. Multiplicity, Missing Data and Analysis Populations

The registry lists a primary immunological-response outcome and additional outcomes covering Anti-HBs concentration, solicited local and general symptoms, unsolicited adverse events, and serious adverse events. Testing or estimating many outcomes can raise multiplicity concerns: when several statistical questions are addressed, the chance of at least one apparently notable result can increase unless the analysis plan accounts for that. This is a general statistical consideration, not evidence that a particular adjustment was needed or used in this study.

The analysis record lists no analyses, and the registry does not report a multiplicity strategy, outcome hierarchy beyond the stated primary endpoint, or planned adjustment. These details are not reported in the registry record. No claim about confirmatory or exploratory status beyond the listed primary and secondary outcome labels is warranted.

The primary analysis population is identified as the According-to-Protocol cohort for immunogenicity, and Australian-center data were excluded after data quality issues. The cohort size, participant flow, and endpoint-specific denominators are not reported in the registry record. The record does not describe imputation, complete-case analysis, sensitivity analyses, or other missing-data methods. It is therefore not possible to tell how unavailable or excluded observations were handled beyond the stated center exclusion.

These omissions matter for interpretation because outcome estimates depend on who was analyzed and how incomplete observations were treated. They do not, by themselves, establish bias or invalidate the study. They limit what readers can verify from the registry record.

12. 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.

Most frequent non-serious adverse event terms

The serious adverse event counts are reported separately by prior primary vaccination schedule. In the group with a prior 2-dose schedule, 0 serious adverse events were reported among 55 participants at risk. In the group with a prior 3-dose schedule, 0 were reported among 22 participants at risk. The record also reports 0 deaths in each group.

Zero observed events does not mean the underlying possibility of an event is zero. It means no serious adverse event was counted in the stated groups during the period represented by the registry safety record. The registry record does not give further details about event definitions, follow-up completeness, or uncertainty around these counts. The figures therefore should not be interpreted as a precise estimate of rare-event risk.

The record separately reports 39 participants affected in the other adverse-event category among 55 at risk in the prior 2-dose group, and 14 among 22 at risk in the prior 3-dose group. These are counts in a separate category, not serious adverse event counts. The registry lists solicited symptoms and unsolicited adverse events among its secondary outcomes, but the reported group counts should not be used to infer a specific symptom, severity, or causal relationship unless the registry identifies it. The safety counts also do not provide evidence about the primary immunological-response outcome.

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

This study addresses a focused measurement question: whether participants met a prespecified Anti-HBs response rule after a challenge dose, with the rule tailored to initial serostatus. The registry record identifies the vaccine, the earlier vaccination schedules, the timing of the primary endpoint, and the analysis population. It also provides adverse-event counts. These details establish what was intended to be measured and how some safety information was recorded.

The posted record does not include primary response counts, antibody concentration results, or a statistical comparison. It therefore does not show the observed frequency of response or how precisely that frequency was estimated. It cannot establish whether the two prior-schedule groups differed in response, and it cannot establish equivalence between them. A missing estimate is not a null result.

The design also does not answer a randomized question about challenge dose versus no challenge dose. Both described groups received the challenge dose, and their earlier primary schedules differed. A contrast between them, if reported, would be observational with respect to those prior schedules. It would not isolate the effect of giving a challenge dose.

The safety counts describe what was recorded in the listed groups, including zero serious adverse events and zero deaths. They do not prove that serious events cannot occur, nor do they quantify risk beyond the observed study record. The other adverse-event counts are separate and should be read as counts, not as a causal comparison.

Overall, the registry record is informative about study structure and endpoint definition, but limited for judging the magnitude or precision of immunological outcomes. It does not report a statistical analysis or enough outcome results to support conclusions about response rates or group differences.

14. Why This Trial Matters Statistically

The record illustrates why an endpoint definition and an endpoint result are not interchangeable. A clear threshold tells readers how a response would be classified, but it does not reveal how many participants met that threshold. Without response counts, denominators, and uncertainty estimates, the primary question remains quantitatively unanswered in the registry record.

It also highlights the importance of distinguishing a treatment contrast from a comparison between participant histories. The study is non-randomized, and both groups received a challenge dose. Their defining difference is the earlier primary vaccination schedule. This makes it essential to avoid interpreting a possible contrast as the isolated effect of the challenge dose.

The stated exclusion of Australian-center data from the immunogenicity analysis demonstrates why analysis populations and data quality decisions matter. Knowing that an exclusion occurred is useful, but readers also need cohort sizes, participant flow, and details of the consequences to evaluate how the analysis population relates to enrollment. Those details are not reported in the registry record.

Finally, safety counts offer a practical lesson in interpreting zero events. A zero count is an observation in a defined study population, not proof of zero underlying risk. Counts should be read with their at-risk denominators and with awareness that the registry does not report uncertainty intervals or fuller follow-up details. Together, these issues show how design, outcome reporting, and analysis transparency shape the strength of statistical conclusions.

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 study has no randomized control group and is described as single-group and non-randomized. Both prior-schedule groups received the challenge dose, so the record does not support a randomized estimate of challenge dose versus no challenge dose. The groups also differ in the number and timing of doses received in the earlier primary course.

The primary outcome results are not reported. The registry describes the endpoint and the immunogenicity analysis population, but gives no responder counts, denominators, antibody summaries, or inferential estimates. The analysis record lists no analyses. Consequently, the magnitude and precision of the primary outcome cannot be evaluated from the registry record.

The primary analysis excluded data from the Australian center after data quality issues detected at the investigator site. The registry record does not describe the issue in detail, report the number of observations affected, or state how the exclusion influenced the analysis. It also does not report the size of the According-to-Protocol cohort or explain handling of missing outcome data.

The enrollment figure is not sufficient to establish the denominator for each outcome, particularly because the primary outcome used a specified analysis cohort and the center exclusion. The registry does not report participant flow or endpoint-specific missingness. It also does not state a sample-size justification, statistical analysis plan, multiplicity approach, or interim analyses. These limitations describe gaps in the posted record; they do not establish that the study procedures were inadequate.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Challenge Dose of Hepatitis B Vaccine in Subjects Who Previously Received Engerix™-B Vaccine — NCT00524576.
• Registry record last updated 2018-09-07; status last verified 2016-11.
• Results first posted 2009-07-08.
• 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.

19. Record Summary

NCT00524576 is a completed Phase 4, non-randomized, single-group study sponsored by GlaxoSmithKline. It evaluated an Engerix™-B challenge dose in participants who had received a primary Engerix™-B course approximately 72-78 months earlier. The registry lists an enrollment of 144 and two prior-schedule groups: a 2-dose course and a 3-dose course, with both groups receiving one challenge dose.

The primary endpoint was the number of participants meeting an Anti-HBs response definition 30 days post-challenge. The analysis used the According-to-Protocol cohort for immunogenicity, excluding Australian-center data after data quality issues. Primary outcome results and statistical analyses are not reported in the registry record.

The safety record reports zero serious adverse events and zero deaths in each group, alongside separate counts for other adverse events. These safety counts do not substitute for the unreported immunogenicity results.