Study to Evaluate an Influenza Vaccine Candidate: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Study to Evaluate an Influenza Vaccine Candidate (NCT00321373), a randomized phase 2 study. The primary outcome is Titers for Serum Hemagglutination Inhibition (HI) Antibodies Against 3 Strains of Influenza Virus.

2. Clinical Question

3. Trial Design

This was a randomized, single-masked, parallel-group Phase 2 study sponsored by GlaxoSmithKline. The registry record lists an enrollment of 1220 and describes three groups: participants receiving the AS03-adjuvanted GSK1247446A candidate, participants receiving non-adjuvanted GSK1247446A, and participants receiving Fluarix. Each group description specifies one intramuscular dose in the deltoid region of the non-dominant arm. The participants were aged 60 years or older at vaccination.

The study began on 2006-05-11, had a primary completion date of 2006-07-01, and is recorded as completed. Its stated purpose was to evaluate immune response and safety for the candidate formulations compared with Fluarix. The primary outcome concerned antibody titers; secondary outcomes addressed antibody titers and several categories of solicited and unsolicited adverse events, including serious adverse events.

Randomization is intended, in general, to make treatment groups comparable at the start of a trial by assigning participants according to a chance-based process. A parallel-group design assigns participants to a group whose intervention remains the basis for comparison. The record identifies the study as single-masked but does not state which parties were masked or describe how masking was maintained. Details of the randomization sequence, allocation concealment, and group assignment procedure are not reported in the registry record.

The study title specifies a non-inferiority objective, but the registry record does not report the non-inferiority margin, the analysis plan for that objective, or numerical primary outcome results. Although the record gives an overall enrollment figure, it does not give enrollment by treatment group in the registry record for this account. The adverse-event records do provide group-specific counts at risk and counts with events; these are described separately and should not be treated as the primary-outcome analysis populations.

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: The analysis was based on the ATP cohort for immunogenicity at Day 21 including subjects for whom assay results were available for antibodies against at least one study vaccine antigen component after vaccination at this 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 Titers for Serum Hemagglutination Inhibition (HI) Antibodies Against 3 Strains of Influenza Virus, measured over At Days 0 and 21 post-vaccination.

The primary outcome was serum hemagglutination inhibition antibody titer against three influenza strains: A/New Caledonia, A/New York, and B/Malaysia. Titers were summarized as geometric mean titers (GMTs), with 95% confidence intervals, at Days 0 and 21 post-vaccination. The record also identifies a reference seropositivity threshold of 1:10.

In general, an antibody titer is a laboratory measure of the level of an antibody response. GMTs are used for titer data because such measurements are commonly analyzed on a logarithmic scale: averaging on that scale and converting back produces a geometric rather than arithmetic mean. This summary can describe the central level of titers while respecting their multiplicative scale. A confidence interval expresses uncertainty around an estimated quantity; it does not by itself establish equivalence or non-inferiority.

The endpoint assesses immune response, not clinical influenza cases. The registry record does not report the GMT estimates, confidence interval limits, or numerical comparisons for the candidate formulations and Fluarix. It therefore does not allow a reader to quantify the observed antibody response or judge whether a prespecified non-inferiority criterion was satisfied. The stated threshold gives a reference for seropositivity, but no group-specific seropositivity results are reported in the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a geometric mean titer represent?

General explanation: Titer measurements often vary over a multiplicative scale, so analysis may use logarithms to make their distribution more suitable for statistical summaries and comparisons. The geometric mean is obtained by averaging on the logarithmic scale and transforming back. It describes a central level of titers, but it does not show every participant's response or the full distribution. In this trial, the registry record says that titers were presented as GMTs for antibodies against A/New Caledonia, A/New York, and B/Malaysia, at Days 0 and 21 post-vaccination.

The record specifies 95% confidence intervals for the GMTs. General explanation: A confidence interval conveys uncertainty in an estimate under the statistical model and analysis used. A narrower interval indicates greater precision than a wider interval, but neither the interval nor the point estimate alone establishes clinical importance. For a non-inferiority assessment, interpretation depends on the interval in relation to a prespecified non-inferiority margin. That margin and the numerical intervals are not reported in the registry record.

How should the non-inferiority objective be understood?

General explanation: A non-inferiority trial asks whether an intervention's effect is not unacceptably worse than a comparator by more than a specified margin. The margin must be chosen in advance and the statistical analysis must address it directly. A failure to detect a difference in an ordinary superiority test is not, by itself, evidence of non-inferiority. In this record, the title states a non-inferiority objective comparing the candidate with Fluarix, but the margin, decision rule, and numerical comparison results are not reported in the registry record. The stated objective can therefore be described, but its success cannot be evaluated.

What does the analysis population mean?

The primary outcome description identifies the ATP cohort for immunogenicity at Day 21. It includes subjects for whom assay results were available for antibodies against at least one study vaccine antigen component after vaccination at that time point. This definition is part of the record's description of the analysis population. The record does not give the numbers included in that cohort by group, the reasons participants may have been excluded, or whether a separate intention-to-treat or per-protocol analysis was performed.

General explanation: An analysis population determines which participants contribute to a particular estimate. An analysis restricted to people with available post-vaccination assay results may differ from an analysis that includes all randomized participants, especially if missing measurements or exclusions are related to treatment or outcome. The record's cohort definition alone does not reveal whether such differences affected results. Reasons for missing assay data and methods for handling missing values are not reported in the registry record.

Why do outcome multiplicity and several strains matter?

The primary outcome covers antibodies against three strains and measurements at two stated time points. The record also lists secondary immune and safety outcomes. When an analysis considers multiple outcomes, strains, time points, or treatment comparisons, the chance of obtaining a seemingly notable result by chance can increase if each is assessed separately. General explanation: Multiplicity methods can control or otherwise address that issue, but the appropriate approach depends on the planned questions and analysis hierarchy. The registry record does not report a multiplicity strategy or the numerical analyses for the multiple strain and time-point results.

The primary outcome was not reduced to one comparison in the registry record. This means the information available does not identify one numerical treatment contrast as the sole primary result. It would be inappropriate to select a favorable strain, time point, or formulation comparison without a documented analysis plan and full results.

What do the planning assumptions mean?

The record describes a sample-size planning illustration based on an observed effect, with a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation; the target effect is stated as a fraction of the observed effect. Such calculations are planning illustrations, not evidence about the achieved precision or power of this completed trial. They are not the trial's own design rationale, and they are not observed power. The registry record does not report the original sample-size justification or enough information to assess how the trial's enrollment target was chosen.

General explanation: A sample-size calculation depends on its assumptions, such as the target effect, variability, significance level, allocation, and analysis method. If the assumptions differ from what occurs in a trial, the resulting precision may also differ. A post hoc calculation based on an observed effect should not be read as a direct measure of whether the trial was adequately designed. The planning assumptions recorded for the illustration do not replace a prespecified protocol or statistical analysis plan.

How should adverse-event counts be read statistically?

The registry record reports at-risk counts and counts with serious adverse events for each group, along with counts for other adverse events. General explanation: These counts are descriptive summaries. Interpreting them comparatively requires attention to the outcome definition, follow-up period, event ascertainment, and analysis method. The number of participants with at least one event is not the same as the total number of event occurrences, and the registry entries available do not give event-level timing or participant-level details. The record does not report comparative statistical tests or adjusted estimates for these counts.

The serious adverse-event terms are listed with proportions, but the registry entries do not specify a group for each term. They should therefore not be assigned to a treatment arm. Death counts are not reported in the registry record. These limits prevent a more detailed statistical comparison of the safety information.

10. Multiplicity, Missing Data and Analysis Populations

The primary outcome spans antibody titers for three influenza strains and measurements at Days 0 and 21. The registry record also lists secondary outcomes covering antibody titers and multiple adverse-event categories. Those multiple outcomes and comparisons raise questions about how the study controlled or interpreted the accumulation of statistical tests. The multiplicity strategy is not reported in the registry record. The registry entries available also list no analyses, so no adjustment method, hierarchy of hypotheses, or prespecified primary contrast can be assessed.

The primary immunogenicity analysis is described as using the ATP cohort at Day 21, defined by the availability of post-vaccination assay results for antibodies against at least one study vaccine antigen component. The registry record does not give group-specific sizes for this cohort, explain exclusions from it, or report how missing assay measurements were handled. It does not state whether missing values were imputed, whether participants with incomplete measurements were excluded from particular strain analyses, or whether sensitivity analyses were conducted. Those procedures are not reported in the registry record.

The record also does not provide the statistical analysis plan, the detailed randomization procedure, the planned handling of protocol deviations, or the sample-size justification for the trial itself. These omissions matter because the analysis population, missing-data approach, and multiplicity decisions can affect the interpretation of confidence intervals and hypothesis tests. Their absence from the registry record does not establish that the procedures were not performed; it means they cannot be evaluated from this record.

Because numerical primary results are not reported in the registry record, readers cannot examine consistency across strains, time points, or treatment comparisons. Nor can they determine whether any missing results would change the interpretation. The registry record therefore leaves both the multiplicity approach and the consequences of missing data unresolved.

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.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: cerebral infarction (0.5%), grand mal convulsion (0.2%), breast cancer (0.2%), cholecystitis (0.2%), cholelithiasis (0.2%).

The registry record gives group-specific counts for serious adverse events. In the GSK1247446A-AS03 group, 407 participants were at risk and 8 had a serious adverse event. In the GSK1247446A group, 406 were at risk and 13 had a serious adverse event. In the Fluarix group, 407 were at risk and 13 had a serious adverse event. These are observed counts as recorded; no comparative effect estimate or statistical test is reported in the registry record.

The record lists cerebral infarction at 0.5%, and grand mal convulsion, breast cancer, cholecystitis, cholelithiasis, and diverticulitis at 0.2% each among the top serious terms. The registry entries do not link each listed term to a particular group, so these terms should not be interpreted as arm-specific. The record does not report death counts.

General explanation: Serious adverse-event counts are descriptive safety information. They do not by themselves show that an intervention caused an event or that event rates differ between groups. Interpretation also depends on definitions, follow-up, event review, and the population under observation. Those details and a comparative statistical analysis are not reported in the registry record.

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

The record establishes that this was a randomized Phase 2 study comparing an AS03-adjuvanted GSK1247446A formulation and a non-adjuvanted formulation with Fluarix in adults aged 60 years or older. It identifies the primary outcome as GMTs of serum hemagglutination inhibition antibodies to three influenza strains, assessed at Days 0 and 21 post-vaccination. It also documents group-specific counts for serious and other adverse events.

Those design and outcome details do not provide the primary result. The numerical GMTs, confidence intervals, and comparisons are not reported in the registry record. The title states a non-inferiority objective, but the record does not provide the margin or the results needed to judge the objective. Accordingly, the available information does not establish whether either candidate formulation met the non-inferiority criterion. It also does not show the magnitude or precision of any difference in antibody response.

The endpoint concerns an immune response measured in a laboratory, not clinical influenza illness. The reference seropositivity threshold of 1:10 is reported, but group-specific seropositivity findings are not reported in the registry record. Without those results, no inference about the distribution of responses relative to the threshold can be made.

The adverse-event counts describe what the record reports for each group, but they do not establish causation or comparative safety. The registry record does not provide a statistical comparison of those counts, death counts, or group assignments for the listed serious-event terms. They should be interpreted as descriptive information rather than as proof of a treatment effect.

General explanation: A trial result is interpreted in light of its estimand, analysis population, uncertainty, and prespecified decision criteria. Missing outcome estimates and methods limit what can be concluded, even when the study design and intended question are clear. Here, the record supports an account of the comparison that was planned and some reported safety counts; it does not support a conclusion about non-inferiority, clinical protection, or a definitive safety difference.

13. Why This Trial Matters Statistically

This trial illustrates the distinction between a stated statistical objective and evidence that can be evaluated from reported results. The title specifies non-inferiority, and the primary measure is antibody GMT. Yet a non-inferiority conclusion requires the margin and the treatment comparison with its uncertainty. Without those elements, readers cannot determine whether the objective was met, regardless of the study's randomized design or the type of endpoint selected.

It also illustrates how an endpoint may encompass several related outcomes. Antibody responses were assessed for three strains and at two time points, while the record lists additional immune and safety outcomes. A statistical interpretation needs to know which comparisons were primary, how multiple tests were handled, and how incomplete measurements were treated. These procedures are not reported in the registry record. Their absence limits reproducibility and makes it difficult to distinguish confirmatory findings from exploratory ones.

The study also demonstrates why an analysis population should be read alongside an endpoint definition. The primary outcome description refers to an ATP cohort with available post-vaccination assay results. That criterion defines who contributes to the specified analysis, but the record does not explain exclusions or missing measurements. In general, differences between randomized participants and the analysis population can affect interpretation if they are related to treatment or outcome.

Finally, safety counts are useful as descriptive evidence but do not automatically provide a comparative estimate. The group-specific serious adverse-event counts are reported, while the registry record does not provide comparative statistical analyses or death counts. Readers can identify what was recorded without treating the counts as causal evidence. These distinctions are central to responsible interpretation of clinical trial statistics.

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 for interpretation is that the registry record does not report numerical primary outcome results. No GMT estimates, confidence intervals, or treatment comparisons are available in the registry record for this account. The record states a non-inferiority objective but does not report the margin, so the primary question cannot be evaluated.

The primary outcome covers several strains and time points, but the record does not identify a single numerical comparison or report the results for each. It also does not report a multiplicity strategy. The number of participants included in the Day 21 ATP cohort by group, reasons for exclusions, and handling of missing assay results are not reported in the registry record. This prevents assessment of the completeness and robustness of the immunogenicity analysis.

Although the trial is described as randomized, the randomization procedure, allocation concealment, and masking details are not reported in the registry record. The record says the study was single-masked but does not specify which parties were masked. The original sample-size justification and full statistical analysis plan are also not reported, so the planning assumptions cannot be matched to the trial's own design decisions.

The safety information is limited to reported counts and listed serious-event terms. Comparative analyses, event timing, and group assignment for the listed terms are not reported in the registry record. Death counts are not reported in the registry record. These limitations mean that safety counts can be described but not used to establish causation or a reliable difference between groups.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Study to Evaluate an Influenza Vaccine Candidate — NCT00321373.
• Registry record last updated 2018-06-08; status last verified 2016-09.
• Results first posted 2013-05-01.
• 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

NCT00321373 is a completed randomized, single-masked, parallel-group Phase 2 study sponsored by GlaxoSmithKline. It enrolled 1220 participants and compared one dose of AS03-adjuvanted GSK1247446A, non-adjuvanted GSK1247446A, and Fluarix in adults aged 60 years or older. The study began on 2006-05-11 and had a primary completion date of 2006-07-01.

The primary outcome was serum hemagglutination inhibition antibody GMT against A/New Caledonia, A/New York, and B/Malaysia at Days 0 and 21 post-vaccination, with 95% confidence intervals. The study title states a non-inferiority objective. Numerical primary results and the non-inferiority margin are not reported in the registry record, so the primary objective cannot be assessed from the information available. Group-specific serious adverse-event counts are reported, but the record does not provide comparative safety analyses or death counts.