Evaluation of the Immune Response and the Safety of a Pandemic Influenza Candidate Vaccine (H1N1): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Evaluation of the Immune Response and the Safety of a Pandemic Influenza Candidate Vaccine (H1N1) (NCT00971425), a randomized phase 3 study. The primary outcome is Geometric Mean Titers (GMTs) of Antibodies Against Pandemrix Vaccine Strain.

2. Clinical Question

That distinction shapes the statistical interpretation. The registered endpoint describes antibody levels within each group at a specified time; it does not, on its own, establish the size or precision of the difference between groups. The observed GMTs can be read as summaries of the antibody responses in the analysed cohort, while the confidence intervals convey uncertainty around each group estimate. A reader interested in a direct comparison should distinguish these separate group summaries from an estimated contrast.

The registration identifies a randomized, single-masked, parallel-group phase 3 study. Its stated objective includes both immunogenicity and safety. The primary question is narrower: antibody titers against the specified Pandemrix strain at Day 42. The record also lists secondary immunogenicity and safety outcomes, but those outcomes address different questions and should not be treated as interchangeable with the primary endpoint.

3. Trial Design

The registry describes a randomized, single-masked, parallel-group phase 3 study. Its enrollment is 145, its status is completed, and the registered condition is influenza. The listed study start is 2009-09-08, with primary completion on 2009-11-16. The sponsor is GlaxoSmithKline. These record details describe the registered study, but they do not explain how the randomization sequence was generated or concealed, or which people were masked; those details are not reported in the registry record.

The two arm names are Placebo-Pandemrix-Fluarix Group and Fluarix-Pandemrix-Placebo Group. The intervention list includes Pandemrix (GSK investigational influenza GSK2340272A vaccine), Fluarix, and placebo. The arm descriptions indicate that participants received different sequences of these products. One description states that the group received placebo at Day -21, two Pandemrix doses at Day 0 and Day 21, and Fluarix; the description is truncated before all its schedule details are complete. The other states that the group received Fluarix at Day -21, two Pandemrix doses at Day 0 and Day 21, and placebo at Day 42. Further details of the incomplete schedule are not reported in the registry record.

The parallel-group structure means participants were assigned to one of the study groups rather than crossing between group regimens. Randomization is intended, in general, to support comparability between groups, but the registry record does not report the allocation procedure or enough baseline information to assess balance. Single masking indicates that the study was registered as single-masked; the identity of the masked party is not reported in the registry record.

The primary outcome uses the According-To-Protocol cohort for immunogenicity: evaluable subjects for whom immunogenicity results were available. Its reported group counts are 64 and 67. These are analysis counts, not a description of all enrolled participants. The record does not explain exclusions or missing outcome data in detail. The study also registered multiple secondary outcomes on antibody responses and adverse events, so its scope extended beyond the primary Day 42 GMT.

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 performed on the According-To-Protocol (ATP) cohort for immunogenicity , which included all evaluable subjects for whom immunogenicity results were available.

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 Titers (GMTs) of Antibodies Against Pandemrix Vaccine Strain, measured over At Day 42.

The primary endpoint is the geometric mean titer (GMT) of antibodies against the Pandemrix vaccine strain at Day 42. The strain is identified as A/Cal/7/09, and the outcome is expressed as a titer with a 95% confidence interval. A titer quantifies antibody activity in the assay used by the study; the GMT summarizes such measurements using a geometric rather than arithmetic average.

In general, geometric summaries are useful for measurements that are positive and can vary multiplicatively across a broad range. Taking logarithms makes multiplicative differences easier to analyze with methods designed for additive variation, and transforming the summary back produces a GMT. This makes the result a compact description of the group’s antibody response. It does not show the full distribution of individual titers, how many subjects met a threshold, or how durable responses were beyond the stated assessment time.

The registry reports a GMT of 309.8, with a 95% confidence interval from 246.4 to 389.6, for the Placebo-Pandemrix-Fluarix Group. For the Fluarix-Pandemrix-Placebo Group, it reports a GMT of 227.5, with a 95% confidence interval from 181.5 to 285.3. These are group-specific estimates in the primary analysis population. The record does not report a primary-endpoint contrast, such as a ratio of GMTs or a difference on the log-titer scale, or a corresponding inferential test.

GMT is used as an immunogenicity measure, not as a direct clinical endpoint. It describes an antibody response in the assay and at the registered time point. It does not by itself establish protection against illness or describe clinical effectiveness. The registry’s secondary outcomes include threshold-based and seroconversion measures, which address other aspects of antibody response, but they remain distinct from the primary GMT measure.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is a geometric mean titer, a summary suited to antibody measurements that commonly vary multiplicatively. In general, a geometric mean is obtained by transforming measurements to a logarithmic scale, averaging on that scale, and transforming the result back. This approach gives less influence to very large observations than an arithmetic mean would and expresses the typical level in the original measurement scale. It remains a summary: it does not reveal the shape of the distribution or the responses of individual participants.

The registry reports a 95% confidence interval for each group’s GMT. In general, a confidence interval describes the uncertainty associated with an estimate under the statistical model and sampling process. It is not a guarantee that the true value lies within the particular interval, nor does it describe the spread of individual titers. The group estimates and their intervals are 309.8 (246.4 to 389.6) for the Placebo-Pandemrix-Fluarix Group and 227.5 (181.5 to 285.3) for the Fluarix-Pandemrix-Placebo Group. No direct primary-endpoint contrast is reported, so these intervals should not be read as an interval for a group difference.

When the aim is to compare groups, an analysis typically estimates a contrast that matches the scale of the outcome. For GMTs, that may involve comparing log-scale means or expressing a ratio of geometric means. In general, the contrast has its own estimate and uncertainty interval. A comparison of separate group summaries cannot supply that information by itself. The registry record does not report a primary contrast, its confidence interval, or a test statistic for the GMT endpoint. It also does not report the model or method used to calculate the GMT intervals.

The registered analysis population is the According-To-Protocol cohort for immunogenicity, comprising evaluable subjects with immunogenicity results available. This defines who contributes to the primary outcome estimate. In general, an analysis restricted to evaluable participants can be informative about measured responses among those participants, but it may not preserve all the advantages of randomization if eligibility for analysis is affected by post-randomization events. The record does not report the exclusion rules, the number excluded, or a comparison of included and excluded participants. It does not describe an intention-to-treat analysis for this endpoint.

The primary outcome is assessed at Day 42. That fixed time point makes the group summaries interpretable as responses at a common scheduled assessment, but it does not describe the time course of response. No serial primary-endpoint trajectory is reported in the record. The secondary outcome list includes other antibody summaries and threshold-based measures, which can complement a GMT by addressing different features of the immune response. Such endpoints answer different questions and should be interpreted separately.

What does randomization contribute?

In general, random assignment aims to make treatment groups comparable, on average, with respect to both measured and unmeasured baseline characteristics. That supports causal comparisons when allocation is maintained, outcome measurement is appropriate, and analysis follows a suitable plan. Here the registry labels the study randomized, but the sequence-generation method, allocation concealment, baseline characteristics, and handling of deviations are not reported in the registry record. Randomization is therefore an important design feature, but it does not alone establish that every potential source of bias was controlled.

What does single masking tell the reader?

The registry identifies the study as single-masked. In general, masking can reduce the risk that knowledge of assignment influences participant behavior, clinical care, or outcome assessment. The practical effect depends on who was masked and whether masking was maintained. The identity of the masked party and details of the masking procedures are not reported in the registry record, so the label cannot be interpreted more specifically.

How should the safety comparison be read?

For adverse events of specific interest, the posted secondary result reports a risk difference of 0.014, with an interval from -0.013 to 0.040 and p = 0.319. The interval includes zero, which represents no difference on the risk-difference scale. In general, an interval crossing zero is compatible with a range of effects in either direction; it is not proof that the groups are identical. The p-value is also not the probability that a treatment effect exists or that the null hypothesis is true. This is a secondary outcome, and the record does not state whether its analysis was adjusted for testing multiple outcomes.

The event counts for this comparison are 0 of 72 in the Placebo-Pandemrix-Fluarix Group and 1 of 73 in the Fluarix-Pandemrix-Placebo Group. The estimate is consequently based on very few events. In general, sparse outcomes can yield imprecise estimates and make statistical results sensitive to individual events and analysis choices. The reported interval reflects uncertainty in the comparison; it should not be interpreted as establishing equivalence or absence of a potentially meaningful difference.

The registry’s planning illustration is described as based on the observed effect, with a two-sided significance level of 0.05, equal allocation, and a normal approximation. It is explicitly a planning illustration, not the trial’s own design justification or observed power. Sample-size illustrations depend on assumptions about the effect to detect and other design choices. The registry record does not report the trial’s original sample-size justification, so the illustration should not be mistaken for evidence that the study was designed to detect a particular effect.

More generally, the registered record does not report an interim-analysis plan, the full statistical analysis plan, or the methods for addressing missing data. These omissions matter because analytic choices determine which participants contribute, how uncertainty is calculated, and how multiple outcomes are interpreted. The numerical results can still be described as posted, but their inferential context is incomplete.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists a primary immunogenicity endpoint and a broad set of secondary outcomes. Several secondary measures concern antibodies against more than one vaccine strain; others concern symptoms and adverse events. When many outcomes are assessed, there are multiple opportunities for apparently unusual results to occur by chance. In general, multiplicity procedures control or describe this risk by defining which hypotheses are confirmatory, adjusting significance thresholds, or using a planned hierarchy. The multiplicity strategy is not reported in the registry record.

The record identifies the primary endpoint as the GMT against the Pandemrix strain at Day 42, but it does not report a formal primary between-group test or contrast. It also does not report an interim-analysis plan or the statistical analysis plan. Without those details, it is not possible to determine how the trial prioritized endpoints, whether secondary tests were intended to be confirmatory, or how their p-values should be interpreted as a family of tests. The posted secondary comparison with p = 0.319 should therefore be read as an outcome-specific result, not as an overall assessment of safety.

For the primary endpoint, the analysis population is the According-To-Protocol cohort for immunogenicity, including evaluable subjects with available immunogenicity results. The record does not report the number or reasons for exclusions, the handling of missing titers, or whether a complementary analysis included all randomized participants. Missing outcome data can affect both precision and interpretation, especially if the probability of having a measured result differs between groups or relates to the response. No missing-data assumptions or sensitivity analyses are reported in the registry record.

These omissions do not invalidate the posted group summaries, but they limit assessment of how robustly those summaries represent the randomized groups and how confidently multiple endpoint results can be interpreted. The analysis population and the primary endpoint definition are reported; important details of the statistical plan and data handling are not reported in 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.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: osteoarthritis (2.7%), sciatica (2.7%), atrial fibrillation (1.4%), breast mass (1.4%), cerebrovascular accident (1.4%).

The registry reports serious adverse events affecting 11 of 72 at risk in the Placebo-Pandemrix-Fluarix Group and 14 of 73 at risk in the Fluarix-Pandemrix-Placebo Group. These are counts of subjects recorded with serious adverse events relative to the stated at-risk groups. The registry also lists serious-event terms with reported percentages: osteoarthritis and sciatica at 2.7% each, and atrial fibrillation, breast mass, cerebrovascular accident, and eyelid ptosis at 1.4% each.

These figures describe events recorded during the study; they do not establish that a product caused an event. The terms are varied, and the registry record does not provide a causal analysis or a comparative statistical test for serious adverse events. The percentages should be read as the registry’s reported values, not recalculated from the counts. Deaths are not reported in the registry record.

In general, serious adverse event counts are important safety observations, but they do not alone quantify comparative risk with precision or establish causality. Interpretation depends on event definitions, follow-up, attribution procedures, and the completeness of reporting. The available record does not report all of those details. The counts should therefore be treated as a descriptive account of recorded serious events rather than a definitive comparison of safety between regimens.

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

The primary result means that the registry reports different group-specific GMT estimates for antibodies against A/Cal/7/09 at Day 42 in the According-To-Protocol immunogenicity cohort. The reported values are 309.8, with a 95% confidence interval from 246.4 to 389.6, for the Placebo-Pandemrix-Fluarix Group, and 227.5, with a 95% confidence interval from 181.5 to 285.3, for the Fluarix-Pandemrix-Placebo Group. The intervals describe uncertainty around each group estimate.

Those figures do not by themselves answer whether the groups differ statistically or clinically. The registry does not report a direct primary-endpoint contrast, its confidence interval, or a test. Separate group estimates are not the same as an estimated difference or ratio. They also do not demonstrate that any antibody level prevents infection, reduces illness, or predicts an individual participant’s outcome. Such conclusions are not established by a GMT alone.

The result is specific to one strain and one assessment time. It does not show how titers changed over time, how long any response persisted, or how responses compared for every strain listed among the secondary outcomes. The primary analysis is restricted to the stated evaluable cohort with available immunogenicity results. The record does not report enough information about exclusions and missing values to determine how those features might affect generalization to all enrolled or randomized participants.

Safety counts provide a separate descriptive picture. Serious adverse events were recorded in both groups, and several serious-event terms are listed. Those observations do not establish that the vaccine caused the events, and they do not provide a definitive comparative safety estimate. The record does not report deaths. The secondary specific-interest comparison is uncertain and should not be interpreted as proof of no difference.

Overall, the posted results describe immunogenicity and safety observations within this trial’s groups. They are not a complete basis for a clinical recommendation or a claim about effectiveness. A full assessment would require direct comparative analyses, transparent methods for missing data and multiplicity, and additional evidence beyond these registry summaries.

13. Why This Trial Matters Statistically

This trial illustrates why the form of a reported endpoint matters as much as its numerical value. A GMT is a useful summary of antibody levels, but it is a group-level measure on a particular scale. Two GMTs can describe the observed groups without providing the estimate or uncertainty for a direct comparison. Readers should distinguish descriptive summaries from inferential contrasts, especially when the study question is comparative.

It also demonstrates the importance of knowing the analysis population. The primary result uses an According-To-Protocol immunogenicity cohort, not simply the enrollment count. The distinction between enrollment and the group counts contributing to an endpoint is essential when interpreting precision and generalizability. Without details on exclusions and missing outcomes, the reader cannot fully assess how the analysis cohort relates to the randomized groups.

The study’s secondary outcomes span immune responses and safety. Such breadth can provide a richer view of the outcomes collected, but it also raises questions about multiplicity and endpoint prioritization. A clearly reported hierarchy and multiplicity plan helps readers separate confirmatory findings from exploratory observations. Those statistical-plan details are not reported in the registry record, making restraint important when considering isolated secondary results.

Finally, safety results show why counts and causal conclusions must not be conflated. Recorded serious events are relevant observations, but attribution, follow-up, and comparative uncertainty matter. Reporting both the counts and the limits of what they establish supports a more careful reading than treating event listings as proof of harm or proof of safety. The record is useful for identifying the trial design and the reported summaries, while also showing how incomplete analysis details can constrain interpretation.

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 registry record identifies the design as randomized, single-masked, and parallel-group, but does not report the randomization procedure, allocation concealment, or which party was masked. It also does not give enough information to assess baseline comparability. These details would help readers judge the safeguards supporting a fair group comparison.

The primary outcome is reported as separate GMTs with confidence intervals, without a primary between-group estimate or test. Consequently, the comparative effect and its uncertainty cannot be read directly from the reported primary result. The methods used to calculate the GMTs and intervals are not reported in the registry record.

The primary analysis is limited to the According-To-Protocol immunogenicity cohort of evaluable subjects with available results. The record does not report detailed exclusion counts, reasons for exclusion, missing-data handling, or a complementary all-randomized analysis. This limits assessment of possible selection effects and the extent to which the analyzed participants represent everyone assigned to a group.

The trial includes multiple secondary immunogenicity and safety outcomes, but the multiplicity strategy and full statistical analysis plan are not reported. The registry also does not report a sample-size justification or interim-analysis plan. The posted planning illustration is based on the observed effect and is not evidence of the trial’s original design rationale or its observed power.

The arm description for the Placebo-Pandemrix-Fluarix Group is truncated, so its complete schedule is not available in the registry record. The record also does not report deaths, and serious-event counts do not establish causality. Finally, the primary endpoint is an antibody measure at Day 42; it does not itself report clinical outcomes, longer-term persistence, or protection from illness.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Evaluation of the Immune Response and the Safety of a Pandemic Influenza Candidate Vaccine (H1N1) — NCT00971425.
• Registry record last updated 2018-08-17; status last verified 2017-01.
• Results first posted 2011-03-04.
• 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

NCT00971425 is a completed phase 3, randomized, single-masked, parallel-group study sponsored by GlaxoSmithKline. The registered condition is influenza, and enrollment is 145. The study began on 2009-09-08, with primary completion on 2009-11-16. Its stated objective was to evaluate the immunogenicity and safety of the investigational vaccine GSK2340272A.

The primary outcome is the GMT of antibodies against the Pandemrix strain, A/Cal/7/09, at Day 42. The registry reports group-specific estimates and 95% confidence intervals in the According-To-Protocol immunogenicity cohort. Secondary outcomes address antibody responses and safety, including adverse events. The record does not report a formal primary GMT comparison, the full statistical analysis plan, or a multiplicity strategy.