Safety and Immunogenicity of Two Doses of H5N1 Influenza Vaccine in Healthy Adults: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Safety and Immunogenicity of Two Doses of H5N1 Influenza Vaccine in Healthy Adults (NCT01776541), a randomized phase 2 study. The primary outcome is Percentages Of Subjects Achieving Hemagglutinin Inhibition (HI) Titers ≥40 Against A/H5N1 Strain..

2. Clinical Question

The clinical question combines an immunogenicity measure with a prespecified regulatory benchmark described in the registry. It is not simply a question of whether the high dose produced a larger response than the low dose: the primary-outcome description evaluates the percentage achieving the HI threshold in each group against a criterion. The registry also lists geometric mean ratios, the percentage with HI titers of at least 40, and seroconversion among secondary outcomes. Arm-specific primary outcome percentages and confidence intervals are not reported in the registry record, so the comparison and the criterion cannot be evaluated from the posted result information.

3. Trial Design

The registry describes NCT01776541 as a randomized, double-masked, parallel Phase II study. Its official title characterizes it as observer-blind and multicenter. It enrolled 979 healthy adults and compared an adjuvanted cell culture-derived H5N1 vaccine in high-dose and low-dose formulations. Participants in both groups were scheduled to receive 2 injections three weeks apart. The record identifies Novartis Vaccines as the sponsor.

The study began in 2013-01, had primary completion in 2013-06, and is listed as completed. These dates describe the registry timeline; they do not establish when individual participants were vaccinated or when analyses were conducted. The primary outcome was assessed three weeks after the second vaccination, at day 43. The registry’s primary-outcome population is the Full Analysis Set, defined as participants who received at least one dose and had evaluable serum samples both at baseline and after vaccination.

The listed arms differ by vaccine dose. The record does not identify a placebo or an unvaccinated comparison arm, nor does it state the allocation ratio, randomization procedure, concealment method, or masking procedures in detail. Those details are not reported in the registry record. The completed status and enrollment total are design and conduct descriptors, not evidence about the immune response or safety findings. Although the registry lists outcome measures, it records 0 analysis entries; specific statistical comparisons and their numerical results are therefore not available in the posted record.

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: All enrolled subjects were included in the trial.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Analysis was done on the Full Analysis Set (FAS) i.e., the subjects who actually receive at least one dose of study vaccination and provide at least one evaluable serum sample both before (baseline) and after vaccination.

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 Percentages Of Subjects Achieving Hemagglutinin Inhibition (HI) Titers ≥40 Against A/H5N1 Strain., measured over Three weeks after 2nd vaccination (day 43).

The primary endpoint is the percentage of participants with an HI titer of at least 40 against the homologous A/H5N1 strain, measured three weeks after the second vaccination, at day 43. HI testing measures inhibition of the interaction between influenza virus and red blood cells. In general, an HI titer is a laboratory marker of antibody response; a threshold-based endpoint classifies participants according to whether their measured titer reaches a specified level. It does not directly count clinical cases of influenza.

The registry states that the endpoint was used to assess the vaccine formulation under the CBER criterion for adults. That criterion is met when the lower limit of the two-sided 95% confidence interval for the percentage achieving an HI titer of at least 40 meets or exceeds 70%. This criterion therefore considers both the estimated percentage and its uncertainty. A percentage alone would not establish whether the stated criterion was met; the confidence interval is also needed.

The registry describes measurement after either dose formulation, but arm-specific endpoint estimates and confidence intervals are not reported in the registry record. The record consequently does not establish whether either group met the criterion or how the groups compared on this endpoint.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry identifies a percentage as the primary endpoint and gives a confidence-interval criterion for interpreting it. The corresponding statistical ideas can be explained without assuming that a particular analysis was performed. The registry record does not report a detailed statistical analysis plan, numerical primary results, or analysis entries.

What does a percentage endpoint represent?

For this endpoint, each participant is classified according to whether the measured HI titer reaches the threshold of at least 40. The percentage is the proportion of eligible participants in an arm who meet that definition. In general, a proportion is an intuitive summary of how common a binary outcome is in the analyzed group. Its interpretation depends on a clear numerator and denominator. The registry defines the Full Analysis Set, but the number analyzed in each arm for the primary outcome is not reported in the registry record.

A threshold endpoint simplifies a continuous laboratory measurement into a yes-or-no classification. This makes it possible to describe the share of participants who attain the specified response level. The simplification also means that values on the same side of the threshold are treated alike for this endpoint, even if their measured titers differ. The registry separately lists geometric mean ratios, which address antibody response on a different scale, and seroconversion, another defined immunogenicity outcome. Their observed results are not reported in the registry record.

Why is a confidence interval relevant?

In general, a confidence interval communicates sampling uncertainty around an estimate. A percentage calculated from a study group is an estimate of a broader underlying response probability, not a perfectly precise value. The interval’s width is influenced by the amount of information and by the observed distribution, among other factors. The registry specifies a two-sided 95% confidence interval and makes the lower limit central to the CBER criterion. Without the interval limits, the stated regulatory comparison cannot be reproduced or judged from the posted record.

The criterion is not equivalent to asking whether the point estimate is at least 70%. It requires the lower confidence limit to meet or exceed 70%. In general, using a lower confidence limit incorporates uncertainty conservatively: a result with a high point estimate but substantial uncertainty may have a lower limit that does not reach the benchmark. The actual confidence limits are not reported in the registry record, so no determination about the criterion can be made from the available result information.

What does randomization contribute?

In general, random allocation is intended to make treatment groups comparable, on average, with respect to characteristics that could influence outcomes. It supports interpretation of a between-group contrast, subject to the conduct of randomization, follow-up, measurement, and analysis. The registry labels the allocation randomized and the design parallel. It does not report the allocation ratio, sequence generation, concealment, baseline characteristics, or an adjusted analysis. The randomized label alone cannot establish that all sources of bias were avoided.

Masking can reduce the risk that knowledge of assigned group affects participant behavior, outcome assessment, or other aspects of trial conduct. The registry classifies masking as double and describes the study as observer-blind in its official title. The specific parties masked and the practical procedures are not reported in the registry record. This distinction matters because a broad masking label does not show how masking was implemented or maintained.

What can the planning assumptions tell a reader?

The registry’s sample-size assumptions describe a planning illustration based on the observed effect, with a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The illustrated effect to detect is a stated fraction of the observed effect. For non-time-to-event outcomes, the planning quantity is participants per arm. These are planning assumptions, not evidence about the original trial’s design rationale. The trial’s sample-size justification is not reported in the registry record.

In general, a sample-size calculation asks what amount of information would be needed to detect a specified effect under stated assumptions. Its result depends on the effect size, variability or event probability, error rate, allocation, and analysis method. A calculation that uses an effect observed in a completed study is retrospective planning context; it is not observed power and does not establish that the original study was adequately planned. No numerical planning values are available in the registry entries described for this analysis, and no additional values should be inferred.

The normal approximation is a mathematical approach for estimating uncertainty or planning sample size. Its suitability depends on the data and assumptions. The registry’s stated method does not, by itself, show whether the approximation was checked, whether an alternative method was used for the primary analysis, or whether any adjustment was made for covariates. These details are not reported in the registry record.

What analysis results are absent?

The record lists outcome measures but reports 0 analysis entries. It does not give the arm-specific primary percentages, confidence intervals, analyzed denominators, between-arm contrast, or inferential test. It also does not report a detailed model, handling of missing observations, or interim-analysis procedures. These absences limit statistical interpretation independently of the randomized design. A reader can identify the intended endpoint and benchmark, but cannot reconstruct the primary analysis or evaluate its numerical result from the registry record.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 7 outcome measures, including the primary HI threshold outcome and secondary measures of geometric mean ratios, HI titers of at least 40, and seroconversion. When a study evaluates multiple outcomes, time points, groups, or analyses, the chance of obtaining at least one apparently unusual result can increase. In general, multiplicity procedures address that issue by defining a testing hierarchy, adjusting significance thresholds, or distinguishing confirmatory outcomes from exploratory ones.

The primary outcome description identifies a specific time point, day 43, and a CBER confidence-interval criterion. However, a complete account of planned hypothesis testing, outcome hierarchy, multiplicity adjustment, or interpretation of secondary outcomes is not reported in the registry record. It is not appropriate to infer from the listed outcomes that adjustments were or were not made in an analysis that is not posted. The record lists 0 analysis entries, and no p-values or inferential results for the outcomes are reported.

The Full Analysis Set is defined as participants who received at least one dose and provided at least one evaluable serum sample both before and after vaccination. That definition identifies an analysis population, but it does not describe how missing samples were handled, whether participants were excluded after enrollment, or how many contributed to each outcome. Detailed participant flow, reasons for missing data, imputation methods, and sensitivity analyses are not reported in the registry record.

These omissions matter because missing outcome measurements can affect both the denominator and the interpretation of a percentage. In general, an analysis based only on participants with evaluable samples may be affected if missingness is related to response or group assignment. The registry definition alone does not establish whether such concerns arose. Interim analyses, stopping rules, and the statistical plan are also 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: pneumonia (0.4%), sepsis (0.4%), nephrolithiasis (0.4%), myocardial infarction (0.2%), pancreatitis (0.2%).

The registry reports serious adverse event counts for the high-dose and low-dose groups, alongside counts for other adverse events. The high-dose group lists 20 serious adverse event-affected participants among 485 at risk; the low-dose group lists 8 among 490 at risk. The total is 28 among 975 at risk. For other adverse events, the record lists 371 among 485 in the high-dose group, 324 among 490 in the low-dose group, and 695 among 975 overall.

These are descriptive counts in the registry record. They do not establish that events were caused by vaccination, and the record does not report a statistical comparison or event-specific time course. The listed serious-event terms include PNEUMONIA, SEPSIS, and NEPHROLITHIASIS at 0.4% each, and MYOCARDIAL INFARCTION, PANCREATITIS, and CHOLECYSTITIS at 0.2% each. The registry does not state whether these terms represent all serious events or provide enough detail to interpret their clinical context.

Death counts are not reported in the registry record. The at-risk totals for the safety groups should not be assumed to equal the primary immunogenicity analysis denominators, which are not reported. These safety figures are useful as a record of reported affected participants, but they do not by themselves establish comparative risk or causality.

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

The posted record establishes the study’s question and design: a randomized Phase II comparison of high- and low-dose adjuvanted H5N1 vaccine in healthy adults, with an HI threshold outcome measured at day 43. It also states the CBER adult benchmark, which depends on the lower limit of a two-sided 95% confidence interval. These details describe what investigators intended to assess, not the result of that assessment.

Arm-specific percentages, confidence intervals, and a high-dose versus low-dose estimate are not reported in the registry record. Consequently, the record does not show whether either dose group met the stated criterion, how responses differed, or how precise the estimates were. The absence of posted analyses should not be interpreted as evidence that there was no effect, nor as evidence that there was an effect. It means the numerical outcome needed to answer the primary question is unavailable in this record.

In general, an immunogenicity endpoint is a laboratory measure and should be distinguished from a clinical outcome such as illness. The primary endpoint records whether a titer reaches a defined threshold; it does not directly report influenza cases or demonstrate a clinical benefit. The registry also provides safety counts, but does not establish causality or a comparative safety effect. Neither the immune-response descriptions nor the adverse-event counts support medical advice.

The interpretation is therefore bounded. The trial is identifiable as a completed randomized study with 979 enrolled participants, two dose groups, and a defined primary measurement. The registry record does not report the outcome estimates, analysis details, or missing-data information needed to quantify efficacy, precision, or group differences. Conclusions beyond those design and reporting details would go beyond the posted evidence.

13. Why This Trial Matters Statistically

This trial illustrates why an endpoint definition and its analysis results are both needed for statistical interpretation. The registry gives a clear primary measure: the proportion reaching an HI titer of at least 40 against A/H5N1 at day 43. It also gives a benchmark that depends on the lower limit of a confidence interval. That combination distinguishes an observed percentage from a conclusion about whether the estimate is sufficiently precise relative to a criterion.

The randomized, parallel design provides a framework for comparing two dose formulations, while the double-masked classification and observer-blind description signal efforts to limit bias. Those design features are important, but they do not reveal the response estimates or guarantee that the analysis was free of bias. The registry’s lack of posted analysis entries makes the difference between planned measurement and interpretable results especially visible.

The study also shows why population definitions matter. The primary analysis is described for the Full Analysis Set, whose membership depends on vaccination and evaluable pre- and post-vaccination serum samples. Without analyzed denominators and missing-data details, readers cannot fully assess how the analysis population relates to the 979 enrolled participants.

More broadly, a registry record can document the clinical question, intended endpoint, timing, and some safety information without supplying enough numerical outcome evidence to answer the question. Statistical interpretation requires estimates, uncertainty, denominators, and transparent analysis methods—not only a trial design or an endpoint threshold. In this case, the distinction is consequential: the registry states the decision rule but does not report the values required to apply it.

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 principal limitation for interpreting this trial from the registry record is the absence of numerical primary immunogenicity results. Arm-specific percentages and confidence intervals are not reported, so the stated CBER criterion cannot be assessed. A comparative estimate between high and low dose is also not reported. The registry lists 0 analysis entries, which leaves the inferential analysis unavailable in the posted record.

Important analysis details are also absent. The record defines the Full Analysis Set but does not report the number analyzed in each group, the distribution of missing serum samples, reasons for missingness, or any handling of missing outcomes. The randomization ratio, sequence generation, concealment procedures, masking details, baseline characteristics, and covariate adjustment are not reported in the registry record. Nor is a detailed statistical analysis plan, including multiplicity handling or interim-analysis procedures.

Safety information is limited to affected-participant counts and selected serious-event terms. Event timing, duration, severity detail, attribution, and statistical comparisons are not reported. Death counts are not reported in the registry record. The counts therefore cannot establish causality or support a complete comparison of safety between dose groups.

The enrollment total of 979 does not identify the denominators used for each outcome. It should not be substituted for the primary analysis population or treated as evidence of precision. The registry status is completed, but completion does not resolve the missing results or analysis details. These limitations concern what can be established from the registry record; they do not show that the study lacked additional analyses outside that record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Safety and Immunogenicity of Two Doses of H5N1 Influenza Vaccine in Healthy Adults — NCT01776541.
• Registry record last updated 2015-02-03; status last verified 2015-01.
• Results first posted 2015-02-03.
• 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

NCT01776541 is a completed randomized, double-masked, parallel Phase II study sponsored by Novartis Vaccines. It enrolled 979 healthy adults and compared high- and low-dose adjuvanted cell culture-derived H5N1 vaccine, with 2 injections three weeks apart. The primary endpoint was the percentage reaching an HI titer of at least 40 against A/H5N1 at day 43. The registry states a CBER criterion based on the lower limit of a two-sided 95% confidence interval meeting or exceeding 70%.

The record lists 7 outcome measures and 0 analysis entries. Arm-specific primary estimates, confidence intervals, analyzed denominators, and comparative results are not reported in the registry record. Safety counts are listed, but they do not establish causality or comparative risk. The record describes the intended study question and some safety information, while leaving the primary numerical result unavailable.