Safety and Immunogenicity of Two Trivalent Subunit Influenza Vaccines in Healthy Adult Subjects: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Safety and Immunogenicity of Two Trivalent Subunit Influenza Vaccines in Healthy Adult Subjects (NCT02256488), a randomized phase 3 study. The primary outcome is Immunologic Equivalence of 3 Consecutive Influenza Vaccine (TIVc) Production Lots..

2. Clinical Question

The study was designed to evaluate whether three consecutive lots of a cell-based vaccine produced comparable immune-response measurements. The primary outcome focuses on antibody titers measured with a hemagglutination inhibition antigen assay after vaccination. This is a laboratory measure of the immune response, rather than a clinical endpoint such as influenza illness.

The record also describes secondary assessments comparing immunogenicity and safety of TIVc and TIVf. These include the percentage of subjects achieving HI seroconversion and an HI titer of at least 1:40 against each strain, as well as reported solicited local and systemic adverse events and unsolicited adverse events. These are distinct questions from whether the three TIVc lots are equivalent to one another.

The registry record does not report the primary GMT estimates, their confidence intervals, or a conclusion about equivalence. It therefore documents the question and outcome definition, but not enough primary-result information to determine how similar the lot-specific responses were.

3. Trial Design

This was a Phase III, randomized, double-blind, controlled, parallel-group study. The official title describes it as multicenter. The record lists 1561 enrolled participants and a completed status. The study start is recorded as 2014-09, and primary completion as 2014-12.

Participants aged 18 through 49 years received one vaccination in one of four groups: TIVc Lot A, TIVc Lot B, TIVc Lot C, or the egg-based control vaccine TIVf. The main design feature is the comparison among the three consecutive TIVc production lots. TIVf serves as a control for the broader evaluation of immunogenicity and safety. The record does not report allocation counts for each vaccination group or describe the randomization procedure in further detail.

Double masking means that treatment assignment was concealed from specified study participants and personnel, but the registry record does not identify which roles were masked or describe how masking was maintained. Randomization helps reduce systematic differences between groups, while masking can limit bias in care, reporting, and outcome assessment. These design features do not, by themselves, establish that groups were comparable in every respect or that outcome measurement was free of bias.

The brief description states that participation involved one vaccination, two clinic visits, three reminder calls, and two blood draws, with total participation of about three weeks per subject. The primary immunogenicity assessment was scheduled for Day 22. The record does not provide further details about visit procedures, the timing of each safety assessment, or the handling of participants who did not complete follow-up.

The safety results use pooled TIVc and TIVf groupings, whereas the primary question concerns the three separate TIVc lots. The record does not explain the lot-specific enrollment or how pooling for the reported safety summaries was determined. Although the total enrollment is 1561, the safety summary lists 1560 as at risk for its total group; the registry record does not explain this difference.

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: Per Protocol Set(PPS) All subjects in the FAS (Full Analysis Set) Immunogenicity Population who were not excluded due to reasons defined prior to unblinding or analysis.

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 Immunologic Equivalence of 3 Consecutive Influenza Vaccine (TIVc) Production Lots., measured over Day 22.

The primary endpoint is the hemagglutination inhibition geometric mean titer against each of the three vaccine strains, assessed on Day 22 after one vaccination. Titers are laboratory measurements of antibodies that inhibit influenza hemagglutination. The endpoint is expressed as a geometric mean because antibody titer measurements commonly have a skewed distribution; analyzing or summarizing them on a logarithmic scale can make comparisons more suitable than relying on an arithmetic mean. This is a general statistical rationale, not a statement about the exact analysis used in this trial, which is not reported in the registry record.

The endpoint is used to assess immunogenicity: whether vaccination is followed by measurable antibody responses. For this study, the primary purpose is not simply to establish that titers are detectable, but to evaluate immunologic equivalence among three TIVc production lots. Equivalence is a specific statistical claim. It ordinarily requires defining acceptable bounds for the difference between treatments and showing that uncertainty around the comparison is contained within those bounds. The registry record does not report the equivalence margins, the lot-specific estimates, or the confidence intervals needed to assess that criterion.

The registry record specifies a 95% confidence interval as the dispersion measure. In general, a confidence interval conveys the uncertainty associated with an estimate; a narrower interval indicates greater precision, while a wider interval indicates less precision. The interval alone does not establish equivalence unless it is interpreted against prespecified equivalence bounds.

The primary endpoint is measured in the per-protocol set described in the record. That population consists of subjects in the full analysis set for immunogenicity who were not excluded for reasons defined before unblinding or analysis. Details of the exclusions and the resulting analysis population are not 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?

A geometric mean summarizes measurements that are positive and may vary multiplicatively. In general, the values are transformed to a logarithmic scale, averaged, and transformed back. This approach is often useful for antibody titers because a small number of high measurements can make an arithmetic mean unrepresentative of the typical response. The endpoint in this study is explicitly a geometric mean titer; the exact transformation and calculation details are not reported in the registry record.

A geometric mean is a summary of a group, not a description of every participant's response. It does not show the spread of individual titers or the proportion meeting a threshold. Those are different features, and the listed secondary outcomes include threshold-based measures such as HI titer of at least 1:40 and seroconversion.

How should the confidence interval be understood?

The primary outcome specifies a 95% confidence interval. In general, a confidence interval represents uncertainty in an estimated quantity under the assumptions of the statistical method. It is not a guarantee that the true value lies within the particular interval, and it does not describe the range of individual participant responses. A narrow interval generally indicates a more precise estimate than a wide interval.

For an equivalence question, precision must be considered in relation to the equivalence margins. The margins define the largest differences that would still count as acceptably small for the question being studied. The confidence interval for the relevant comparison must be evaluated against those bounds. Neither the margins nor the comparison intervals are reported in the registry record, so the equivalence assessment cannot be reconstructed from the endpoint description alone.

What does randomization contribute?

Random allocation is intended to make treatment groups comparable, on average, with respect to factors that could affect outcomes. It can reduce selection bias in group assignment. It does not guarantee identical groups, prevent missing outcomes, or correct problems in how outcomes are measured. The record identifies the trial as randomized but does not report the random sequence generation method, allocation concealment procedure, or group assignment counts.

Double masking can reduce the risk that knowledge of assignment affects participant reports or study personnel's assessment and management. Its actual protection depends on which people were masked and whether masking was maintained. Those operational details are not reported in the registry record.

Why distinguish equivalence from difference testing?

In general, a conventional test for a difference asks whether the data provide evidence against equality. Failure to detect a difference does not demonstrate that two treatments are sufficiently similar. Equivalence testing instead asks whether the possible difference is confined to a prespecified acceptable range. This distinction is important for a study whose stated primary objective is immunologic equivalence across production lots.

The registry record does not report the statistical test, the estimand, the comparison structure, or the equivalence margins. It also does not give the primary results. No conclusion about passing or failing an equivalence criterion can be drawn from the design description or the confidence-interval label alone.

What can the safety counts tell a reader?

The safety summary gives counts of subjects with serious and other adverse events, with separate at-risk counts for pooled TIVc, TIVf, and a total group. These counts describe events recorded during the study according to the registry summary. They are not automatically estimates of treatment-caused harm. A causal interpretation would require consideration of event definitions, timing, background occurrence, follow-up, and comparative analysis. Those details are not fully reported in the registry record.

The term percentages listed for serious events are also descriptive. The registry record gives APPENDICITIS and ABORTION SPONTANEOUS, each at 0.3%, but does not provide the context needed to connect these terms to a treatment effect. In general, rare events are difficult to compare precisely, especially when the event count is small. The record does not report confidence intervals for the safety counts or a formal between-group analysis.

What is not available for checking the analysis?

The registry record identifies the primary population as a per-protocol set within the full analysis set for immunogenicity, excluding subjects for prespecified reasons. It does not list the exclusion reasons, the number analyzed, or how missing Day 22 measurements were handled. The detailed statistical analysis plan, exact model, adjustment factors, and missing-data procedures are not reported in the registry record.

The record lists four outcome measures and three analyses, but does not supply enough detail to reconstruct how each analysis relates to the primary and secondary questions. It also does not report interim analyses or a sample-size justification. General statistical methods can explain what these elements would ordinarily address, but their use in this trial cannot be assumed.

10. Multiplicity, Missing Data and Analysis Populations

The study includes a primary outcome assessed against each of three vaccine strains, comparisons among three consecutive TIVc lots, and secondary outcomes involving immunogenicity and adverse events. Testing multiple outcomes or contrasts can increase the chance of at least one apparently notable result arising by chance. In general, a statistical plan may address this through a prespecified testing hierarchy or another multiplicity procedure. The registry record does not report whether any adjustment or hierarchy was used, so the interpretation of any unreported secondary findings cannot be assessed on that basis.

The primary outcome description identifies a per-protocol set drawn from the full analysis set for immunogenicity. It says subjects were excluded for reasons defined before unblinding or analysis, but does not list those reasons or give the number included in the primary analysis. The record does not report whether an intention-to-treat analysis or other supportive population was used for the lot-equivalence question.

The handling of missing blood draws, unavailable titers, protocol deviations, and incomplete follow-up is not reported in the registry record. In general, missing-data methods matter because the results can depend on whether participants with missing measurements differ systematically from those with observed measurements. For an equivalence objective, analysis-population choices can also affect the interpretation, so reporting both exclusions and sensitivity analyses can be informative.

The record lists 1561 enrolled participants, while the safety summary lists 1560 at risk in its total group. It does not explain the difference. The registry record also does not report whether participants were analyzed according to randomized assignment, vaccination received, or another rule for the safety summaries. No sample-size justification or detailed multiplicity plan is reported.

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: appendicitis (0.3%), abortion spontaneous (0.3%).

The registry safety summary reports 0 serious adverse events among 1169 subjects at risk in the pooled TIVc group and 2 among 391 subjects at risk in the TIVf group. For other adverse events, it reports 709 subjects affected in pooled TIVc and 237 in TIVf. These are counts in the registry record, not estimates of the probability that vaccination caused an event.

The summary also lists APPENDICITIS and ABORTION SPONTANEOUS as top serious-event terms, each at 0.3%. The record does not give additional case descriptions or explain how the term-specific percentages relate to the group-level counts. Deaths are not reported in the registry record.

The total safety group is listed as 1560 at risk. The enrollment figure is 1561, and the registry record does not explain why those figures differ. The record also does not report confidence intervals or a formal comparison for the adverse-event counts. Descriptive differences should therefore not be interpreted as evidence of a causal difference between vaccines.

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

The registry record establishes that the study was designed to compare immune responses across three consecutive lots of a cell-based trivalent influenza vaccine and to assess immunogenicity and safety in relation to an egg-based vaccine. It specifies a Day 22 HI geometric mean titer endpoint and a per-protocol analysis population. These details clarify the planned question and the type of measurement used.

The record does not report the primary lot-specific GMTs, their confidence intervals, the equivalence margins, or the conclusion of the equivalence analysis. It therefore does not show whether the lots met the study's stated equivalence objective, nor how precisely any estimated differences were measured. A study description is not itself evidence that its primary objective was achieved.

The secondary outcomes include seroconversion, an HI titer threshold, and adverse-event summaries. The registry record does not provide the secondary immunogenicity results. Its safety counts are descriptive and do not establish that a reported event was caused by vaccination or that one vaccine has a different safety profile from the other.

More generally, a laboratory immune-response endpoint is not the same as a direct measure of prevention of influenza illness. The study's primary endpoint concerns antibody titers, and the registry record does not report a clinical influenza outcome. Any broader inference would require evidence beyond the endpoint description and safety counts listed in this record.

The appropriate reading is therefore limited: this is a randomized, double-blind Phase III study with a stated lot-equivalence objective and defined immunogenicity and safety assessments, but the record does not include the results needed to determine the primary finding. The absence of those details is not evidence for either equivalence or non-equivalence.

13. Why This Trial Matters Statistically

Production consistency is an important statistical question for vaccines: a comparison of consecutive lots asks whether the measured immune response is sufficiently similar across manufacturing batches. Unlike a simple search for any difference, an equivalence objective requires both a meaningful bound for acceptable differences and estimates precise enough to evaluate that bound. This trial's primary endpoint and objective are framed around that type of question.

The study also illustrates why endpoint definition and result reporting need to be read together. A geometric mean titer and a confidence interval identify the form of the summary and an uncertainty measure, but they do not reveal the size or precision of the observed responses without the actual estimates. For equivalence, the margins and analysis method are also central. These details are not reported in the registry record.

The randomized, double-blind design is relevant to interpretation because it can reduce some sources of bias in assignment and assessment. However, design labels alone cannot establish the adequacy of the analysis or resolve missing-data and multiplicity questions. The record does not describe those methods in enough detail for an independent reconstruction.

The safety summary offers a separate example of the difference between descriptive and inferential evidence. Counts of serious and other adverse events document what was recorded in the registry summary, but do not provide causal attribution or the precision of comparative estimates. The pooled safety grouping also differs from the lot-specific structure of the primary question.

Statistically, the trial matters as an example of how a production-lot consistency question differs from a conventional superiority question. It also shows why complete reporting should include estimates, confidence intervals, equivalence margins, analysis populations, and missing-data methods. Without these, the stated objective can be identified, but its statistical resolution cannot be judged from the registry record.

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 main limitation for interpreting this record is that the primary outcome results are not reported. There are no lot-specific GMT estimates, confidence intervals, equivalence bounds, or reported conclusion. The registry entry therefore cannot establish whether the primary objective was met or how precisely the lot comparisons were estimated.

The analysis methods are also incompletely described. The record identifies a per-protocol population but does not state the number analyzed, list the exclusions, or explain how missing Day 22 titers were handled. The statistical model, comparison procedure, multiplicity approach, and any sensitivity analyses are not reported in the registry record. The reason for the stated sample size and any interim analyses are also not reported.

The secondary immunogenicity outcomes are named but their results are not included in the record. Safety reporting is limited to group-level counts and selected serious-event terms. The record does not provide event timing, case-level context, confidence intervals, or a formal comparison. Causal attribution cannot be determined from these counts alone.

The safety summary pools the three TIVc lots, so it does not show lot-specific adverse-event results. The total safety group is listed as 1560 at risk, compared with 1561 enrolled, and the registry record does not explain the difference. The masking roles, randomization procedure, allocation counts, and full details of follow-up are also not reported. These limitations restrict assessment of the analysis and its precision; they do not establish that the trial itself was inadequately conducted.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Safety and Immunogenicity of Two Trivalent Subunit Influenza Vaccines in Healthy Adult Subjects — NCT02256488.
• Registry record last updated 2019-06-11; status last verified 2019-05.
• Results first posted 2015-08-07.
• 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

NCT02256488 is a completed, randomized, double-blind, parallel-group Phase III study described as multicenter. It enrolled 1561 healthy adult subjects aged 18 through 49 years and compared three consecutive lots of a cell-based trivalent subunit influenza vaccine with an egg-based trivalent subunit control vaccine.

The primary objective was immunologic equivalence among the three TIVc lots, assessed using HI geometric mean titers against each of three strains on Day 22. The record specifies a 95% confidence interval and a per-protocol population within the full analysis set for immunogenicity. It also lists secondary immunogenicity and adverse-event outcomes.

The registry record does not report the primary estimates, confidence intervals, equivalence margins, or conclusion. Safety summaries report 0 serious events in pooled TIVc and 2 in TIVf, with other adverse events reported for both groups. These summaries are descriptive and do not establish causation or a comparative effect.