Immunogenicity of Influenza Vaccinations: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Immunogenicity of Influenza Vaccinations (NCT06518577), a randomized phase 4 study. The primary outcome is Percent of Participants With a Seroprotective HAI Titer (≥1:40).

2. Clinical Question

The research question is about immunogenicity: measured immune responses after vaccination. It is not a direct comparison of prevention of influenza illness. The study also included active surveillance for influenza-like illness, but the primary endpoint described in the record is antibody-based. Blood samples were intended to support measurement of immune biomarkers, including antibody responses; saliva collection was optional. Participants reporting illness could be asked for a respiratory swab and additional blood draws. Those procedures provide context for the study, but do not change what the stated primary endpoint measures.

3. Trial Design

NCT06518577 is a completed, randomized, phase 4, parallel-group study sponsored by Duke University. The registry lists planned enrollment of 606 and describes two approved influenza vaccines: Flucelvax® (ccIIV3) and Flublok® (RIV3). Participants were assigned to receive one vaccine at Visit 1. Masking is listed as none, so the study is described as unmasked. The registry record does not describe further details about how the random sequence was generated or how allocation was implemented.

The study began on 2024-09-09, with primary completion recorded as 2024-12-09. Its status is completed. These dates describe the registry’s study milestones; they do not by themselves establish when all analyses were conducted or when results became available.

Blood collection was planned at baseline (Visit 1; day 1), post-vaccination (Visit 2; day 29), and post-season (Visit 3; day 181). The primary endpoint specifies Visit 2 as days 28-42 after vaccination. The record says HAI was used for cell-grown A(H1N1)pdm09 and B/Victoria, while A(H3N2) titers were measured by microneutralization. The endpoint therefore involves antigen-specific measurements rather than a single undifferentiated immune marker.

Participants were also to receive weekly electronic surveys about changes in health and new influenza-like symptoms. For participants reporting illness, the study could request a respiratory swab for laboratory testing and additional blood draws under the conditions described in the record. The primary analysis population was narrower than everyone enrolled: it required vaccine receipt, eligible paired blood results, and no influenza infection between the specified visits. The registry record does not report the number included in that population or the number excluded from it.

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: 7 participants were not randomized to a vaccine group; 2 due to protocol violations and 5 were considered screen fails.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Immunogenicity Population: A subset of the mITT population that includes only participants who received the study vaccine, provided Visit 1 and Visit 2 blood draws available with HAI titer results for analysis within the protocol-defined time frame, did not have an influenza infection between Visits 1 and 2, and had no protocol violations affecting immunogenicity.

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.

Population: Intention-to-treat population: Any participant who was enrolled and randomized.

6. Primary Endpoint

The registry lists the primary outcome as Percent of Participants With a Seroprotective HAI Titer (≥1:40), measured over Visit 2 (Days 28-42, Post-vaccination).

The primary endpoint is the percentage of participants with a seroprotective HAI titer of at least 1:40 at Visit 2, described as days 28-42 after vaccination. The registry specifies a 95% confidence interval for this percentage. The measure is assessed for each influenza vaccine antigen, with HAI used for cell-grown A(H1N1)pdm09 and B/Victoria and microneutralization used for A(H3N2).

A threshold-based endpoint converts a measured antibody response into a binary classification: each eligible participant either meets the stated titer threshold or does not. The resulting percentage describes how common that classification was in the analyzed group. It is not the same as the average titer, and it does not show the full distribution of antibody measurements. The cutoff offers a consistent way to describe seroprotection, while the underlying antibody levels may still vary among participants who fall on the same side of it.

Immune-response measurements are used in immunogenicity studies to characterize antibody responses after vaccination. Such a measure is not itself a count of influenza cases, and it should not be read as a direct estimate of clinical protection. The registry record does not report the numerical percentage or confidence interval for any antigen or arm, so the magnitude and precision of the observed responses cannot be described from this record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a percentage endpoint represent?

The primary endpoint classifies participants according to whether their antibody titer meets the seroprotective threshold of at least 1:40. A percentage summarizes the share of the analyzed population meeting that criterion. This is a binary-outcome summary even though the underlying titer is measured on a scale. The percentage does not retain all information about the size of titers above or below the cutoff. Two groups could have similar percentages while having different distributions of measured titers, or different percentages while having overlapping distributions.

For interpretation, the denominator matters: the percentage pertains to those included in the defined analysis population, not automatically to everyone enrolled or vaccinated. Here, the immunogenicity population is a subset of the mITT population with specified vaccination, blood-draw, assay-result, timing, and infection-status criteria. The registry record does not give the number analyzed for each antigen or arm. It is therefore not possible to reconstruct the percentages or judge how much the analysis population differs from the enrolled cohort.

Why report a confidence interval?

In general, a confidence interval describes the precision of an estimate, taking account of sampling variability under a statistical model. For a percentage, it conveys uncertainty about the underlying proportion represented by the observed group. The interval is most useful when read alongside the estimate, the population definition, and the analysis method. A confidence interval is not a measure of the range of individual participants’ antibody titers, nor is it a substitute for an explicit comparison between groups.

The record specifies a 95% confidence interval, but does not report the interval limits or say which calculation method was used. It also does not report the numerical percentages. Thus, the confidence interval’s width and the precision of the estimated seroprotection percentages cannot be evaluated. The record does not state whether an interval for a between-group contrast was calculated, or how any such contrast would have been modeled.

Why are the assay methods relevant?

HAI and microneutralization are laboratory methods for measuring antibody response. The registry assigns HAI to cell-grown A(H1N1)pdm09 and B/Victoria and microneutralization to A(H3N2). This means that a result should be interpreted in relation to its antigen and assay; the endpoint is not based on one interchangeable measurement across all antigens. The record does not provide additional assay validation information or a method for combining measurements across antigens.

General statistical practice treats laboratory measurements according to their scale and distribution. Converting titers to a threshold-based outcome may make a percentage comparison easier to communicate, but it discards detail about the magnitude of values. If the analysis also reports continuous titers, those would answer a related but distinct question. The registry record lists the seroprotective percentage as the primary endpoint and does not provide numerical findings that would allow either type of result to be assessed.

What does randomization contribute?

Randomization is intended to balance measured and unmeasured participant characteristics between comparison groups on average, reducing confounding as a basis for group differences. It does not ensure identical groups in a particular study, and it does not remove the need to define the analysis population and handle missing measurements transparently. The record states randomized allocation and parallel groups, but does not report the randomization procedure, allocation concealment, or any balance results.

The registry lists no masking. In general, lack of masking can affect behaviors, symptom reporting, or other assessments influenced by knowledge of treatment assignment. The stated primary outcome relies on laboratory antibody measurements, but the record does not say whether specimen processing or laboratory assessment was masked. It would be inappropriate to assume either that the outcome measurement was affected or that masking occurred.

What can be learned from the reported statistical details?

The endpoint includes multiple antigen categories, and the registry notes that the primary outcome has several categories or time points rather than being reduced to one comparison. Multiple comparisons can increase the chance that at least one result appears notable by chance when each is interpreted separately. Statistical methods sometimes address this through a prespecified hierarchy or adjustment, or by clearly treating analyses as descriptive. The registry record does not report which approach, if any, was used.

The record lists 4 outcome measures and 15 analyses, but these counts do not reveal which tests were conducted, their hypotheses, or how their results were interpreted. The statistical analysis plan, including model choice, handling of missing data, confidence-interval calculation, and multiplicity strategy, is not reported in the registry record. General methodological explanations cannot fill in those study-specific details.

Planning calculations are conceptually different from trial findings. The described sample-size illustration assumes a two-sided significance level of 0.05, equal allocation, a normal approximation, and an effect defined as a stated fraction of the observed effect. Such calculations are planning illustrations, not observed power and not evidence about the endpoint result. The registry record does not report a sample-size justification for the trial itself. No conclusion about the trial’s design rationale or statistical power should be inferred from the illustration assumptions.

10. Multiplicity, Missing Data and Analysis Populations

The primary outcome is defined across several influenza antigens, and the registry record says it has several categories or time points rather than being reduced to one comparison. When multiple outcomes or comparisons are examined, interpreting each as if it were the only test can overstate the strength of evidence. A study may manage this issue with a prespecified hierarchy, multiplicity adjustment, or descriptive interpretation, but the registry record does not report a strategy. The number of listed analyses does not identify which comparisons were confirmatory or how they were handled.

The analysis population is specified as a subset of the mITT population. Inclusion requires receipt of study vaccine, Visit 1 and Visit 2 blood draws with HAI titer results within the protocol-defined time frame, and no influenza infection between visits. These rules define who contributes to the primary immunogenicity analysis, but the registry record does not report the number eligible, the number analyzed by arm or antigen, or the reasons and counts for exclusions.

Missing blood samples or unavailable assay results could affect which participants contribute to an endpoint percentage. In general, conclusions can depend on whether missingness is related to participant characteristics or outcomes. The registry record does not report a missing-data method, sensitivity analysis, or imputation approach. It also does not describe how participants with incomplete follow-up were handled for this endpoint.

The record does not report the statistical analysis plan, including the prespecified analysis model or the handling of multiplicity and missing data. These omissions limit assessment of whether the posted endpoint analyses followed a prospective plan and how robust results would be to alternative choices. Since numerical endpoint results are also not reported, neither the impact of missingness nor the effect of any multiplicity approach can be evaluated.

11. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent serious adverse event terms

Serious events with the highest arm-level rate: small cell lung cancer stage iv (0.3%), kidney stones (0.3%), whiplash (0.3%).

The registry lists serious adverse event counts for the two vaccine groups. For Flucelvax® (ccIIV3), 301 participants were at risk and 3 serious adverse events were reported; for Flublok® (RIV), 298 were at risk and 0 serious adverse events were reported. The record also lists 1 death in the Flucelvax® group and 0 in the Flublok® group. These are counts as recorded, not evidence by themselves that an event was caused by a vaccine.

The serious adverse event terms listed are Small cell lung cancer stage IV, Kidney stones, and Whiplash, each with a reported value of 0.3%. The record does not connect these terms to a particular vaccine arm in the listed information, and it does not state whether they were considered related to vaccination. The term list should not be interpreted as a complete description of every safety event unless the record says so.

Counts and percentages answer different questions. Counts indicate how many events were recorded; a percentage relates a count to a specified denominator. The registry does not provide enough information to calculate or compare any additional safety rates, and none should be inferred. Serious events and deaths are also distinct listings in the record; it does not establish whether any death overlapped with a serious adverse event. These figures are descriptive and require the registry’s definitions and follow-up context for fuller interpretation.

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

The study is designed to compare immune responses after two approved influenza vaccines in US adults aged 18-64. Its stated primary measure is the percentage meeting an antibody titer threshold for each influenza antigen at the post-vaccination visit. The record identifies the analysis population and the assays used. Those design details clarify what the endpoint is intended to describe, but they are not a substitute for results.

Numerical primary endpoint results are not reported in the registry record. As a result, the record does not establish whether the measured percentages differed between vaccine groups, how large any difference was, or how precise either estimate was. It also does not support a statement that either vaccine generated a higher or lower response. The endpoint description alone cannot show whether an observed difference would be statistically persuasive or clinically important.

Seroprotection is an antibody-based outcome. It should not be equated with a direct count of influenza infections, illness severity, hospitalization, or prevention of disease. Although the study used active surveillance for influenza-like illness and could collect swabs and additional blood samples after reported illness, the primary outcome is immunogenicity. The record does not report clinical illness comparisons that would allow those outcomes to be interpreted.

The randomized, parallel-group design provides a framework for comparing assigned vaccines, but the analysis applies to a defined subset with eligible post-vaccination blood results and no influenza infection between visits. The relationship between that subset and the full enrolled cohort cannot be assessed without analysis denominators and missing-data information. The unmasked design is also relevant context, while the record does not clarify whether laboratory assessment was masked.

Safety counts should be read as recorded events, not as causal conclusions. The registry lists serious adverse events and deaths by group, but does not report attribution in the information available. Overall, the study’s question and endpoint are clear, while the numerical immunogenicity results and several analysis details needed to judge the comparison are not reported in the registry record.

13. Why This Trial Matters Statistically

This trial illustrates a common distinction in clinical research: a clearly specified endpoint is not the same as a reported result. The registry describes an antigen-specific, threshold-based immune measure and names a confidence-interval type. Without the corresponding percentages, interval limits, and analysis denominators, readers cannot judge either the estimated response or its precision. A study’s design can be informative while its result remains unavailable in the registry record.

The endpoint also highlights how measurement choices shape interpretation. A seroprotection percentage simplifies an antibody titer into whether a participant crosses a cutoff. That summary is accessible, but it does not convey the full spread or magnitude of titers. The use of HAI for some antigens and microneutralization for another further underscores why antigen-specific results should be read in their assay context rather than collapsed into an unsupported overall claim.

Eligibility for the immunogenicity population depends on vaccination, timely blood draws, assay results, and infection status between visits. Such criteria can be appropriate to the stated question, but they also make the analysis denominator central to interpretation. Reporting how many participants contribute to each antigen-specific estimate and how missing measurements were handled would help readers understand precision and potential selection effects. That information is not reported in the registry record.

Finally, the record lists multiple outcome categories and analyses but does not report a multiplicity strategy or statistical analysis plan. This matters because inferential claims depend not only on a numerical estimate, but also on which comparisons were planned and how uncertainty was evaluated. The case demonstrates why statistical transparency includes endpoint definitions, analysis populations, missing-data handling, multiplicity, and numerical results—not merely the enrollment figure or study design label.

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 the primary comparison is that numerical seroprotection results are not reported in the registry record. The record gives no arm-specific percentages, confidence-interval limits, analysis denominators, or between-group estimate. It is therefore not possible to describe the observed effect or its precision, or to assess a statistical comparison.

The primary measure covers several antigens, and the registry does not reduce them to one comparison. Antigen-specific estimates may need to be considered separately, but no results are available to establish their pattern. The assay differs by antigen as specified in the record, which is relevant context when interpreting measurements. The record does not provide further assay-performance information or a method for synthesizing responses across antigens.

The primary population excludes participants who did not meet the stated criteria for vaccination, paired blood draws, assay results, timing, and infection status. The number included and the extent of missing or excluded data are not reported in the registry record. Without that information, the representativeness of the analyzed subset and the effects of incomplete measurements cannot be judged.

The study is listed as unmasked, and the record does not state whether laboratory personnel were masked. It also does not report the randomization procedure, allocation concealment, the statistical analysis plan, the model used for the endpoint, or the method used to calculate confidence intervals. A strategy for multiplicity and a missing-data approach are not reported in the registry record. These are uncertainties in the available documentation, not evidence that any particular method was or was not used.

The registry’s safety listings do not provide causal attribution for serious adverse events or deaths. The clinical illness surveillance procedures are described, but numerical clinical illness results are not included among the registry entries available for interpretation. Thus, the immunogenicity endpoint should not be extended to claims about clinical protection or comparative safety.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Immunogenicity of Influenza Vaccinations — NCT06518577.
• Registry record last updated 2026-07-28; status last verified 2026-06.
• Results first posted 2026-07-28.
• 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

NCT06518577, titled “Immunogenicity of Influenza Vaccinations,” is a completed, randomized, unmasked, parallel-group phase 4 study sponsored by Duke University. It compares Flucelvax® (ccIIV3) with Flublok® (RIV3) in US adults aged 18-64. The registry lists enrollment of 606 and study dates from 2024-09-09 to primary completion on 2024-12-09.

The primary endpoint is the percentage of participants with a seroprotective antibody titer of at least 1:40 at Visit 2, with a 95% confidence interval. HAI is specified for cell-grown A(H1N1)pdm09 and B/Victoria, and microneutralization for A(H3N2). The defined immunogenicity population is a subset of the mITT population with specified vaccine receipt, blood results, timing, and infection-status criteria.

The registry record does not report numerical primary endpoint results or key analysis details such as the multiplicity and missing-data strategies. Serious adverse events and deaths are listed by group, but the record does not establish causality.