Safety, Immunogenicity, and Relative Efficacy of H1N1 Vaccines in Adults Aged 18 Years and Older: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Safety, Immunogenicity, and Relative Efficacy of H1N1 Vaccines in Adults Aged 18 Years and Older (NCT00979602), a randomized phase 3 study. The primary outcome is Number of Seroconverted (SCR) Subjects for Hemagglutination Inhibition (HI) Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain.

2. Clinical Question

The trial was designed to characterize safety, immunogenicity, and relative efficacy of the two vaccines in adults. The primary outcome posted in the registry is immunogenicity: a count of participants meeting a prespecified antibody-response definition. The registry record does not report a primary clinical illness endpoint in the registry record for this analysis.

The comparisons include the reference group and several other age-labeled groups. They therefore combine differences in vaccine formulation and age-group label for some contrasts. In particular, the record includes separate age-band definitions of 18-60 and >60 years, and 18-64 and >64 years. The reason for reporting both sets of labels, and whether they correspond to distinct analyses or overlapping populations, is not reported in the registry record. Comparisons across those labels should not be read as a single direct comparison that isolates vaccine formulation alone.

The trial asks about measured antibody responses under the study conditions. It does not, by itself, answer how well either formulation prevents infection, illness, transmission, or complications. Those are different outcomes from the Day 21 seroconversion count and require evidence based on those clinical endpoints.

3. Trial Design

The registry identifies this as a Phase 3, randomized, triple-masked, parallel-group study. It lists enrollment of 4048 and a status of completed. The listed start date is 2009-11-09, and the primary completion date is 2011-01-10. GlaxoSmithKline is named as the sponsor.

Participants were assigned to vaccine groups. The registry describes GSK2340274A as adjuvanted and GSK2340273A as unadjuvanted. Each arm description states that participants received one dose on Day 0, administered intramuscularly in the deltoid region of the non-dominant arm. The arm descriptions are truncated in the registry record, so additional administration details are not reported in the registry record.

The registry labels the study model as parallel, meaning that the listed treatment groups are compared as separate groups rather than as a crossover sequence. Random allocation can help balance participant characteristics across groups on average, while masking can reduce the influence of treatment knowledge on care or outcome assessment. These are general design principles; the specific allocation procedures and masking roles are not reported in the registry record.

The posted primary endpoint analysis population is the according-to-protocol cohort for immunogenicity at Day 21. The record describes this as evaluable subjects who received the assigned vaccine and had antibody assay results, but the complete eligibility language is truncated. The exact population used for each other endpoint, and the handling of participants who were not evaluable, are not reported in the registry record.

The trial includes age-labeled groups under two sets of thresholds: 18-60 and >60 years, and 18-64 and >64 years. The registry does not explain the relationship between these classifications. Nor does it report the original sample-size justification, the planned interim analyses, or a full statistical analysis plan. Those details should not be inferred from the enrollment figure or from the posted results.

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 at Day 21, which included all evaluable subjects who received the study vaccine according to their treatment assignment and for whom the assay results for antibodies against A/California-like HA antigen for the blood sample taken on Day 21 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 Number of Seroconverted (SCR) Subjects for Hemagglutination Inhibition (HI) Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain, measured over At Day 21.

The primary endpoint is the number of participants who seroconverted for hemagglutination inhibition (HI) antibodies against the A/California/7/09 (H1N1)v-like influenza strain at Day 21. The registry defines seroconversion as a vaccinated participant having a post-vaccination titer of at least 1:40, or having at least a 4-fold increase from a pre-vaccination titer of at least 1:10. The reported outcome is a count of participants meeting either part of that definition.

HI antibody measurements are used to describe an immune response to a vaccine. A count-based endpoint makes the number meeting the stated response definition visible within each reported group. In general, a clear operational definition helps specify which assay results qualify as an event; the registry’s definition supplies that rule for this endpoint.

Seroconversion is a laboratory outcome. It is not the same as a diagnosis of influenza or a direct measure of protection from clinical illness. The endpoint can therefore inform interpretation of immunogenicity, but it should not be treated as a clinical efficacy result. The registry’s primary analysis population is the according-to-protocol cohort for immunogenicity at Day 21, rather than all participants enrolled. The full cohort criteria are not reported in the registry record because the population description is truncated.

7. Results Posted in the Registry

• GSK2340274A (>60 y) Group: 76.8% (411/535) versus 90.0% (1210/1345) on control; risk difference -13.1 percentage points (95% CI -17.1 to -9.2), risk ratio 0.85 (0.81–0.90); unadjusted two-proportion z-test p = <0.001.
• GSK2340273A (18-60 y) Group: 75.6% (1012/1338) versus 90.0% (1210/1345) on control; risk difference -14.3 percentage points (95% CI -17.1 to -11.5), risk ratio 0.84 (0.81–0.87); unadjusted two-proportion z-test p = <0.001.
• GSK2340273A (>60 y) Group: 56.7% (301/531) versus 90.0% (1210/1345) on control; risk difference -33.3 percentage points (95% CI -37.8 to -28.8), risk ratio 0.63 (0.58–0.68); unadjusted two-proportion z-test p = <0.001.
• GSK2340274A (18-64 y) Group: 89.7% (1279/1426) versus 90.0% (1210/1345) on control; risk difference -0.3 percentage points (95% CI -2.5 to 2.0), risk ratio 1.00 (0.97–1.02); unadjusted two-proportion z-test p = 0.813.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Number of Seropositive Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • GSK2340274A (>60 y) Group: 92.3% (501/543) versus 98.2% (1282/1306) on control; risk difference -5.9 percentage points (95% CI -8.3 to -3.5), risk ratio 0.94 (0.92–0.96); unadjusted two-proportion z-test p = <0.001.
  • GSK2340273A (18-60 y) Group: 95.7% (1257/1314) versus 98.2% (1282/1306) on control; risk difference -2.5 percentage points (95% CI -3.8 to -1.2), risk ratio 0.97 (0.96–0.99); unadjusted two-proportion z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically lower for GSK2340274A (>60 y) Group than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically lower for GSK2340273A (18-60 y) Group than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Number of Seroconverted (SCR) Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • GSK2340274A (>60 y) Group: 72.0% (36/50) versus 98.0% (49/50) on control; risk difference -26.0 percentage points (95% CI -39.0 to -13.0), risk ratio 0.73 (0.62–0.88); unadjusted two-proportion z-test p = <0.001.
  • GSK2340273A (18-60 y) Group: 79.5% (35/44) versus 98.0% (49/50) on control; risk difference -18.5 percentage points (95% CI -31.0 to -5.9), risk ratio 0.81 (0.70–0.95); unadjusted two-proportion z-test p = 0.004.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically lower for GSK2340274A (>60 y) Group than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically lower for GSK2340273A (18-60 y) Group than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Number of Seroprotected (SPR) Subjects for HI Antibodies Against the Flu A/CAL/7/09 H1N1 Virus Strain

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • GSK2340274A (>60 y) Group: 76.0% (38/50) versus 98.0% (49/50) on control; risk difference -22.0 percentage points (95% CI -34.5 to -9.5), risk ratio 0.78 (0.66–0.91); unadjusted two-proportion z-test p = 0.001.
  • GSK2340273A (18-60 y) Group: 93.2% (41/44) versus 98.0% (49/50) on control; risk difference -4.8 percentage points (95% CI -13.2 to 3.6), risk ratio 0.95 (0.87–1.04); unadjusted two-proportion z-test p = 0.248.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically lower for GSK2340274A (>60 y) Group than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between GSK2340273A (18-60 y) Group and the control. The interval gives the range of effects that remain compatible with the data.

The posted secondary outcomes extend the immunogenicity picture beyond the Day 21 primary endpoint. They include seropositivity and antibody titers, as well as seroconversion and seroprotection outcomes at other time points. The registry lists 27 outcome measures overall and 22 posted secondary outcomes. Different outcomes and time points use different analysis denominators, so their counts are not directly interchangeable.

Broadly, the secondary records include differences between some groups and less conclusive comparisons in others. This is not a single uniform pattern across every antibody outcome, age label, and time point. The secondary results relate to the primary endpoint because they also assess antibody responses, but they use different definitions or measurement times. Agreement in general direction can support a coherent description of immunogenicity; it does not make each outcome an independent confirmation, particularly when the outcomes are related.

The secondary comparisons are unadjusted recalculations, and the registry lists no reported analyses for these secondary outcomes. No multiplicity strategy is reported in the registry record. When many outcomes and comparisons are considered, unadjusted p-values can make isolated findings appear more decisive than they would under a prespecified adjustment or hierarchy. Accordingly, the secondary findings carry less weight than a well-documented primary analysis and should be treated as descriptive evidence rather than as a set of separate confirmatory claims.

These antibody outcomes do not establish clinical protection from influenza. They describe laboratory measures recorded at the specified time points. The registry record does not provide a basis for translating these secondary results into a clinical efficacy estimate or for assuming that antibody levels have the same meaning across all age-labeled groups.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Number of Seropositive Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Titers for Serum HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Number of Seropositive Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Titers for Serum HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Number of Seropositive Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Titers for Serum HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Number of Seroconverted (SCR) Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain
• Number of Seroconverted (SCR) Subjects for HI Antibodies Against Flu A/CAL/7/09 H1N1 Virus Strain

9. Statistical Interpretation

10. Statistical Methods Explained

The outcome is binary at the participant level: a participant either met the registry’s seroconversion definition or did not. The posted group values are counts of participants meeting the definition, alongside the number evaluated. A comparison of these counts can be expressed as a difference in event risks, a risk ratio, or an odds ratio. These measures answer related but not identical questions. A risk difference describes the difference between the two group risks; a risk ratio compares them as a ratio; and an odds ratio compares their odds. The calculated comparisons include these effect measures, but the interpretation emphasizes the risk difference because it expresses the contrast on an absolute scale.

What does a risk difference describe?

For a binary endpoint, each group’s risk is the proportion of evaluated participants with the event. The risk difference subtracts the reference-group risk from the comparison-group risk. A negative estimate means that the event risk in the comparison group is lower than in the reference group under that coding. A value of zero corresponds to equal risks. This is a general explanation of the statistic, not a statement that the difference is causal.

The registry’s comparison set uses GSK2340274A aged 18-60 years as the reference. That makes the direction of each reported risk difference dependent on the stated reference and comparison group. The age labels matter: contrasts involving a differently labeled age group may reflect both treatment-group and population differences. A contrast between groups with the same age label is closer to a formulation comparison, but it remains an unadjusted comparison based on the posted summaries.

How should the p-values be read?

The p-values are unadjusted. In general, a p-value evaluates the compatibility of observed data with a null hypothesis under a statistical model; it is not the probability that the null hypothesis is true, nor does it measure the probability that the result occurred by chance in a broad everyday sense. It also does not measure the size of an effect. Interpretation should consider the estimated contrast and its confidence interval alongside the p-value.

The record does not report a multiplicity adjustment for the many outcomes and group comparisons. When many comparisons are examined, the chance of finding one or more small p-values can increase even if the underlying null hypotheses are true. This general concern means that unadjusted secondary or exploratory comparisons should receive less weight than a prespecified primary analysis interpreted under a documented plan. The statistical plan is not reported in the registry record, so the prespecification status of the listed contrasts cannot be established.

Why do confidence intervals matter?

A confidence interval gives a range of estimates compatible with the data and the method’s assumptions at its stated confidence level. In general, a narrower interval indicates greater precision, while a wider interval indicates less precision. For a risk difference, an interval that includes zero does not rule out no difference at the corresponding confidence level. An interval entirely on one side of zero indicates that the values in that interval share a direction, but it does not eliminate uncertainty about the size of the difference.

The posted risk-difference intervals are unadjusted recalculations. They reflect uncertainty from the summary counts under the calculation used; they do not account for every possible source of uncertainty, including unreported design choices, differences in analysis populations, or multiple comparisons. They should therefore be read together with the trial’s population and outcome definitions, rather than as a complete measure of evidential strength.

What is the role of the reported analysis population?

The primary endpoint is based on the according-to-protocol immunogenicity cohort at Day 21. In general, restricting an analysis to participants who met protocol and evaluability criteria can make the population relevant to an assay-based outcome, but it can also mean that the analyzed participants are not identical to everyone randomized. The complete criteria and a comparison of analyzed with excluded participants are not reported in the registry record. No assumptions about the impact of exclusions should be made without those details.

The primary endpoint is a count of participants, and no dispersion measure is listed. For a binary outcome, uncertainty is usually represented through a confidence interval for a chosen contrast rather than through a standard deviation for the count. The posted comparisons provide risk-difference intervals and other effect measures. Those are recalculated, unadjusted contrasts, not identified as the sponsor’s prespecified inferential analysis.

What cannot be recovered from the record?

The registry record does not report the original sample-size justification, the detailed statistical analysis plan, interim-analysis procedures, missing-data methods, or multiplicity strategy. It also does not provide a sponsor-reported analysis listing for the primary endpoint. Those omissions prevent a full assessment of whether the posted contrasts match the trial’s original inferential hierarchy and analysis decisions. They do not change the values posted, but they limit how strongly those values can be interpreted as confirmatory findings.

11. Confidence Intervals and Precision

The confidence intervals for the primary risk-difference comparisons convey how precisely the posted summary counts estimate the contrasts under the unadjusted calculation. Most intervals lie entirely below zero, while the interval for the GSK2340274A group labeled 18-64 years crosses zero. That interval therefore includes no risk difference as well as small differences in either direction. Its associated unadjusted p-value is 0.813. The other posted primary comparisons have unadjusted p-values of <0.001, with intervals on the negative side of zero.

In general, a confidence interval is not a guarantee that the true effect lies within its endpoints. It is a range produced by a method with stated assumptions and a coverage interpretation over repeated use. Narrower intervals generally indicate more precision than wider intervals. The intervals should be judged against the scale of the outcome and the clinical question, not just by whether they cross zero.

The comparison groups have different endpoint denominators, and the primary analysis uses the according-to-protocol immunogenicity cohort. Precision depends in part on the amount of information in each comparison, but the registry record does not report a prespecified precision target for these contrasts. The original sample-size assumptions and rationale are not reported in the registry record.

These intervals are unadjusted recalculations from the posted summary statistics. They do not incorporate a documented adjustment for the number of outcomes or comparisons, and they do not resolve the uncertainty created by incomplete reporting of analysis choices. A confidence interval around a laboratory response contrast also does not quantify uncertainty about clinical influenza outcomes that were not measured in this primary endpoint.

12. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning estimates are expressed as participants per arm, not as a total across arms or as a total number of events. They are illustrative calculations based on the observed effect and are not the trial’s original design justification. The registry record does not report the original sample-size assumptions.

The stated planning assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The calculations consider detecting the stated fraction of the observed effect. The planning rows for 100% of the observed effect use the full observed contrast as the target; the rows for 75% and 50% use smaller target effects. Smaller effects need more participants to detect under otherwise unchanged assumptions.

The 80% power column gives the estimated participants per arm for a design with an 80% chance of detecting the target effect if that effect is present and the calculation assumptions hold. The 90% power column gives the corresponding planning estimate for a 90% chance. Higher target power generally requires more participants. These are planning illustrations, not observed power: observed data cannot tell whether a completed trial had a particular probability of detecting its own result.

Very small observed differences can produce very large planning requirements when the target is to detect a similar difference. Such estimates should not be read as recommendations without considering the clinical importance of the target effect, the chosen endpoint, and the original trial design. The original justification, if any, is not reported in the registry record.

13. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.900; treatment proportion p₁ = 0.768; average p̄ = 0.834.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.5263 = 1.0315.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.5180 = 0.4360.
4. n = (1.0315 + 0.4360)² / (-0.1314)² = 124.7, rounded up to 125 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
p0 <- 0.899628252788104                          # control proportion
p1 <- 0.768224299065421                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(979602)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 125 (80% power), 167 (90% power)
 75% of observed effect: n per arm = 205 (80% power), 274 (90% power)
 50% of observed effect: n per arm = 419 (80% power), 561 (90% power)
power.prop.test: 124.7 
Simulated power at n = 125 per arm: 0.809 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

14. Multiplicity, Missing Data and Analysis Populations

The registry record does not report a multiplicity-adjustment strategy. This matters because the study lists numerous outcomes and the posted results include several group contrasts. In general, testing many hypotheses without adjustment increases the chance that at least one result will appear statistically notable even when no corresponding effect exists. Whether the trial used a prespecified hierarchy, adjusted thresholds, or another strategy is not reported in the registry record.

The registry record also does not report detailed missing-data methods. The primary endpoint is analyzed in an according-to-protocol immunogenicity cohort at Day 21, described as evaluable participants who received the assigned vaccine and had assay results. The full cohort definition is truncated. The reasons participants might be excluded from this cohort, the number excluded from each randomized group, and any method for handling unavailable assay results are not reported in the registry record.

The record does not report a full statistical analysis plan or interim-analysis procedures. It lists no reported analysis entries for the primary endpoint and no reported analyses for the secondary outcomes. The comparisons discussed in the statistical interpretation are unadjusted recalculations from the posted counts, not identified as sponsor-reported adjusted analyses. This limits the ability to determine whether the posted analyses correspond to the original prespecified plan.

These reporting gaps do not prove that missing data or multiplicity affected the results. They mean that the relevant methods cannot be evaluated from the registry record. Readers should distinguish the observed counts and recalculated contrasts from conclusions that would require knowledge of the trial’s prespecified analysis decisions.

15. 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 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 (0.3%), atrial fibrillation (0.2%), coronary artery disease (0.2%), chest pain (0.2%), pancreatitis (0.1%).

The registry posts serious adverse event counts for the GSK2340274A and GSK2340273A groups, with the number at risk listed as 2025 and 2023, respectively. Serious adverse events were recorded for 84 participants in the GSK2340274A group and 73 participants in the GSK2340273A group. The listed death counts are 6 and 1, respectively. These are descriptive registry counts; they do not establish that a vaccine caused an event or death.

The record also lists other adverse events for 1680 participants in the GSK2340274A group and 1063 participants in the GSK2340273A group, with the same group-specific at-risk counts as reported for those entries. The registry fields do not supply further event-level detail in the registry record for this analysis. A participant count is not necessarily the same as an event count, and the posted summaries do not specify whether participants could contribute multiple events.

The serious-event terms listed include pneumonia at 0.3%, atrial fibrillation at 0.2%, coronary artery disease at 0.2%, chest pain at 0.2%, pancreatitis at 0.1%, and pulmonary embolism at 0.1%. These entries describe reported events, not comparative causal effects. Detailed definitions, follow-up windows, adjudication methods, and adjusted safety comparisons are not reported in the registry record.

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

The posted primary result describes how many participants in each reported group met a defined HI antibody seroconversion criterion at Day 21. It is evidence about an immunogenicity measure within the stated according-to-protocol cohort. The group counts and unadjusted recalculated comparisons show that the observed response counts differ across some of the registry’s age-labeled groups, while one listed comparison has an interval that includes no risk difference.

The result does not establish whether either vaccine prevents influenza illness. Seroconversion is a laboratory endpoint, not a clinical infection outcome. It also does not establish that any observed difference was caused by the vaccine formulation, especially for comparisons where the reference and comparison groups have different age labels. The posted contrasts are unadjusted, and no sponsor-reported analysis is listed for this primary endpoint.

A small p-value does not establish clinical importance, and a confidence interval does not remove uncertainty. Interpretation depends on the outcome definition, analysis population, comparison groups, and methods. The record provides the endpoint definition and some cohort information, but the full cohort criteria, multiplicity approach, missing-data methods, and detailed analysis plan are not reported in the registry record.

The safety summaries likewise describe reported adverse events and deaths in the listed groups. They are not proof that an event was or was not caused by vaccination. The registry record does not provide enough detail to determine how event timing, severity, or other factors were assessed.

Overall, the results support a statistical description of antibody response counts and their unadjusted contrasts at specified time points. They do not justify a broader conclusion about clinical efficacy, long-term antibody persistence, or comparative safety beyond the reported summaries. Those questions require outcomes and analyses that are not established by the primary seroconversion count alone.

17. Why This Trial Matters Statistically

This trial illustrates why the outcome definition and analysis population matter when interpreting a clinical study. The primary endpoint is not a broad label such as immune response; it has a specific HI antibody threshold and fold-rise rule, measured at Day 21 in an according-to-protocol immunogenicity cohort. A reader can therefore distinguish what the endpoint counts from clinical outcomes it does not measure.

The record also illustrates how group labels shape comparisons. The posted contrasts use GSK2340274A aged 18-60 years as a reference for groups with several age labels and vaccine assignments. A comparison across different age labels cannot isolate a vaccine-formulation contrast as cleanly as a comparison that holds the age label constant. Reporting those distinctions helps prevent an unadjusted numerical contrast from being interpreted as a simple head-to-head treatment effect.

Finally, the registry contains counts and outcome summaries but does not report the complete statistical plan, multiplicity strategy, missing-data methods, or sponsor-reported primary analyses. This is a useful reminder that numerical precision and a small p-value do not replace transparent reporting of how analyses were selected and conducted. The recalculated estimates are informative descriptions of the posted data, but they cannot supply design and analysis details that the registry record does not report.

18. 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 primary outcome is an antibody response rather than a clinical influenza outcome. The results therefore cannot directly quantify prevention of infection or illness. The registry also reports the primary endpoint in an according-to-protocol immunogenicity cohort, and its full criteria are truncated. The composition of that cohort and the handling of participants without evaluable assay results are not reported in the registry record.

The comparison set uses GSK2340274A aged 18-60 years as the reference. Other groups include different vaccine formulations and age labels, so several contrasts do not isolate formulation alone. The record lists both 18-60 and >60 year labels and 18-64 and >64 year labels, without explaining their relationship. The reason for these parallel age classifications is not reported in the registry record.

The reported comparisons are unadjusted recalculations from summary counts. A sponsor-reported primary analysis, multiplicity adjustment, missing-data approach, and detailed statistical analysis plan are not reported in the registry record. These omissions limit assessment of prespecification, analysis choices, and the combined interpretation of multiple outcomes.

The registry’s arm descriptions are truncated, and detailed safety information is limited in the registry record. The safety counts do not provide a causal comparison, and event definitions, follow-up details, and adjudication procedures are not reported in the registry record. The posted information should therefore be interpreted as a record of observed summaries rather than a complete account of all trial methods and findings.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Safety, Immunogenicity, and Relative Efficacy of H1N1 Vaccines in Adults Aged 18 Years and Older — NCT00979602.
• Registry record last updated 2018-03-21; status last verified 2018-03.
• Results first posted 2017-09-21.
• 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.

22. Record Summary

This completed, randomized, triple-masked, parallel Phase 3 study evaluated adjuvanted GSK2340274A and unadjuvanted GSK2340273A in adults aged 18 years and older. The registry lists enrollment of 4048. The primary endpoint was the number of participants meeting the HI antibody seroconversion definition at Day 21 in the according-to-protocol immunogenicity cohort.

The posted counts and unadjusted recalculated comparisons show differing seroconversion results across the age-labeled groups, with uncertainty varying across contrasts. The primary endpoint is an immunogenicity measure and does not itself establish clinical efficacy. Detailed analysis methods, multiplicity handling, missing-data methods, and the original sample-size justification are not reported in the registry record.