Sanofi H1N1 + TIV - Adults and Elderly: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Sanofi H1N1 + TIV - Adults and Elderly (NCT00943878), a randomized phase 2 study. The primary outcome is Number of Participants Age 18 to 64 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the H1N1 2009 Virus 21 Days Following the First Dose of H1N1 Vaccine.

2. Clinical Question

The question is about the observed immune-response measure under these schedules, not about whether participants developed influenza or whether a schedule prevented illness. The registry record describes the study as assessing safety and immune response, and lists additional outcomes for antibody responses after later vaccination timepoints and for seasonal influenza strains.

3. Trial Design

This was a randomized, triple-masked, parallel-group Phase 2 study. The registry record lists an enrollment of 805 and describes four groups, with about 200 participants planned in each group and adult and elderly participants represented in each. The study was completed. The record lists a start in 2009-08 and primary completion in 2010-04.

The schedules used three study visits on Day 0, Day 21 and Day 42. Group 1 received H1N1 vaccine plus placebo at the first two visits, followed by TIV. Group 2 received H1N1 vaccine plus TIV at the first visit, H1N1 vaccine plus placebo at the second, and placebo at the third. Group 3 received H1N1 vaccine plus placebo, then H1N1 vaccine plus TIV, then placebo. Group 4 received TIV plus placebo, followed by H1N1 vaccine plus placebo and then H1N1 vaccine.

The primary endpoint is reported for adults age 18 to 64 years, not for the full enrolled population. Its assessment was 21 days after the first H1N1 vaccination; for Group 4, that assessment occurred on Study Day 42, while for the other groups it occurred on Study Day 21. Thus the assessment is anchored to first H1N1 vaccination, even though its calendar study day differs in Group 4. The registry record does not report the trial's original sample-size justification or detailed masking procedures.

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

Analysis population for the primary outcome, as defined in the registry: Participants who received the H1N1 vaccination and from whom blood was collected at the timepoint, both within 4 days of the window, are included. Seven participants were excluded due to eligibility deviations, vaccine administration error or other protocol deviations. Analyses are as treated. This outcome restricts to age stratum.

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 Participants Age 18 to 64 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the H1N1 2009 Virus 21 Days Following the First Dose of H1N1 Vaccine, measured over Day 21 after first H1N1 vaccination.

The primary endpoint counts participants age 18 to 64 years whose serum HAI antibody titer against the 2009 H1N1 virus was 1:40 or greater at the assessment 21 days after the first H1N1 vaccination. Blood was tested using the HAI assay, and each sample was tested at least twice. A participant counted when the geometric mean of replicate results met or exceeded the stated threshold.

HAI is a laboratory assay used to quantify antibody activity against a specified virus strain. A threshold-based endpoint converts those measurements into a count of participants meeting a defined antibody criterion. This can make group results straightforward to describe, but it discards information about how far titers lie above or below the threshold. The endpoint is an immune-response measure; it is not itself a count of influenza infections or a direct measure of clinical protection.

The analysis population included participants who received H1N1 vaccination and had blood collected within 4 days of the specified timepoint window. The record says analyses were as treated and notes that seven participants were excluded for eligibility deviations, vaccine administration error or other protocol deviations. The registry record does not report a more detailed handling rule for participants without an evaluable blood sample.

7. Results Posted in the Registry

• Group 2: H1N1+TIV; H1N1+Placebo; Placebo: 86.1% (87/101) versus 91.8% (90/98) on control; risk difference -5.7 percentage points (95% CI -14.3 to 3.0), risk ratio 0.94 (0.85–1.03); unadjusted two-proportion z-test p = 0.200.
• Group 3: H1N1+Placebo; H1N1+TIV; Placebo: 91.9% (91/99) versus 91.8% (90/98) on control; risk difference 0.1 percentage points (95% CI -7.5 to 7.7), risk ratio 1.00 (0.92–1.09); unadjusted two-proportion z-test p = 0.983.
• Group 4: TIV+Placebo; H1N1+Placebo; H1N1: 87.2% (82/94) versus 91.8% (90/98) on control; risk difference -4.6 percentage points (95% CI -13.3 to 4.1), risk ratio 0.95 (0.86–1.05); unadjusted two-proportion z-test p = 0.297.

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 Participants Age 18 to 64 Years With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Influenza H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine

• 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:
  • Group 2: H1N1+TIV; H1N1+Placebo; Placebo: 83.7% (77/92) versus 90.4% (85/94) on control; risk difference -6.7 percentage points (95% CI -16.3 to 2.9), risk ratio 0.93 (0.83–1.03); unadjusted two-proportion z-test p = 0.171.
  • Group 3: H1N1+Placebo; H1N1+TIV; Placebo: 94.4% (85/90) versus 90.4% (85/94) on control; risk difference 4.0 percentage points (95% CI -3.6 to 11.6), risk ratio 1.04 (0.96–1.13); unadjusted two-proportion z-test p = 0.304.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 2: H1N1+TIV; H1N1+Placebo; Placebo and the control. The interval gives the range of effects that remain compatible with the data.
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 3: H1N1+Placebo; H1N1+TIV; Placebo and the control. The interval gives the range of effects that remain compatible with the data.

Number of Participants Age 65 Years and Older With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Influenza H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine

• 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:
  • Group 2: H1N1+TIV; H1N1+Placebo; Placebo: 79.3% (73/92) versus 83.3% (80/96) on control; risk difference -4.0 percentage points (95% CI -15.1 to 7.2), risk ratio 0.95 (0.83–1.09); unadjusted two-proportion z-test p = 0.483.
  • Group 3: H1N1+Placebo; H1N1+TIV; Placebo: 77.4% (72/93) versus 83.3% (80/96) on control; risk difference -5.9 percentage points (95% CI -17.2 to 5.4), risk ratio 0.93 (0.81–1.07); unadjusted two-proportion z-test p = 0.306.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 2: H1N1+TIV; H1N1+Placebo; Placebo and the control. The interval gives the range of effects that remain compatible with the data.
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 3: H1N1+Placebo; H1N1+TIV; Placebo and the control. The interval gives the range of effects that remain compatible with the data.

Number of Participants Age 18 to 64 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine

• 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:
  • Group 2: H1N1+TIV; H1N1+Placebo; Placebo: 89.1% (82/92) versus 94.7% (89/94) on control; risk difference -5.6 percentage points (95% CI -13.4 to 2.3), risk ratio 0.94 (0.86–1.03); unadjusted two-proportion z-test p = 0.165.
  • Group 3: H1N1+Placebo; H1N1+TIV; Placebo: 96.7% (87/90) versus 94.7% (89/94) on control; risk difference 2.0 percentage points (95% CI -3.9 to 7.8), risk ratio 1.02 (0.96–1.09); unadjusted two-proportion z-test p = 0.509.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 2: H1N1+TIV; H1N1+Placebo; Placebo and the control. The interval gives the range of effects that remain compatible with the data.
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Group 3: H1N1+Placebo; H1N1+TIV; Placebo and the control. The interval gives the range of effects that remain compatible with the data.

The secondary outcomes extend the immune-response assessment to two related areas: four-fold or greater increases in HAI antibody titer, and HAI titers meeting the 1:40 threshold. They examine responses to H1N1 after the second H1N1 dose and responses to strains in the 2009-2010 TIV after the last vaccination, with outcomes reported separately for adults age 18 to 64 years and participants age 65 years and older.

Across the posted secondary results, the observed directions are not uniform. Some group contrasts are negative and others positive, and the calculated intervals for the reported comparisons span zero. This broad pattern is consistent with the primary result in one important respect: the available recalculated contrasts do not give a precise, consistent separation between the schedules. The secondary outcomes add information about different timepoints and age groups, but they are not interchangeable with the primary endpoint, which concerns the first H1N1 dose in adults age 18 to 64 years.

The registry record lists 29 outcome measures but reports no analyses for the secondary endpoints. The numerical comparisons displayed for them are therefore unadjusted recalculations from posted arm summaries, not sponsor-reported analyses. No adjustment for the many outcomes or group comparisons is reported. When many outcomes are examined, some apparently notable differences can arise by chance even if there is no underlying difference. Unadjusted secondary findings consequently carry less weight than a clearly prespecified, multiplicity-aware primary analysis.

The secondary results are best treated as descriptive context about the observed antibody measures. They can help readers see whether the broad pattern varies by age group, assay criterion or assessment time, but they do not establish a confirmatory conclusion for any individual secondary comparison. The registry record does not report a hierarchy that would give selected secondary outcomes confirmatory status.

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

• Number of Participants Age 18 to 64 Years With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Virus Strains in the 2009-2010 Trivalent Influenza Vaccine (TIV) 21 Days Following the Last Vaccination
• Number of Participants Age 65 Years and Older With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Virus Strains in the 2009-2010 Trivalent Influenza Vaccine (TIV) 21 Days Following the Last Vaccination
• Number of Participants Age 18 to 64 Years With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Influenza H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine
• Number of Participants Age 65 Years and Older With 4-fold or Greater Hemagglutination Inhibition Assay (HAI) Antibody Titer Increases Against the Influenza H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine
• Number of Participants Age 18 to 64 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Virus Strains in the 2009-2010 Trivalent Influenza Vaccine (TIV) 21 Days Following the Last Vaccination
• Number of Participants Age 65 Years and Older With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Virus Strains in the 2009-2010 Trivalent Influenza Vaccine (TIV) 21 Days Following the Last Vaccination
• Number of Participants Age 18 to 64 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine
• Number of Participants Age 65 Years and Older With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the H1N1 2009 Virus 21 Days Following the Second Dose of H1N1 Vaccine

9. Statistical Interpretation

10. Statistical Methods Explained

What does a risk difference describe?

A risk difference subtracts the proportion meeting an endpoint in one group from the corresponding proportion in a reference group. In this analysis, the event is meeting the HAI antibody threshold, and Group 1 is the reference. A negative estimate means the observed proportion was lower in the compared group; a positive estimate means it was higher. The estimate is expressed as a proportion difference, not a difference in the raw participant counts.

The reported comparison estimates are recalculated, unadjusted contrasts based on the posted arm counts. They do not incorporate covariate adjustment or a model-based correction. The registry record does not report a sponsor analysis for the primary endpoint, so these contrasts should not be described as the trial's prespecified inferential results. The record's discrepancy indicator is false, but that does not turn the recalculations into sponsor-reported results.

Why use a binary threshold?

The endpoint classifies each participant according to whether the geometric mean of replicate HAI results reached 1:40 or greater. A binary outcome can be communicated as a count meeting the criterion among those assessed. However, participants just below the threshold and participants with very low titers are grouped together, as are participants just above the threshold and those with much higher titers. That loss of detail is inherent in reducing a continuous laboratory measure to two categories.

The endpoint also depends on the stated assay antigen, collection timing and replicate rule. The primary assay used the 2009 H1N1 virus, and blood was collected 21 days after the first H1N1 vaccination. Because Group 4 had its first H1N1 vaccination later in the sequence, its corresponding assessment occurred on Study Day 42. The timing relative to first H1N1 vaccination is aligned, while the study-day number differs.

How should the p-values be read?

A p-value measures how surprising data at least as discrepant as those observed would be under a specified null model. It is not the probability that the null hypothesis is true, and it does not quantify the size or clinical importance of a difference. The displayed p-values are unadjusted recalculations. The registry record does not report a statistical analysis plan, the test used for sponsor analyses, or any multiplicity adjustment. They should therefore be interpreted cautiously and alongside the effect estimates and intervals, rather than as a definitive decision rule.

What does an interval add?

A confidence interval gives a range of values compatible with the data under the calculation's assumptions and method. For a risk difference, an interval containing zero includes no difference among the values compatible with the data. When an interval extends to both negative and positive values, the data do not precisely determine the direction of the contrast. A wide interval signals that a broad range of differences remains plausible; it does not mean that every value in the interval is equally likely.

The intervals shown with the recalculated contrasts are not identified in the registry record as sponsor-reported intervals. Their interpretation is conditional on the calculation used for the posted comparisons. The record does not report a model, covariate structure, or detailed inferential method for an official primary comparison. The listed arm counts and the participant eligibility rules are the clearest basis for understanding the endpoint results.

Why do group definitions matter?

The groups differ by the sequence and co-administration of H1N1 vaccine, TIV and placebo. A group comparison therefore concerns the complete assigned schedule, not a single isolated factor unless the schedules support that distinction. The primary outcome is measured after the first H1N1 vaccination, so comparisons describe the antibody threshold at that assessment rather than later responses after the second dose. Separate secondary outcomes address later H1N1 and TIV antibody measurements.

Random allocation is intended to support fair comparisons by assigning participants to groups without relying on participant or investigator choice. The registry record identifies the study as randomized and triple masked, but does not report the randomization procedure or masking roles. Those details, as well as any stratification or adjustment, are not reported in the registry record.

11. Confidence Intervals and Precision

The primary comparisons have limited precision. Each reported confidence interval extends from a negative risk difference to a positive one, so none identifies a single direction of difference with confidence. For Group 2 versus Group 1, the interval is -0.143 to 0.029. For Group 3 versus Group 1, it is -0.075 to 0.077. For Group 4 versus Group 1, it is -0.133 to 0.041. These intervals are recalculated rather than sponsor-reported.

The widths matter as much as whether an interval includes zero. The Group 2 and Group 4 intervals permit potentially meaningful differences in either direction, while the Group 3 interval also spans differences on both sides of zero. The observed Group 3 estimate is close to zero, but its uncertainty does not establish equivalence. Demonstrating equivalence would require a defined margin and an appropriate analysis; such a margin is not reported in the registry record.

Confidence intervals are not guarantees that the true value lies inside a particular realized interval. In general, they describe the behavior of a calculation method over repeated studies under its assumptions. Here, they communicate the uncertainty associated with the recalculated comparisons and the available participant counts. They do not account for unreported analysis choices or any unreported multiplicity procedure.

The arm denominators for the primary endpoint are 98, 101, 99 and 94, substantially smaller than the overall enrollment of 805 because the primary endpoint is limited to adults age 18 to 64 years with the required vaccination and blood collection. The registry record notes seven exclusions for protocol-related reasons but does not provide a full accounting of all participants not included in this endpoint population. This limits how completely the primary counts can be reconciled with enrollment.

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 figures are participants per arm, not total participants and not total events. They are illustrations calculated from the observed effect, rather than the trial's original design calculation. The registry record does not report the assumptions used to choose the original enrollment target or group sizes.

The planning illustration assumes a two-sided significance level of 0.05, equal allocation between the two arms and a normal approximation. It considers effects set at 100%, 75% or 50% of the observed effect. The 80% power column describes the number of participants per arm that the calculation would require to have an 80% chance of detecting the stated effect under those assumptions, if that effect were the true difference. The 90% power column uses a higher detection probability, so it generally calls for more participants per arm.

The rows that plan for a smaller fraction of the observed effect require more participants, because smaller differences are harder to distinguish from random variation. When the observed effect is very close to zero, planning around that estimate can produce extremely large participant requirements. Such values should not be read as a recommendation or as evidence that a particular sample size is appropriate; they are especially sensitive to the observed estimate.

These calculations are not observed power. They are hypothetical planning illustrations based on the observed effect and stated assumptions, not a measure of what the completed study achieved. The sample-size assumptions used by the trial itself, including its original target effect and planned power, are 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 = 0.918; treatment proportion p₁ = 0.861; average p̄ = 0.890.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.4427 = 0.8677.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.4409 = 0.3710.
4. n = (0.8677 + 0.3710)² / (-0.0570)² = 472.6, rounded up to 473 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 <- 0.918367346938776                          # control proportion
p1 <- 0.861386138613861                          # 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(943878)
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 = 473 (80% power), 633 (90% power)
 75% of observed effect: n per arm = 793 (80% power), 1,062 (90% power)
 50% of observed effect: n per arm = 1,676 (80% power), 2,243 (90% power)
power.prop.test: 472.6 
Simulated power at n = 473 per arm: 0.805 (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 trial reports one primary endpoint and a large set of outcome measures, including secondary antibody outcomes across age groups, virus targets and assessment times. The registry record lists 29 outcome measures. It does not report whether the investigators controlled the overall type I error across the primary and secondary comparisons, or whether a hierarchy or other multiplicity strategy was prespecified. Without that information, unadjusted p-values from multiple comparisons should not be treated as if each were the sole planned test.

The record lists no reported analyses for the primary endpoint or for the posted secondary endpoints. The displayed between-group estimates and p-values are unadjusted recalculations from summary counts. The registry record does not report the trial's detailed statistical analysis plan, the original sample-size justification, interim analyses, covariate adjustment, or a formal missing-data strategy. It is therefore not possible to determine from this record whether all analysis decisions followed a prespecified plan.

The primary endpoint population is defined by receipt of H1N1 vaccine and blood collection within 4 days of the timepoint window. Analyses are described as treated, and seven participants were excluded due to eligibility deviations, vaccine administration error or other protocol deviations. This is not the same description as a full intention-to-treat analysis. The record does not explain how other missed assessments, unevaluable specimens or protocol deviations were handled, beyond the stated population rule.

The same limitations affect interpretation of secondary outcomes, for which denominators vary by endpoint. Missingness can matter if participants with available samples differ systematically from those without them. The registry record does not provide enough detail to assess whether that occurred. These reporting gaps do not invalidate the posted counts, but they limit the strength of conclusions that can be drawn from unadjusted comparisons.

15. 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: wound infection (0.5%), abdominal pain (0.5%), cervicobrachial syndrome (0.5%), malignant melanoma (0.5%), peripheral vascular disorder (0.5%).

The registry record reports serious adverse events by group, with 202 participants at risk in Group 1, 200 in Group 2, 203 in Group 3 and 200 in Group 4. The recorded serious adverse event counts were 5, 7, 12 and 13, respectively. These are group-level counts and do not, on their own, establish that an event was caused by a vaccine or that differences between groups are statistically reliable.

The most frequently listed serious-event terms were wound infection, abdominal pain, cervicobrachial syndrome, malignant melanoma, peripheral vascular disorder and pneumonia; each is listed at 0.5%. The record does not provide a causal assessment for these terms in the information summarized. Death counts are not reported in the registry record.

These figures should be interpreted in light of the study's group sizes and the absence of a reported comparative safety analysis. They describe recorded serious events during the study, not a complete characterization of risk or a formal test of differences. Other adverse events were also recorded, but event counts alone do not indicate severity, relatedness or comparative safety.

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

The posted primary data describe the proportion of adults age 18 to 64 years meeting a laboratory antibody threshold 21 days after their first H1N1 vaccination. Counts meeting the criterion were high in each group, ranging from 82 of 94 to 91 of 99. The recalculated risk differences versus Group 1 are imprecise, and their confidence intervals include both negative and positive values.

That pattern does not establish that the schedules are equivalent, nor does it establish a clear difference among them. A result that is not statistically distinguishable from zero is not proof that the true difference is zero. The registry record does not report an equivalence or non-inferiority margin, and the recalculated comparisons are not sponsor-reported analyses.

The endpoint is a laboratory measure of antibody response. It does not directly report influenza infection, symptoms, hospitalization or prevention of disease. The study's registry summary describes a goal of assessing safety and immune response, but the posted primary endpoint cannot by itself answer whether a vaccination sequence changes clinical outcomes. No such conclusion should be inferred from the antibody threshold counts alone.

The trial includes adults age 18 years and older, but the primary endpoint concerns only participants age 18 to 64 years. Separate secondary outcomes concern people age 65 years and older and later immune-response timepoints. Those results broaden the descriptive evidence but do not replace a prespecified primary analysis or resolve the limitations of unadjusted multiple comparisons.

In practical statistical terms, the results show substantial numbers of participants meeting the specified antibody criterion in all four schedules, while leaving considerable uncertainty about the size and direction of between-group differences. The record does not support a precise ranking of schedules or a claim about clinical protection.

17. Why This Trial Matters Statistically

This study illustrates why the estimand and timing of an endpoint matter in vaccine research. The primary outcome is measured after each participant's first H1N1 dose, not on a common study day for every group. Group 4's assessment takes place on Study Day 42, while the other groups are assessed on Study Day 21. Describing the result as 21 days after the first H1N1 vaccination preserves the biological timing relevant to the stated endpoint.

It also shows how a simple count can conceal important differences in what is being measured. The threshold outcome communicates how many participants reached an HAI criterion, but it does not retain the full distribution of antibody titers. The endpoint is useful as a defined laboratory measure, while remaining distinct from clinical outcomes.

The comparisons also demonstrate the importance of uncertainty and reporting completeness. The recalculated risk differences have intervals spanning zero, and the registry record reports no sponsor analyses for the primary or secondary outcomes. The numbers support description, but detailed analysis methods, multiplicity handling and missing-data rules would be needed to understand the intended inferential framework fully.

Finally, planning illustrations based on observed effects can be highly sensitive to the size of the observed contrast. They help explain the relationship between target effect and required sample size, but they do not reconstruct the original design rationale. Distinguishing planning calculations from the completed trial's observed data is essential when interpreting a registry record.

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 result is restricted to adults age 18 to 64 years and does not represent the entire enrolled population. Although the study included an elderly group, the primary endpoint and its comparisons do not summarize that group. Secondary outcomes address older participants, but their analyses are also unadjusted recalculations and have no sponsor-reported analyses in the registry record.

The endpoint is based on a threshold applied to a laboratory measurement. The record describes replicate testing and a geometric mean rule, but the threshold count does not show the distribution of titers or how close participants were to the cutoff. The primary result also concerns one timepoint after the first H1N1 vaccination; it does not describe persistence of antibody response.

Group comparisons are based on modest endpoint denominators, and the recalculated intervals are broad enough to include differences in either direction. The record does not report the original statistical analysis plan, sample-size justification, adjustment for multiple comparisons, or details of missing-data handling beyond the endpoint population definition. It also does not report a sponsor analysis for the listed endpoints.

The schedules differ in the timing and co-administration of H1N1 vaccine, TIV and placebo. The resulting contrasts compare complete schedules, and should not be attributed to one isolated component unless the design and analysis support that interpretation. Finally, the primary outcome is immunogenicity, not a clinical influenza outcome; the posted counts do not establish disease prevention or clinical effectiveness.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Sanofi H1N1 + TIV - Adults and Elderly — NCT00943878.
• Registry record last updated 2012-08-01; status last verified 2010-04.
• Results first posted 2011-05-10.
• 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

The registry record describes a completed, randomized, triple-masked, parallel-group Phase 2 study of H1N1 vaccine schedules with seasonal TIV in healthy adults and older adults. Enrollment was 805. Four groups received different sequences of H1N1 vaccine, TIV and placebo across visits on Day 0, Day 21 and Day 42.

The primary endpoint counted adults age 18 to 64 years with an HAI antibody titer of 1:40 or greater against the 2009 H1N1 virus 21 days after their first H1N1 vaccination. The posted group counts ranged from 82 of 94 to 91 of 99. Recalculated unadjusted comparisons against Group 1 have confidence intervals that include zero.

The registry record lists additional antibody outcomes and serious adverse event counts, but no reported endpoint analyses. It does not report the trial's original sample-size assumptions, analysis plan, multiplicity approach or detailed missing-data strategy. The posted results support descriptive interpretation of antibody counts and uncertainty, not a conclusion about clinical influenza outcomes.