Safety and Immunogenicity of GPO Seasonal Trivalent Inactivated Influenza Vaccine in Healthy Thai Adults: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Safety and Immunogenicity of GPO Seasonal Trivalent Inactivated Influenza Vaccine in Healthy Thai Adults (NCT02894840), a randomized phase 1, phase 2 study. The primary outcome is Number of Participants With Adverse Events.

2. Clinical Question

The statistical question is how the recorded frequency of participants with adverse events differed across these groups, and how precisely the registry results estimate those differences. The posted risk-difference estimates are unadjusted comparisons; they are not adjusted for other participant characteristics. The Phase I vaccine comparison uses the Phase I placebo group, while both Phase II estimates are also referenced to that Phase I placebo group. The registry record does not post a direct Phase II vaccine-versus-Phase II placebo estimate. This matters when interpreting the estimates: they are not all contrasts between groups enrolled in the same phase.

Immunogenicity was also part of the study aim. The registry lists three secondary outcomes concerning seroconversion or significant antibody-titer increases and geometric mean immune responses. Numerical results for those outcomes are not reported in the registry record. The interpretation of posted results therefore centers on the adverse-event counts and their uncertainty, rather than on vaccine-induced antibody responses.

3. Trial Design

The study is listed as a randomized, double-blind, controlled, parallel-group Phase I/II trial. Its title identifies the study population as healthy Thai adults aged 18 to 49 years, and the stated study aim was to assess safety and immunogenicity of a single dose of seasonal trivalent inactivated split virion influenza vaccine. The registry status is completed. The recorded start is 2015-11 and the primary completion is 2016-07.

The registry lists enrollment of 340 participants. The four reported groups comprise 20 participants assigned to vaccine in Phase I, 20 assigned to placebo in Phase I, 200 assigned to vaccine in Phase II, and 100 assigned to placebo in Phase II. These group sizes sum to the recorded enrollment, but the record does not give further details about the sequence of enrollment, randomization procedures, or how allocation was implemented. Those procedures are not reported in the registry record.

Double masking indicates that the record describes the trial as double-blind, but it does not identify which individuals were masked or how masking was maintained. The placebo description says it was administered intramuscularly, with the deltoid of the non-dominant arm preferred. The intervention description does not separately state the vaccine administration route in the available record. The vaccine is described as one dose containing 15 μg HA per strain per dose.

Phase I and Phase II results are reported as separate groups, but the posted statistical comparisons use the Phase I placebo group as the reference for the vaccine group in Phase I and both groups in Phase II. The record does not provide a direct Phase II comparison of vaccine with Phase II placebo. Nor does it explain the rationale for the group sizes or the relationship between the phases beyond their labels and arm assignments. Original design assumptions, including any planned sample-size justification, are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: A total of 340 participants (40 in phase I and 300 in phase II) who met all inclusion criteria and not of the exclusion criteria were enrolled in the study.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: All participants who receive at least one vaccination would be included in the safety population. The analysis will be conducted based on intent-to-treat (ITT) analysis. The population for the ITT analysis is defined as all individuals in the trial. The ITT analysis would include individuals who may not complete follow-up.

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 With Adverse Events, measured over 90 days.

The primary outcome is the number of participants with adverse events, assessed over 90 days. Its unit is participants, so the endpoint counts people who experienced an event rather than the total number of events. The registry description says all adverse events during the 90-day period would be analyzed as a percentage and considered in relation to study vaccine. The posted results give the number with events in each group: 5 of 20 in Phase I vaccine, 8 of 20 in Phase I placebo, 69 of 200 in Phase II vaccine, and 30 of 100 in Phase II placebo.

A participant-based safety outcome is useful because it describes how many people in each group experienced an event, without allowing multiple events in one person to count as multiple participants. It can therefore communicate the frequency of participants affected during the specified follow-up period. It does not, on its own, describe the number, severity, timing, or type of each event, nor establish whether a particular event was caused by the intervention.

The registry does not give a detailed definition of which events qualified for this primary outcome, how event severity or relatedness was assessed, or how participants with incomplete follow-up were handled. Those details are not reported in the registry record. The separately posted adverse-event summaries list no serious adverse events, but those summaries do not replace the primary count of participants with any adverse event.

7. Results Posted in the Registry

• Inactivated Influenza Vaccine Phase I: 25.0% (5/20) versus 40.0% (8/20) on control; risk difference -15.0 percentage points (95% CI -43.7 to 13.7), risk ratio 0.62 (0.25–1.58); unadjusted two-proportion z-test p = 0.311.
• Inactivated Influenza Vaccine Phase II: 34.5% (69/200) versus 40.0% (8/20) on control; risk difference -5.5 percentage points (95% CI -28.0 to 17.0), risk ratio 0.86 (0.49–1.52); unadjusted two-proportion z-test p = 0.623.
• Placebo Phase II: 30.0% (30/100) versus 40.0% (8/20) on control; risk difference -10.0 percentage points (95% CI -33.3 to 13.3), risk ratio 0.75 (0.41–1.39); unadjusted two-proportion z-test p = 0.380.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is binary at the participant level: each participant either had at least one adverse event during 90 days or did not, according to the outcome definition used for the posted results. The arm counts are the number of participants recorded with events, alongside the group sizes. For this type of outcome, a risk is the proportion of participants with the event in a group. The posted comparisons express the difference between the two group risks as a risk difference.

What does a risk difference describe?

A risk difference compares event risks by subtracting the reference-group risk from the other group's risk. A value below zero means the estimated risk in the group being compared is lower than in the reference group; a value above zero means it is higher. The registry comparisons report risk differences of -0.15 for the Phase I vaccine group, -0.055 for the Phase II vaccine group, and -0.1 for the Phase II placebo group. Each comparison uses the Phase I placebo group as its reference. These are unadjusted estimates based on the posted group summaries.

Risk differences are expressed on the absolute risk scale. They can be useful for describing how far apart the estimated event frequencies are, but their precision depends on the amount of information in both groups. Small groups generally leave more uncertainty in estimates than larger groups. The Phase I comparison has 20 participants in each group, while the Phase II comparisons use a Phase I reference group of 20 participants. The width of the reported intervals reflects that limited reference-group information, among other factors.

How should confidence intervals be read?

A confidence interval gives a range of values reasonably compatible with the observed data under the statistical method and its assumptions. It conveys uncertainty about an estimate, not a guarantee that the true value lies within the particular interval. If an interval for a risk difference includes zero, the data are compatible with no difference as well as with differences in either direction at the interval's stated confidence level. All three posted intervals include zero.

For Phase I vaccine versus Phase I placebo, the estimated risk difference is -0.15 and the interval is -0.437 to 0.137. For Phase II vaccine versus Phase I placebo, the estimate is -0.055 and the interval is -0.280 to 0.170. For Phase II placebo versus Phase I placebo, the estimate is -0.1 and the interval is -0.333 to 0.133. The intervals are broad, so the results do not pinpoint a narrow range of plausible risk differences.

What do the p-values add?

The posted unadjusted p-values are 0.311, 0.623, and 0.38 for the three comparisons, respectively. A p-value is calculated under a null hypothesis, commonly that the compared risks are equal, and summarizes how incompatible the observed data are with that hypothesis according to a specified statistical procedure. It is not a measure of the probability that chance alone caused the results, nor does it quantify the size of an effect. The registry does not describe the test procedure in detail; the analysis method underlying these p-values is not reported in the registry record.

Risk ratios and odds ratios are also listed with confidence intervals. These are alternative ways to compare binary outcomes. A risk ratio compares the risks, while an odds ratio compares the odds of an event. They are not interchangeable with risk differences: the risk difference gives an absolute contrast, while the ratio measures express a relative contrast. Because the intervals for these measures also span values consistent with no relative difference, they likewise do not provide a precise estimate of a group contrast. Their values should be read only for the specific comparisons and reference groups stated in the registry results.

Why does the comparison structure matter?

The Phase I vaccine group is compared with Phase I placebo. In contrast, the Phase II vaccine and Phase II placebo groups are each compared with Phase I placebo, rather than with one another. This distinction is important because a direct randomized contrast within Phase II would ordinarily use the Phase II placebo group as its comparator. No such Phase II vaccine-versus-placebo result is posted. Comparisons across different phases may also reflect differences in the populations or circumstances of the phases; the registry record does not supply enough information to assess that possibility.

The outcomes are counts of participants with events. The number of participants with an event is not the same as an event rate per unit of follow-up time, and no time-to-event estimate is posted. The record does not report the timing or recurrence of events. It also does not report enough detail to determine whether every participant completed the full 90-day observation period or how incomplete observations were handled.

The registry lists four outcome measures and three secondary outcomes, but it posts no analyses for the secondary outcomes. It does not describe any adjustment for multiple comparisons, the exact statistical model, or other prespecified analysis details. These elements should not be inferred from the posted counts or p-values. In general, a statistical result depends not only on the numerical data but also on the outcome definition, comparison, analysis population, missing-data handling, and method used to quantify uncertainty.

10. Confidence Intervals and Precision

Precision concerns how narrowly the results locate the underlying group differences. The posted risk-difference estimates are -0.15 for Phase I vaccine compared with Phase I placebo, -0.055 for Phase II vaccine compared with Phase I placebo, and -0.1 for Phase II placebo compared with Phase I placebo. Their respective confidence intervals are -0.437 to 0.137, -0.280 to 0.170, and -0.333 to 0.133.

Each interval spans zero and values on both sides of zero. Thus the posted data are compatible with lower as well as higher event risks in the compared group. The intervals also cover a range of magnitudes, rather than locating each contrast narrowly. This is especially relevant to comparisons using the Phase I placebo group, which has 20 participants. Even when another group is larger, a small reference group can limit precision for the contrast.

A confidence interval is not a range of individual participant outcomes, and it does not show the probability that the true difference is inside the interval. It describes uncertainty in an estimated group contrast under the method used. Its width depends on factors including the group sizes, the observed event frequencies, and the statistical procedure. The registry record does not give sufficient detail to fully evaluate the procedure used for these intervals.

The relative measures have broad intervals as well. For example, the Phase I risk ratio is 0.625 with an interval from 0.247 to 1.584; the Phase II vaccine risk ratio is 0.863 with an interval from 0.488 to 1.525. These estimates are not a substitute for the absolute risk differences, and their intervals also indicate uncertainty. Taken together, the posted estimates do not tightly determine the direction or size of the contrasts.

11. 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 sample-size figures are planning illustrations based on the observed effects, not the trial's original design justification. For these binary-outcome comparisons, the figures are participants per arm, not total events. The planning assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set at the stated fraction of the observed effect. The original trial sample-size assumptions are not reported in the registry record.

The 80% power column gives the estimated number of participants per arm for a hypothetical study to have an 80% chance of detecting the specified effect under the stated assumptions, if that effect and the assumptions were correct. The 90% power column gives the corresponding planning number for a 90% chance. These are design calculations, not statements about the chance that the trial's conclusions are correct. This is not observed power.

Rows planning for 100% of the observed effect use the full observed estimate as the target difference. Rows planning for 75% or 50% use smaller target effects. Smaller effects are harder to distinguish from random variation and therefore require more participants. This pattern appears across the posted planning figures. For the Phase I comparison, for example, the listed participants per arm for 80% power increase from 152 when planning for 100% of the observed effect to 279 for 75% and 644 for 50%. The corresponding 90% figures are 203, 373, and 862.

The same approach is applied to the other posted contrasts, each with its own observed estimate. Because these are illustrations based on observed estimates, they should not be treated as a recommended design or as evidence that the completed study was adequately or inadequately sized. They also inherit the limitations of the comparisons, including the Phase I placebo reference used for the Phase II estimates.

12. 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 = 0.400; treatment proportion p₁ = 0.250; average p̄ = 0.325.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.6624 = 1.2982.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.6538 = 0.5503.
4. n = (1.2982 + 0.5503)² / (-0.1500)² = 151.9, rounded up to 152 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 <- 0.4                          # control proportion
p1 <- 0.25                          # 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(2894840)
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 = 152 (80% power), 203 (90% power)
 75% of observed effect: n per arm = 279 (80% power), 373 (90% power)
 50% of observed effect: n per arm = 644 (80% power), 862 (90% power)
power.prop.test: 151.9 
Simulated power at n = 152 per arm: 0.800 (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.

13. Multiplicity, Missing Data and Analysis Populations

The registry lists four outcome measures in total and identifies three secondary outcomes concerning antibody responses: seroconversions or significant increases in antihemagglutinin antibody titer, geometric mean immune response at each assessment, and geometric mean immune-response increases greater than 2.5 from baseline for the listed strains. Numerical results for these secondary outcomes are not reported in the registry record. The record lists zero posted analyses.

When several outcomes or comparisons are examined, the chance of obtaining at least one apparently notable result can increase. Statistical methods can account for multiple comparisons, but the registry does not say whether any adjustment was planned or used. The multiplicity strategy is not reported in the registry record. The unadjusted p-values should therefore be understood as individual comparison results, not as results adjusted across outcomes or contrasts.

The registry describes an intent-to-treat analysis population as all individuals in the trial, but the description is truncated and does not fully explain how participants who did not receive vaccination, withdrew, or had missing outcome data were handled. It also states that the safety population includes all participants who received at least one vaccination. How the primary outcome analysis reconciled these population definitions is not reported in the registry record.

The number of participants with adverse events is posted for each group, but information about missing observations, follow-up completeness, and any imputation or exclusion procedures is not reported in the registry record. The record also does not state whether the posted analysis was prespecified, whether interim analyses occurred, or whether a statistical analysis plan was used. None of those details can be inferred from the completed status or from the posted estimates.

These gaps matter because the analysis population and missing-data approach can affect the estimated event frequencies, while multiplicity affects how a collection of tests should be interpreted. The available results support describing the posted counts and estimates, but not reconstructing unreported analysis decisions.

14. 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.

The adverse-event summary lists 0 serious adverse events in each group: among 20 participants in Phase I vaccine, 20 in Phase I placebo, 200 in Phase II vaccine, and 100 in Phase II placebo. It also lists 0 other adverse events in each group and 0 deaths in each group. The corresponding numbers at risk are the same group sizes: 20, 20, 200, and 100.

These safety-summary entries are distinct from the primary outcome counts, which report 5, 8, 69, and 30 participants with adverse events across the four groups. The serious-event summary does not mean that no participant had any adverse event; it records zero in the serious and other categories shown in that summary. The record does not provide further details about how those categories relate to the primary outcome definition.

Zero recorded events in these categories is a description of the posted results, not proof that such events cannot occur. The number of participants observed, the duration of observation, and the event definitions all shape how much can be concluded. The registry does not provide event-level details, adjudication procedures, or further information about the completeness of safety follow-up.

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

The results describe the number of participants recorded with adverse events during the 90-day period and the posted unadjusted contrasts. The Phase I groups had counts of 5 of 20 for vaccine and 8 of 20 for placebo. The Phase II groups had counts of 69 of 200 for vaccine and 30 of 100 for placebo. The estimated risk differences are negative in all three posted comparisons, but each corresponding confidence interval includes zero and extends to values in the opposite direction.

These estimates do not establish that event risks were identical, nor do they precisely establish a particular difference. The confidence intervals indicate considerable uncertainty, and the unadjusted p-values do not give the probability that a treatment effect is present or absent. The data are best read as a set of group counts and imprecise contrasts, not as a definitive estimate of a single vaccine-versus-placebo effect across both phases.

The posted Phase II estimates compare each Phase II group with Phase I placebo, rather than directly comparing Phase II vaccine with Phase II placebo. This limits the interpretation of the Phase II results as treatment contrasts. The registry record does not explain the rationale for those choices or provide a direct Phase II comparison.

The primary outcome concerns whether participants experienced adverse events, not the total number, timing, seriousness, or cause of each event. The separate safety summary records no serious adverse events or deaths, but the available record does not describe the event categories in enough detail to reconcile them fully with the primary outcome counts. Numerical results for immunogenicity are also not reported in the registry record.

Accordingly, the posted results address a limited statistical question about recorded participant-level events and their estimated group contrasts. They do not establish causation, resolve all safety questions, or provide a numerical answer about antibody response. The registry does not report sufficient detail about missing data, outcome definitions, or analysis procedures to remove these uncertainties.

16. Why This Trial Matters Statistically

This trial illustrates why the comparison used to calculate an effect estimate matters as much as the estimate itself. The Phase I vaccine and placebo groups form a comparison within the same phase. The Phase II vaccine and placebo groups, however, are each compared with Phase I placebo in the posted results. Those contrasts answer different questions from a direct Phase II vaccine-versus-placebo comparison, which is not reported.

The example also shows why participant counts should be considered alongside uncertainty. The primary results give counts of participants with adverse events, while the confidence intervals describe how uncertain the corresponding risk differences are. Each interval includes zero and covers differences in both directions. A point estimate alone would not communicate that uncertainty.

Planning calculations based on an observed effect are another useful teaching point. The registry's planning illustrations show that smaller target effects require more participants per arm under the stated assumptions. They are not observed power and do not explain the trial's original design. The original sample-size rationale is not reported in the registry record.

Finally, a completed registry record can contain group counts without enough detail to assess every analytical decision. Here, the registry lists secondary antibody outcomes but no corresponding analyses, and it does not report a multiplicity approach or sufficient detail about missing observations and analysis populations. For clinical researchers, these omissions underscore the value of clearly defined outcomes, explicit comparison groups, complete analysis descriptions, and uncertainty intervals alongside point estimates.

17. 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.

Several limitations of the posted record affect interpretation. The most important is the comparison structure: the Phase II vaccine and placebo groups are each contrasted with Phase I placebo, rather than being directly compared with one another. This means the posted Phase II estimates do not give a direct within-phase randomized contrast. The rationale for using the Phase I placebo group as the reference is not reported in the registry record.

The Phase I placebo group includes 20 participants, and all posted comparisons use it as the reference. The confidence intervals are broad, including zero and values on both sides. Thus the results do not precisely locate the risk differences. The sample-size illustrations use the observed effect and do not substitute for an original design justification, which is not reported in the registry record.

The outcome description is brief. It does not fully define qualifying adverse events, specify how severity or relatedness was assessed, or explain how incomplete follow-up was handled. The primary analysis population description is truncated; the record mentions both an intent-to-treat population and a safety population, but does not completely describe their use in the posted analysis. Missing-data procedures are not reported in the registry record.

Three secondary immunogenicity outcomes are listed, but numerical results are not reported in the registry record. A multiplicity strategy, detailed statistical test, and any analysis plan are also not reported. The adverse-event summary lists zero serious events and deaths, but further event-level details and the relationship between that summary and the primary adverse-event counts are not given.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Safety and Immunogenicity of GPO Seasonal Trivalent Inactivated Influenza Vaccine in Healthy Thai Adults — NCT02894840.
• Registry record last updated 2020-03-17; status last verified 2020-03.
• Results first posted 2018-09-19.
• 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.

21. Record Summary

The registry describes NCT02894840, a completed randomized, double-blind, parallel Phase I/II study of one dose of GPO seasonal trivalent inactivated split virion influenza vaccine in healthy Thai adults aged 18 to 49 years. Recorded enrollment was 340. The primary outcome was participants with adverse events during 90 days.

Posted counts were 5 of 20 in Phase I vaccine, 8 of 20 in Phase I placebo, 69 of 200 in Phase II vaccine, and 30 of 100 in Phase II placebo. Three unadjusted risk-difference comparisons are reported, all using Phase I placebo as the reference. Their confidence intervals include zero. The record also lists no serious adverse events or deaths in the four groups.

Secondary immunogenicity outcomes are listed, but numerical analyses are not reported in the registry record. The Phase II vaccine-versus-Phase II placebo contrast, original sample-size justification, detailed missing-data procedures, and multiplicity approach are also not reported in the registry record.