Safety, Tolerability and Long-term Immunogenicity of Different Formulations of a Chikungunya Vaccine (V184-005): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Safety, Tolerability and Long-term Immunogenicity of Different Formulations of a Chikungunya Vaccine (V184-005) (NCT03635086), a randomized phase 2 study. The primary outcome is Geometric Mean Titer of Anti-Chikungunya Antibodies as Measured by 50% Plaque Reduction Neutralization Test 28 Days After Last MV-CHIK Vaccination.

2. Clinical Question

3. Trial Design

This completed phase 2 trial was randomized, quadruple masked, and used a parallel-group design. It enrolled 60 healthy volunteers. The registry record gives a study start date of 2018-08-22 and a primary completion date of 2019-01-25. The sponsor was Themis Bioscience GmbH. The official title describes an observer-blinded study investigating safety, tolerability, and long-term immunogenicity of different dose regimens and formulations of MV-CHIK.

Five groups received distinct vaccination schedules. Groups A through D received two vaccinations, on day 0 and day 28. These were, respectively, low-dose lyophilized MV-CHIK, low-dose liquid frozen MV-CHIK, low-dose liquid MV-CHIK with SPS®, and high-dose liquid frozen MV-CHIK. Group E received high-dose liquid frozen MV-CHIK on day 0 and placebo on day 28. Thus, the study included both formulation comparisons and a schedule in which the second administration was placebo. The registry record does not report the allocation ratio or the planned number assigned to each group.

The primary endpoint was assessed 28 days after the last vaccination, up to Day 56. The primary analysis included randomized participants who received at least one investigational medicinal product and had no major protocol deviation that could affect immune response. The record lists 11 participants for each of Groups A, B, D, and E, and 9 for Group C in the primary endpoint results. The registry record does not explain the differences between these endpoint counts and the 60 participants enrolled, or report details of participant flow beyond the listed analysis population.

In addition to antibody titers, the study included safety, laboratory, and cellular immune outcomes. The registry record describes assessments of solicited and unsolicited adverse events, serious adverse events, anti-chikungunya antibody titers, and several chikungunya virus-specific T-cell measures. It does not report the full protocol, the planned allocation process, or details of masking procedures beyond the stated quadruple masking.

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: After completion of screening procedures, participants will be randomized to one of five treatment groups (A, B, C, D or E).

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: The analysis population included all randomized participants who received at least one investigational medicinal product (IMP) administration and had no major protocol deviation that could have had an impact on their immune response.

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 Geometric Mean Titer of Anti-Chikungunya Antibodies as Measured by 50% Plaque Reduction Neutralization Test 28 Days After Last MV-CHIK Vaccination, measured over 28 days after last vaccination (Up to Day 56).

The primary endpoint was the geometric mean titer (GMT) of functional anti-chikungunya antibodies measured by the 50% plaque reduction neutralization test, PRNT50. Serum was collected 28 days after the last vaccination, with the assessment described as occurring up to Day 56. PRNT50 measures the serum dilution associated with a specified reduction in viral plaque formation in the test. In this trial, the resulting titer was used as a laboratory measure of neutralizing antibody response.

A geometric mean summarizes titers on a multiplicative rather than an additive scale. This is useful for measurements such as antibody titers, which are commonly analyzed after logarithmic transformation. The registry record says the investigators analyzed log10-transformed titers using an analysis of variance that included treatment group. They then converted estimates back to the original scale to report GMTs, differences between least-squares means, and confidence intervals. Pairwise geometric mean titer ratios were also estimated.

The endpoint helps describe the magnitude of antibody responses among the analyzed participants at the specified assessment time. It is an immunogenicity measure, not a direct clinical outcome. The registry record does not report a protective threshold, a link between these titers and clinical protection, or results from a clinical efficacy endpoint. The word “long-term” appears in the study title, but the primary endpoint described in the record is measured 28 days after the last vaccination; longer-term endpoint timing and results are not reported in the registry record presented for this analysis.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary endpoint analysis used an analysis of variance (ANOVA) on log10-transformed antibody titers, with treatment group as a factor. In general, logarithmic transformation is used when measurements are positive and differences are more naturally interpreted proportionally than additively. On the log scale, multiplicative relationships between original-scale titers become additive relationships, which can be modeled using standard linear-model methods. The registry record does not report additional model terms, assumptions checks, or covariate adjustments.

Why report geometric means?

In general, the arithmetic mean gives equal weight to additive differences, while the geometric mean is suited to quantities that vary multiplicatively. For antibody titers, one group’s level may be meaningfully described as a multiple of another group’s level. Taking logarithms before analysis and converting estimates back to the titer scale produces geometric means and ratios that are easier to interpret on the original scale. The registry record states that the least-squares means and their differences were transformed back by taking anti-logs.

What does a confidence interval describe?

In general, a confidence interval expresses uncertainty around an estimated quantity under the statistical model and analysis procedure. It is not a guarantee that the true value lies inside a particular interval, and a wider interval indicates less precision. The group GMT confidence intervals are broad in relation to their point estimates. For pairwise ratio estimates, a confidence interval that includes 1 is compatible with no ratio difference as well as with values on either side of 1. The reported intervals should therefore be considered alongside the point estimates and adjusted p-values, rather than treating a single number as definitive.

How were groups compared?

The registry record says GMT ratios and related estimates were derived from the ANOVA on log10-transformed data. Pairwise p-values were adjusted for multiple comparisons using the Tukey-Kramer procedure. In general, making several comparisons increases the chance of finding an apparently unusual result by chance if each is treated as an isolated test. A multiple-comparison procedure accounts for the family of comparisons specified for the analysis. The adjusted results are therefore the relevant reported inferential results for these pairwise contrasts. The registry record does not provide further detail on the comparison family beyond the stated Tukey-Kramer adjustment.

The primary result includes both group GMTs with confidence intervals and pairwise ratio estimates with confidence intervals and adjusted p-values. These answer related but distinct questions. The GMTs describe estimated antibody levels within each group; the ratios compare levels between two groups. The group confidence intervals should not be used as a substitute for the reported pairwise analysis. Conversely, a ratio estimate does not communicate the absolute titer levels by itself.

What should be made of the sample sizes?

The primary analysis group sizes were small, with 9 to 11 participants listed per group. In general, a small sample can yield imprecise estimates, reflected in wide confidence intervals, and can limit the ability to distinguish differences between groups. A non-significant test does not establish equivalence or prove that there is no difference. The registry record does not report a sample-size justification or a prespecified target difference for the primary endpoint. Any planning illustrations associated with the posted results are based on assumptions and observed effects, not evidence that the study had a particular probability of detecting an effect before it was conducted.

There are also distinct analysis populations across the record. The primary population is defined as randomized participants who received at least one investigational medicinal product and had no major protocol deviation considered capable of affecting immune response. The adverse-event summaries list 12 participants at risk in each group. These counts should not be treated as interchangeable denominators: they refer to different outcome analyses and stated populations. Reasons for any difference in counts are not reported in the registry record.

What is not specified?

The registry record does not report the complete statistical analysis plan, the handling of missing antibody measurements, the imputation method if any, or sensitivity analyses. It also does not report model diagnostics, the distribution of individual titers, or whether any additional analysis populations were used for the primary endpoint. General statistical interpretation depends on such choices, so the reported estimates are best understood within the analysis description that is available. The endpoint was measured at 28 days after the last vaccination, up to Day 56; the record does not establish the statistical method for any later immunogenicity assessment.

10. Analyses Reported by the Sponsor

The registry record’s primary antibody analysis is the sponsor-reported analysis: log10-transformed titers were analyzed by ANOVA, and pairwise comparisons were adjusted using Tukey-Kramer. The reported pairwise ratio estimates, confidence intervals, and p-values should therefore be read as results from that stated analysis. No separate recalculated primary comparison is reported in the registry record for this interpretation.

The adjusted comparison between Groups D and E has a ratio estimate of 5.2, a 95% confidence interval of 1.2 to 23.1, and p = 0.025. The other reported pairwise comparisons have confidence intervals that include 1 and adjusted p-values above 0.05. The sponsor’s analysis is the relevant basis for these primary comparisons; a simple comparison based only on the displayed group summaries would not reproduce the log-scale ANOVA and Tukey-Kramer procedure.

For secondary outcomes, the posted summaries include some simple unadjusted comparisons, while the registry record does not list sponsor-reported analyses for those outcomes. Those summaries do not supersede the sponsor’s primary analysis, and any unadjusted secondary comparison should be interpreted as exploratory rather than as a confirmatory finding. No discrepancy between a sponsor-reported primary result and a recalculated primary result is reported in the registry record.

11. Multiplicity, Missing Data and Analysis Populations

For the primary antibody endpoint, the registry record states that p-values comparing GMTs between treatment groups were adjusted for multiple comparisons according to Tukey-Kramer. This is relevant because the five-group design allows several pairwise contrasts. In general, adjustment helps control the risk of a false-positive result across a defined family of comparisons. The reported comparison results should be read as adjusted results, not as a set of independent unadjusted tests. The record does not provide further detail on whether other primary analyses or endpoints were included in a broader testing family.

For secondary outcomes, the registry record lists multiple safety and immunogenicity measures, but does not report a comprehensive multiplicity adjustment or testing hierarchy for them. The presence of many outcomes means that isolated secondary findings should be interpreted cautiously. The record does not report which secondary outcomes, if any, were intended as confirmatory or how their analyses were ordered.

The primary analysis population is described as all randomized participants who received at least one investigational medicinal product and had no major protocol deviation that could have affected immune response. This is a defined analysis population, but the registry record does not report a full account of exclusions, protocol deviations, or the number excluded for each reason. It does not report how missing primary endpoint measurements were handled, whether any values were imputed, or whether sensitivity analyses assessed the impact of missing data. Those details matter because missing measurements or exclusions can affect how representative an analyzed group is of the randomized group.

The reported primary endpoint group sizes are 11, 11, 9, 11, and 11. The adverse-event summaries state that 12 participants were at risk in each group. The registry record does not explain these differences, so they should not be interpreted as evidence of a particular pattern of withdrawal or missingness. The sample-size justification, interim analyses, and fuller statistical analysis plan are not reported in the registry record. Without those details, it is not possible to establish from the record whether all analysis choices were specified before outcomes were examined.

12. 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 non-serious adverse event terms

The serious adverse event summaries list 12 participants at risk in each of the five groups. No serious adverse events were reported in any group, and deaths were recorded as 0 in every group. These are counts observed in the stated safety summaries. They describe what was reported during the available assessment period; they do not establish that the risk of a serious event is absent.

When an outcome is uncommon, observing no events in a small group can still leave substantial uncertainty about the underlying event rate. In general, the number of participants and duration of observation both affect how much information a study provides about rare harms. The registry record does not report a longer-term safety follow-up duration in these serious adverse event summaries, nor does it provide detailed serious-event terms, since the list of top serious terms is empty.

The absence of reported serious events should be considered alongside the study’s small group sizes and the fact that this was a phase 2 study. It is not a guarantee of safety, and it does not resolve questions about uncommon or delayed events. The record also lists assessments of solicited and unsolicited adverse events, but those are distinct from the serious adverse event counts.

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

The primary results describe measured PRNT50 antibody titers 28 days after the last vaccination in the analyzed participants. The group GMTs varied, with the highest point estimate in the two-dose high-dose liquid frozen group and the lowest in the group receiving one high-dose vaccination followed by placebo. The sponsor-reported pairwise analysis found a statistically significant adjusted contrast between those two groups; the other reported pairwise contrasts were not statistically significant at the stated threshold. Confidence intervals, especially for ratios, were broad, so the estimated magnitude of differences remains uncertain.

These findings do not establish clinical benefit or lack of benefit. PRNT50 is a laboratory measure of functional antibodies, and the registry record does not report a protective antibody threshold or a clinical efficacy endpoint linking these titers to chikungunya illness. The measured response at this time point also cannot establish how long antibodies persist. Although long-term immunogenicity appears in the study title, longer-term results are not reported in the primary endpoint description.

A non-significant pairwise comparison should not be interpreted as proof that two regimens are equivalent. The study groups were small, and the confidence intervals allow a range of plausible values. Likewise, the statistically significant Group D versus Group E result does not by itself prove a clinical difference or establish the exact size of a true response difference. It is evidence about the estimated antibody ratio under the reported analysis and within this study population.

The safety summaries report no serious adverse events or deaths among the participants at risk, but such observations cannot rule out uncommon or delayed harms. The findings are best understood as a description of a small randomized study’s measured immunogenicity and reported safety outcomes, not as a comprehensive assessment of clinical effectiveness or safety in broader populations.

14. Why This Trial Matters Statistically

This trial is statistically informative because it compares several vaccine formulations and schedules within one randomized parallel-group study. The five-group design permits contrasts across low-dose formulations, a high-dose formulation, and a schedule that replaces the second high-dose vaccination with placebo. That structure makes it possible to examine specific regimen contrasts, while also creating a need to distinguish among multiple comparisons rather than relying on an overall ranking.

The endpoint illustrates why the analysis scale matters. Antibody titers are reported as geometric means, with ANOVA performed on log10-transformed measurements and pairwise ratios estimated from the fitted model. This connects the scientific question about proportional differences in titers to an analysis appropriate for multiplicative measurements. The registry’s use of Tukey-Kramer adjustment also makes the multiplicity issue visible: inference across several pairwise comparisons differs from interpreting each contrast in isolation.

The study also shows why effect estimates and uncertainty should be read together. The Group D versus Group E comparison has a confidence interval that excludes 1, while still spanning a broad range of possible ratios. Other comparisons have intervals that include 1. The contrast between these findings demonstrates that statistical significance and precision are related but separate considerations. Small groups can yield informative estimates, but often with considerable uncertainty.

Finally, the record highlights the boundaries of registry interpretation. It identifies the primary analysis population and method, but does not report a complete statistical analysis plan, missing-data handling, or sample-size justification. Recognizing what is reported and what is not is part of sound statistical reading. The results are useful for understanding the measured immune response under the trial’s stated regimens, while leaving clinical protection, durability, and broader safety questions unresolved in this record.

15. 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 analysis involves small groups, with 9 to 11 participants listed per group. The confidence intervals around group GMTs and pairwise ratios are wide, limiting precision. The record does not report a sample-size justification, so it is not possible to assess the planned sensitivity to a particular difference or the rationale for the enrolled number from the registry information alone.

The primary endpoint is a laboratory immunogenicity measure at 28 days after the last vaccination, up to Day 56. The registry record does not report a clinical efficacy outcome, a validated protective threshold, or a direct connection between measured titers and clinical protection. Nor does the primary endpoint result establish how antibody responses persist beyond the stated time point. The study title refers to long-term immunogenicity, but longer-term endpoint timing and results are not reported in the primary results.

There are several groups and pairwise comparisons. Tukey-Kramer adjustment is stated for the primary comparisons, but the registry record does not report a broader multiplicity plan across primary and secondary outcomes. Secondary outcome testing should therefore be interpreted cautiously. The record also does not report details of missing-data handling, sensitivity analyses, or the full flow from enrollment to each analysis population. Although the primary population is defined, reasons for differences between the primary endpoint counts and the adverse-event denominators are not reported.

The posted summary does not give the full protocol, model diagnostics, individual participant data, or detailed masking and randomization procedures. These limits constrain independent assessment of assumptions and robustness. The conclusions should remain specific to the measured outcomes, analysis populations, and follow-up reported in the registry record.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Safety, Tolerability and Long-term Immunogenicity of Different Formulations of a Chikungunya Vaccine (V184-005) — NCT03635086.
• Registry record last updated 2021-10-22; status last verified 2021-10.
• Results first posted 2021-10-20.
• 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.

19. Record Summary

NCT03635086 is a completed randomized phase 2 parallel-group study of MV-CHIK in 60 healthy volunteers. It compared five dose and formulation schedules, with quadruple masking reported. The primary outcome was PRNT50 geometric mean antibody titer measured 28 days after the last vaccination, up to Day 56.

The primary analysis used log10-transformed titers in ANOVA and reported Tukey-Kramer-adjusted pairwise comparisons. The comparison between two high-dose liquid frozen vaccinations and one high-dose vaccination followed by placebo had a ratio estimate of 5.2, a 95% confidence interval of 1.2 to 23.1, and p = 0.025. Other reported pairwise comparisons were not statistically significant. No serious adverse events or deaths were reported in the safety summaries.

The group sizes were small, and the confidence intervals were broad. Clinical protection, longer-term persistence, missing-data handling, and a sample-size justification are not reported in the registry record.