A Study to Learn About the Safety and Immune Activity of RSVpreF in Children 2 to <18 Years of Age: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study to Learn About the Safety and Immune Activity of RSVpreF in Children 2 to <18 Years of Age (NCT05900154), a randomized phase 1 study. The primary outcome is Percentage of Participants With Local Reactions Within 7 Days After Vaccination.

2. Clinical Question

The clinical question is therefore primarily a dose-finding and early safety question, not a test of whether vaccination prevents RSV illness. The endpoint addresses short-term reactions at the injection site. Immune measures were listed as secondary outcomes: neutralizing antibody titers against RSV A and RSV B before vaccination and one month after vaccination, the geometric mean fold rise in those titers, and frequencies of RSV F antigen-specific CD4+ T cells expressing IFN gamma and IL-4. These measures can describe immune responses, but the registry record does not report their results. The study included children aged 2 to <5 years and children aged 5 to <18 years, with the latter further described as healthy or having chronic high-risk conditions. The record identifies randomized allocation and a sequential model, but does not specify the dose-assignment procedure or the intended statistical comparison. Those details are not reported in the registry record.

3. Trial Design

This was a Phase 1, randomized, open-label, age-descending, dose-finding study of RSVpreF. The registry lists a sequential study model and no masking. Enrollment was 128. The study is marked completed; its recorded start date was 2023-06-22 and its primary completion date was 2024-02-29. The sponsor was Pfizer.

Participants received one injection of RSVpreF at 120 µg or 60 µg. The six described groups distinguish dose, age, and health status: children aged 2 to <5 years, healthy children aged 5 to <18 years, and children aged 5 to <18 years with chronic high-risk conditions. The descriptions identify 120 µg as the standard dose and 60 µg as the low dose. The registry record does not explain how participants were allocated among groups, how the age-descending sequence was implemented, or whether dose assignment was randomized within each age and health-status group.

The brief study description states that there were four visits, including two in-clinic visits and two telehealth visits, with blood samples collected for testing. Each participant’s involvement was described as about six months, and the study was to be conducted in the United States. Local reactions were recorded in an electronic diary from Day 1 through Day 7 after vaccination. The registry does not report further details of follow-up completion, adherence to diary reporting, or the timing of safety assessments beyond the specified endpoint window.

The registry describes a Phase 1 study intended to inform dose selection for later trials in this age cohort. It does not report a formal sample-size justification, allocation ratio, planned statistical model, or a prespecified dose-selection rule. The stated enrollment and the recorded group counts in the safety information should not be interpreted as proof that the study was designed to detect a particular difference.

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: Participants were divided into 2 age groups: 5 to less than (\<) 18 years and 2 to \< 5 years. Participants in 5 to \<18 years age group were further stratified as healthy and with high-risk chronic medical conditions.

Vaccination Period

Follow-Up Period

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Safety population included all enrolled participants who received study intervention.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Safety population included all enrolled participants who received the study intervention.

Only the first 6 of 7 groups are shown.

6. Primary Endpoint

The registry lists the primary outcome as Percentage of Participants With Local Reactions Within 7 Days After Vaccination, measured over Day 1 through Day 7 after Vaccination.

The primary endpoint is the percentage of participants with local reactions during Day 1 through Day 7 after vaccination. Reactions were collected in an electronic diary and included injection-site pain, redness, and swelling. The registry describes severity grading for redness and swelling separately by age: participants aged >=2 years to <12 years and participants aged >=12 years had different size thresholds. Pain was graded according to whether it interfered with activity or prevented daily activity.

A percentage is a measure of how common an event was in the defined population during the specified observation period. In this early-phase setting, recording local reactions can characterize short-term tolerability and support discussion of dose levels for later studies. It does not, by itself, establish whether any reaction was caused by vaccination, nor does it answer whether the vaccine prevents RSV infection or illness.

The endpoint specifies the safety population as enrolled participants who received study intervention. The registry record does not report the primary endpoint’s observed percentages, confidence intervals, or a dose-group comparison. It also does not report how participants with incomplete diary entries were handled. Consequently, the endpoint definition is available, but its observed frequency and precision cannot be described from the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry identifies the study as randomized, open-label, age-descending, and sequential. Those labels describe features of the design, but the registry record does not report the operational details of randomization, the sequence of enrollment across age groups, or how dose selection decisions were made. Open-label means masking was not used, as recorded; the registry does not describe additional procedures intended to reduce measurement or reporting bias.

What does the primary percentage measure?

The primary endpoint is a binary event summarized as a percentage: whether a participant had a local reaction during Day 1 through Day 7 after vaccination. The event definition includes pain at the injection site, redness, and swelling. A participant who meets the event definition contributes to the count of participants with a local reaction; the percentage relates that count to the relevant population. This is a general explanation of a binary percentage, not a reported study estimate. The actual percentages and denominators for this endpoint are not reported in the registry record.

The endpoint also includes grading rules. Redness and swelling use different size thresholds for participants aged >=2 years to <12 years and participants aged >=12 years. Pain is graded according to interference with activity. The record does not provide participant-level results by reaction type or severity. It is therefore not possible to describe which local reaction was most frequent or how many reactions fell into each severity grade.

What would a confidence interval add?

A confidence interval expresses uncertainty around an estimate. For a percentage, it gives a range of values compatible with the observed data under the method used to calculate the interval. General statistical interpretation considers both the estimate and its interval: an estimate can be more or less precise depending on the amount of information and the variability in the data. The primary endpoint names a 95% Confidence Interval as its dispersion measure, but the registry record does not report an interval or identify the calculation method. No claim about the precision of the study’s local-reaction estimates can be made from the endpoint definition alone.

How should group comparisons be approached?

The study includes 120 µg and 60 µg groups across age and health-status categories. Comparing percentages between groups would require attention to the populations being compared, the number of participants assessed, and the analysis method. General statistical practice distinguishes descriptive differences from inferential comparisons: a difference in observed percentages is not automatically evidence of a reliable or causal dose effect. In this case, the registry record does not report a primary dose comparison, an adjusted model, or analysis results. No between-dose effect estimate can be interpreted.

The study is described as randomized, but the record does not say whether allocation occurred within each age and health-status stratum or how the sequence of enrollment interacted with randomization. Without that information, the design labels alone do not establish which groups are directly comparable for a specific analysis. The record also does not describe blinding of outcome assessment; it lists masking as none.

What do the secondary immune measures represent?

Geometric mean titers summarize antibody measurements on a multiplicative scale and are commonly used for antibody concentrations that can vary over a wide range. A geometric mean fold rise describes change between measurements on that scale. These are general interpretations of the named measures, not findings from this study. The listed time points include before vaccination and one month after vaccination. The registry record does not report the corresponding estimates, uncertainty intervals, or comparisons.

The registry also lists median frequencies of RSV F antigen-specific CD4+ T cells expressing IFN gamma and IL-4 before vaccination and one month after vaccination. A median describes the central observation after ordering participant values; it does not by itself show the spread or precision of those values. The record does not report the medians or any accompanying analysis.

What is the role of planning calculations?

The planning description specifies a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. It says the effect to detect is a stated fraction of the observed effect. These are assumptions for planning illustrations, not the trial’s reported analysis plan and not evidence of observed power. The registry record does not report the trial’s own sample-size justification or establish that these assumptions governed enrollment. Any planning illustration should be treated as hypothetical and distinct from the observed results.

Over-dispersion is a general issue when observed variability is greater than a model assumes. If a method assumes less variability than the data contain, uncertainty can be understated and statistical conclusions can be too confident. The registry record does not report an over-dispersion assessment or a model for these outcomes, so this general concern cannot be tied to a reported analysis from this study.

10. Multiplicity, Missing Data and Analysis Populations

The registry record lists several secondary immune outcomes and a primary local-reaction endpoint, but it does not report a multiplicity strategy. Multiplicity concerns arise when several outcomes, time points, groups, or comparisons are tested: the chance of at least one apparently notable result can increase across multiple tests. General approaches may define a hierarchy of outcomes or adjust inference across comparisons. Which, if any, approach was planned for this study is not reported in the registry record. No adjustment method or confirmatory testing sequence is described.

Handling of missing data is also not reported in the registry record. The primary endpoint relies on electronic diary entries during Day 1 through Day 7 after vaccination, but the record does not say how incomplete entries were treated, whether participants needed a minimum amount of diary information, or whether any imputation method was used. The status of missing blood samples or incomplete immune measurements is likewise not reported. This matters because estimates may depend on which participants contribute observations and how unavailable measurements are handled.

The endpoint description defines the safety population as all enrolled participants who received study intervention. That is the only analysis population definition given for the primary safety endpoint. The registry record does not report an intention-to-treat population, a per-protocol population, or the analysis population used for immunogenicity outcomes. It also does not report whether all randomized participants received intervention or contributed endpoint data.

The registry lists no statistical analyses. The statistical analysis plan, prespecified handling of protocol deviations, interim analyses, and sample-size justification are not reported in the registry record. The planning assumptions described for illustrations do not fill these gaps: they are not evidence of the trial’s own analysis plan. Without information on multiplicity, missingness, and analysis populations, readers cannot assess how the reported analyses would have addressed those sources of uncertainty.

11. Safety Results

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

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 5% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: asthma (5.6%), food allergy (4.3%).

The serious adverse event counts are listed separately for each of the six dose, age, and health-status groups. One serious adverse event is reported in the High Risk RSVpreF 120 mcg group and one in the High Risk RSVpreF 60 mcg group. Zero serious adverse events are reported for the Healthy RSVpreF 120 Micrograms (mcg), 2 to < 5 Years RSVpreF 120 mcg, Healthy RSVpreF 60 mcg, and 2 to < 5 Years RSVpreF 60 mcg groups. The registry reports zero deaths in each group.

The listed group sizes at risk for serious adverse events are 25, 23, 24, 17, 18, and 20, respectively, in the order named in the registry record. The event counts are descriptive. They do not establish that an event was caused by RSVpreF, and the record does not provide a statistical comparison between groups. With small group counts, a difference in observed counts may be imprecise; general statistical interpretation would require estimates and uncertainty intervals to assess that precision.

The leading serious adverse event terms are listed as Asthma at 5.6% and Food allergy at 4.3%. The registry record does not explain the denominator or provide further details about these terms, including timing, severity, or attribution. These figures should be read as registry-reported descriptive information, not as evidence that treatment caused either condition.

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

The posted information describes an early-phase study intended to assess safety, tolerability, and immune activity of two RSVpreF dose levels in children aged 2 to <18 years. It defines a short-term local-reaction endpoint and identifies the population for that endpoint. It also records serious adverse event counts by group and lists secondary measures of antibody and cellular immune responses.

The record does not report the observed primary endpoint percentages or their confidence intervals. It therefore does not establish the frequency or precision of local reactions in the study groups. It also does not report results for the secondary immune outcomes, so the magnitude of antibody or cellular responses cannot be described from the registry record.

The serious adverse event counts are limited descriptive information. The record lists one event in each of the two high-risk groups and zero in the other listed groups, with zero deaths in every group. These counts do not establish a causal relationship, and no statistical comparison is reported. The listed event terms do not substitute for detailed event narratives or an analysis of attribution.

The study is not described as an efficacy trial, and the listed outcomes do not measure prevention of RSV illness. Its stated purpose was to identify a dose level for Phase 2/3 trials in this age cohort. The registry record does not say what dose was selected or provide the decision criteria. The data described in the record therefore cannot establish whether RSVpreF prevents infection, reduces illness, or has a particular benefit-risk balance.

In general, a trial’s conclusions depend on its design, outcomes, analysis methods, and the precision of its estimates. Here, the registry provides important design and endpoint descriptions, but no reported statistical analyses or outcome estimates for the primary endpoint. Any interpretation should remain within those limits and distinguish the study’s stated aims from results that are actually reported.

13. Why This Trial Matters Statistically

This study is statistically notable for including children across a broad age range, with groups differentiated by age, health status, and dose. Early-phase dose-finding studies can inform the design of later trials by describing how endpoints are defined and by collecting initial safety and immune-response measurements. In this study, the registry identifies the intended dose-finding role: informing a dose level for Phase 2/3 trials in this age cohort.

The design also makes clear why careful comparisons matter. The registry lists 120 µg and 60 µg doses across children aged 2 to <5 years, healthy children aged 5 to <18 years, and children aged 5 to <18 years with chronic high-risk conditions. A group-level percentage cannot be interpreted without knowing which participants contributed data and how the groups were compared. The registry identifies randomized allocation but does not report enough detail to reconstruct a comparison strategy.

The local-reaction endpoint illustrates the role of precise outcome definitions in safety studies. It specifies an observation window, data collection through an electronic diary, and age-dependent grading for redness and swelling. Such definitions support consistent measurement, but they do not replace reporting the observed counts, percentages, and uncertainty. Those primary outcome results are not reported in the registry record.

The record’s limits are also instructive. A completed study can have a clear stated purpose and detailed outcome definitions while the registry record still lacks analysis results, missing-data procedures, and multiplicity methods. Those omissions constrain what readers can infer. The posted record supports description of the study design and the reported serious adverse event counts, but not a statistical conclusion about dose differences or immune response.

14. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

Several limitations specific to this study constrain interpretation of the registry record. The primary endpoint is defined, but the registry does not report its observed percentages or confidence intervals. The listed secondary immune outcomes likewise have no reported results. No statistical analyses are reported, so effect estimates, uncertainty, and comparisons between dose groups are unavailable.

The study spans age and health-status groups, but the registry does not explain the allocation procedure in enough detail to assess how the groups were formed or which comparisons were planned. Although the record identifies randomized allocation and a sequential model, it does not describe the randomization method, the age-descending process, or how the two design features were combined. It also does not report the criteria used to select a dose for later trials.

The primary safety endpoint depends on diary reporting, but handling of incomplete diary data is not reported. The registry does not describe missing blood-sample handling, analysis populations for immune outcomes, or participant exclusions from analyses. The analysis plan, multiplicity approach, interim analyses, and trial-specific sample-size justification are also not reported.

The serious adverse event information is presented as group counts and selected terms. The registry record does not provide event-level narratives, timing, attribution, or statistical comparisons. These limitations mean that the counts cannot establish causation or support precise comparative safety conclusions. The record also does not report an efficacy outcome, so it cannot answer whether RSVpreF prevents RSV infection or illness.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: A Study to Learn About the Safety and Immune Activity of RSVpreF in Children 2 to <18 Years of Age — NCT05900154.
• Registry record last updated 2025-03-06; status last verified 2025-02.
• Results first posted 2025-03-06.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

18. Record Summary

NCT05900154 is a completed Phase 1, randomized, open-label, age-descending, dose-finding study sponsored by Pfizer. It enrolled 128 children aged 2 to <18 years and evaluated RSVpreF at 120 µg and 60 µg across age and health-status groups. The stated purpose was to assess safety, tolerability, and immune activity and identify a dose level for Phase 2/3 trials.

The primary endpoint was the percentage of participants with local reactions during Day 1 through Day 7 after vaccination. Secondary outcomes included neutralizing antibody measures for RSV A and RSV B and RSV F antigen-specific CD4+ T-cell measures. The registry reports serious adverse event counts by group and zero deaths in each group, but does not report the primary endpoint results, secondary outcome results, or statistical analyses. Missing-data methods, multiplicity handling, and the trial-specific sample-size justification are not reported in the registry record.