Human Papilloma Virus (HPV) Vaccine Immunogenicity and Safety Trial in Young and Adult Women With GSK Biologicals' HPV-16/18: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Human Papilloma Virus (HPV) Vaccine Immunogenicity and Safety Trial in Young and Adult Women With GSK Biologicals' HPV-16/18 (NCT00196937), a non-randomized phase 3 study. The primary outcome is Number of Seroconverted Subjects for Anti-human Papilloma Virus 16 (Anti-HPV-16) and Anti-human Papilloma Virus 18 (Anti-HPV-18) Antibodies, in Women 15 to 25 Years of Age and Women 26 to 45 Years of Age.

2. Clinical Question

The study was designed to describe immune responses and safety across age groups after vaccination, rather than to estimate a vaccine effect against a concurrent control. That distinction matters when interpreting the posted data: an antibody response after vaccination can be described, but a difference attributable to vaccination cannot be separated from other explanations using this design alone.

3. Trial Design

This was a Phase 3, open-label, non-randomized, parallel-group study sponsored by GlaxoSmithKline. The registry lists enrollment of 667. The study began on 2004-10-07, had primary completion on 2005-12-15, and is recorded as completed. Its stated focus was immunogenicity and safety in women aged 15 to 55 years, with long-term follow-up planned.

All three groups received Cervarix according to the same 3-dose intramuscular schedule at months 0, 1 and 6. The groups were defined by age: 15 to 25 years, 26 to 45 years, and 46 to 55 years. The main seroconversion outcome at month 7 was specified for the first two age groups; data for the 46 to 55 years group were designated as a secondary outcome.

The study was open, with no masking recorded. Since allocation was non-randomized and the groups differed by age, comparisons between groups may reflect age-related differences as well as other differences in who entered or remained in each group. They should not be read as if participants had been randomly assigned to different treatments. No untreated or active-comparator arm is described.

The registry lists 14 outcome measures. Beyond seroconversion and antibody titers, the listed outcomes cover solicited local and general symptoms, unsolicited adverse events, serious adverse events, and new-onset chronic diseases. The record describes the intended follow-up and outcome domains, but does not report the full protocol or a detailed statistical analysis plan.

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: Out of the 667 enrolled subjects, 1 subject was excluded for not receiving vaccination and the actual starting number was 666.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: The analysis was based the According-to-Protocol (ATP) cohort for immunogenicity, which included all evaluable subjects for whom immunogenicity data were available at the time of the analysis and who were seronegative before vaccination.

5. Baseline Characteristics

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

6. Primary Endpoint

The registry lists the primary outcome as Number of Seroconverted Subjects for Anti-human Papilloma Virus 16 (Anti-HPV-16) and Anti-human Papilloma Virus 18 (Anti-HPV-18) Antibodies, in Women 15 to 25 Years of Age and Women 26 to 45 Years of Age, measured over At Month 7.

The primary endpoint counts participants who seroconverted for anti-HPV-16 and anti-HPV-18 antibodies at month 7. Seroconversion was defined as the appearance of antibodies above the assay cut-off in a participant who was seronegative before vaccination. The listed cut-offs were 8 EL.U/mL for anti-HPV-16 antibodies and 7 EL.U/mL for anti-HPV-18 antibodies.

This is a binary outcome at the participant level: a person either meets the seroconversion definition or does not. Counting participants makes the result interpretable as the frequency of meeting a prespecified laboratory threshold in the analysis population. It does not describe the size of an antibody response among those who seroconverted; antibody titers were assessed separately as another outcome.

The endpoint is an immunogenicity measure, not a clinical disease endpoint. It indicates whether the specified antibody response was detected at the assessment time. The registry record does not report the primary outcome counts, estimates of uncertainty, or a comparison with an unvaccinated group. As a result, the endpoint definition can be explained, but the magnitude and precision of the reported primary response cannot be assessed from the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The methods information in the registry record is limited. It identifies a Phase 3, open, non-randomized, parallel study and names the primary outcome, its assessment time, and the analysis population. It does not report a full statistical analysis plan, the methods used to calculate uncertainty, or the details of any model used for outcome comparisons. The concepts below are general explanations of how such methods are commonly interpreted, not claims about unreported procedures in this study.

What does a binary endpoint mean?

General: A binary endpoint classifies each participant into one of two categories, such as meeting or not meeting a specified definition. A participant count can be used to describe how often the endpoint occurred in a defined analysis population. When accompanied by a denominator, it gives context for that count. The month 7 seroconversion endpoint uses a laboratory threshold and a baseline seronegative status to define the response.

The count alone does not show how far antibody titers were above the threshold, how titers changed over time, or whether participants remained seropositive at later follow-up. Those are different questions and require other outcome summaries. The registry lists antibody titers at pre-vaccination and at months 2, 7 and 12 as a secondary outcome, but does not include those results in the record.

What does an analysis population tell the reader?

The primary analysis was based on the According-to-Protocol cohort for immunogenicity. The record defines this as evaluable participants with immunogenicity data available at the time of analysis who were seronegative before vaccination. This definition indicates that the primary endpoint was not described as an analysis of every enrolled participant. It depends on evaluability, data availability, and baseline serostatus.

General: Restricting an analysis to participants who meet protocol and data criteria can make an endpoint definition more coherent for an immunogenicity question. It can also affect how broadly results apply. If participants without evaluable measurements differ from those included, the analyzed group may not represent everyone enrolled. The registry record does not report how many participants were excluded from this cohort, why data were unavailable, or how missing measurements were handled.

Why does the lack of randomization matter?

General: Random assignment helps balance measured and unmeasured participant characteristics across treatment groups, on average. In a non-randomized study, treatment groups may differ in ways that influence outcomes. Here, the parallel groups are age strata and all received the same vaccine schedule. A contrast between age groups therefore combines any age-related differences with possible differences in enrollment, health, follow-up, or other characteristics. It is not a randomized comparison of vaccination against a control treatment.

Open-label status means masking was not used according to the registry. For an antibody assay, laboratory measurement may be less dependent on participant expectations than a self-reported symptom, but the registry record does not describe laboratory blinding or other safeguards. For reported symptoms and adverse events, awareness of treatment can in general influence reporting or assessment. That is a methodological consideration, not evidence that reporting was biased in this study.

How should precision be assessed?

General: A confidence interval expresses the range of effect estimates compatible with the data under a specified method and assumptions. Its width indicates precision; it does not correct for bias from a non-randomized design or incomplete data. The registry record does not report confidence intervals for the primary endpoint. Without the primary counts and an uncertainty estimate, the precision of the primary seroconversion result cannot be assessed from this record.

For binary outcomes, uncertainty depends on the number of events and the number of participants analyzed, as well as the chosen estimation method. A statistical test can address compatibility with a specified null hypothesis, but a test result alone would not resolve the design limitations or establish clinical relevance. The registry record does not report a primary hypothesis test, p-value, or adjustment strategy for multiple outcomes.

What do the planning assumptions mean?

General: Sample-size calculations are planning tools. They estimate how many participants might be needed under stated assumptions to detect a target effect with a chosen error rate and power. The planning assumptions recorded for this study use a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and a target effect described as a fraction of the observed effect. These are described as planning illustrations based on the observed effect, not as the trial's original design justification.

Such illustrations do not report the trial's observed power. They also do not establish that the original study was adequately sized for every endpoint, particularly when there is no randomized two-arm treatment comparison. The registry record does not report the original sample-size justification. Any sample-size illustration should therefore be read as a conditional planning calculation, not as evidence about the reliability of the observed study result.

General: Multiple endpoints create multiple opportunities to observe an unusual result by chance. Statistical methods may address that issue by defining a hierarchy, adjusting testing thresholds, or treating outcomes as descriptive. The registry lists many immunogenicity and safety outcomes, but does not report which outcomes, if any, were included in a confirmatory testing sequence or whether multiplicity adjustments were used.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 14 outcome measures, including antibody responses, local and general symptoms, unsolicited adverse events, serious adverse events, and new-onset chronic diseases. It identifies the primary seroconversion outcome but does not report a detailed statistical analysis plan, a testing hierarchy, or a method for controlling the chance of false-positive findings across multiple outcomes. Any multiplicity handling is not reported in the registry record.

General: When many outcomes are examined, some may appear notable by chance even if there is no underlying difference. A prespecified primary endpoint and a clear testing hierarchy can help readers distinguish confirmatory analyses from exploratory descriptions. Without a reported hierarchy or adjustment plan, individual outcome findings should be interpreted in the context of the number and nature of outcomes examined. This is a general caution, not an assertion that any particular result in this study is false.

The record defines the primary analysis population as the According-to-Protocol cohort for immunogenicity: evaluable participants with available immunogenicity data who were seronegative before vaccination. It does not report the number excluded from this population, reasons for exclusion, or details of missing-data handling. It also does not describe whether an intention-to-treat analysis or sensitivity analyses were conducted.

General: Analyses restricted to participants with evaluable measurements may be informative for the endpoint, but missingness can affect interpretation if it is related to participant characteristics or outcomes. A complete account would ordinarily describe the analysis population, exclusions, missing measurements, and any methods used to assess their effect. Those details are not reported in the registry record.

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: breast cancer (1.4%), depression (0.9%), abortion missed (0.9%), abortion spontaneous complete (0.9%), irritable bowel syndrome (0.5%).

The safety summary lists serious adverse events by age group. In the 15 to 25 years group, 6 participants were affected among 229 at risk. In the 26 to 45 years group, 8 were affected among 226 at risk. In the 46 to 55 years group, 11 were affected among 211 at risk. These are descriptive counts from the registry record, not estimates from a randomized comparison.

The death counts recorded by group are 0 in the 15 to 25 years group, 0 in the 26 to 45 years group, and 2 in the 46 to 55 years group. The registry record does not establish that these deaths were caused by vaccination. Event counts observed during follow-up need causal assessment and context; the presence of an event in a vaccinated group alone does not determine its cause.

The record also lists serious adverse event terms with percentages: breast cancer, 1.4%; depression, 0.9%; missed abortion, 0.9%; spontaneous complete abortion, 0.9%; irritable bowel syndrome, 0.5%; and vestibular neuronitis, 0.5%. The denominator and the basis for these percentages are not reported in the registry record. They should not be interpreted as comparative risks or used to infer group-specific counts.

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

The study addresses whether antibody responses and safety outcomes were assessed after Cervarix vaccination in women across age strata. Its primary outcome definition is specific: seroconversion for anti-HPV-16 and anti-HPV-18 antibodies at month 7 among participants who were seronegative before vaccination. The registry record also describes secondary assessments of antibody titers and several categories of symptoms and adverse events.

The posted record does not report the primary seroconversion counts or corresponding uncertainty estimates. It therefore cannot establish from the recorded primary results how often participants met the endpoint or how precisely that frequency was estimated. The endpoint is an immunogenicity measure, and by itself it does not establish a clinical outcome such as prevention of infection or disease. No such conclusion should be inferred from the endpoint definition alone.

The design also sets limits on what comparisons mean. The study was open and non-randomized, and each parallel group was defined by age rather than by assignment to different treatments. There is no described untreated or active-comparator group. Consequently, differences between age groups cannot be interpreted as the effect of vaccination versus no vaccination, and safety observations cannot be compared with a concurrent control arm.

The serious adverse event counts and death counts provide descriptive information during the study, but do not by themselves establish causation. The registry record does not provide a controlled comparison, detailed attribution information, or enough context for the listed serious-event percentages to be interpreted as comparative risks. These limitations apply whether an event is common or rare in the posted summary.

In general, a well-defined endpoint, a transparent analysis population, reported estimates with uncertainty, and an appropriate comparison group each contribute to interpretation. The registry record supplies some design and endpoint definitions, but not the primary results or full analysis details needed for a complete assessment. The appropriate reading is therefore descriptive and bounded by the information recorded, not a definitive statement about clinical benefit or harm.

13. Why This Trial Matters Statistically

This study is statistically informative as an example of an age-stratified immunogenicity and safety design without random allocation. It shows why the type of question and the type of comparison must be kept distinct. Seroconversion can be defined using a laboratory threshold and assessed at a stated time, while age-group results remain observational comparisons rather than randomized contrasts.

It also illustrates why analysis populations matter. The primary outcome was defined for an According-to-Protocol cohort restricted to evaluable participants with available immunogenicity data who were seronegative before vaccination. That population definition is relevant to the endpoint, but readers also need to know how many participants met it and how missing data were handled to judge the scope and possible selection effects of the analysis. Those details are not reported in the registry record.

The record separates immunogenicity outcomes from safety outcomes. Antibody seroconversion and titers address measured immune responses, whereas symptom and adverse-event outcomes describe safety observations. These outcome types have different meanings and should not be treated as interchangeable evidence. A biological response is not itself a clinical disease endpoint, and an event observed during follow-up is not automatically caused by the intervention.

Finally, the record demonstrates the value of reporting estimates and their precision, not only endpoint definitions. The primary count and uncertainty estimates are not reported in the registry record, so the central immunogenicity result cannot be assessed quantitatively. The design and reporting details therefore shape what can be concluded just as much as the outcome label does.

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.

The study has no randomized control group, and allocation is recorded as non-randomized. All three parallel groups received Cervarix; their defining distinction was age. This means the groups do not provide a like-for-like comparison of vaccination against a control condition. Differences between age strata may reflect age and other differences between participants, rather than a treatment effect.

The study was open, with no masking recorded. The registry record does not describe whether laboratory personnel assessing antibody outcomes were blinded, or whether any other protections against assessment or reporting bias were used. This matters especially for subjective safety outcomes, although the record does not establish that bias occurred.

The primary outcome counts and uncertainty estimates are not reported in the registry record. The record also does not report the number included in the primary analysis population, the number with missing immunogenicity data, reasons for exclusions, or missing-data methods. These omissions prevent assessment of the primary result's magnitude, precision, and sensitivity to analysis-population decisions.

The full statistical analysis plan, original sample-size justification, interim-analysis information, and multiplicity strategy are not reported in the registry record. Although planning assumptions are described as an illustration based on the observed effect, they are not the original design justification and do not establish observed power. The listed serious adverse event percentages also lack a reported denominator in the recorded information, limiting their interpretation as rates.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Human Papilloma Virus (HPV) Vaccine Immunogenicity and Safety Trial in Young and Adult Women With GSK Biologicals' HPV-16/18 — NCT00196937.
• Registry record last updated 2019-12-11; status last verified 2019-12.
• Results first posted 2010-08-06.
• 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

NCT00196937 is recorded as a completed Phase 3, open, non-randomized, parallel study sponsored by GlaxoSmithKline. It evaluated Cervarix, administered intramuscularly on a 3-dose schedule at months 0, 1 and 6, in women aged 15 to 55 years. Enrollment was 667; the study began on 2004-10-07 and primary completion was on 2005-12-15.

The primary outcome was month 7 seroconversion for anti-HPV-16 and anti-HPV-18 antibodies in women aged 15 to 25 years and women aged 26 to 45 years. The primary outcome counts are not reported in the registry record. Three age groups are described, but no randomized control group is recorded. The posted safety summary includes serious adverse event counts and deaths by group, which are descriptive and do not establish causation.