Projecting adverse event incidence rates using empirical Bayes methodology.

Although there is considerable interest in adverse events observed in clinical trials, projecting adverse event incidence rates in an extended period can be of interest when the trial duration is limited compared to clinical practice. A naïve method for making projections might involve modeling the observed rates into the future for each adverse event. However, such an approach overlooks the information that can be borrowed across all the adverse event data. We propose a method that weights each projection using a shrinkage factor; the adverse event-specific shrinkage is a probability, based on empirical Bayes methodology, estimated from all the adverse event data, reflecting evidence in support of the null or non-null hypotheses. Also proposed is a technique to estimate the proportion of true nulls, called the common area under the density curves, which is a critical step in arriving at the shrinkage factor. The performance of the method is evaluated by projecting from interim data and then comparing the projected results with observed results. The method is illustrated on two data sets.

The benefit of stratification in clinical trials revisited.

Stratification is common in clinical trials because it can reduce the variance of the estimated treatment effect. The traditional demonstration of variance reduction relies on the assumption of stratum sizes being fixed quantities. However, in practice, to speed up enrollment, and to obtain a study population with a similar distribution as the overall population, the stratum sizes are allowed to vary. Under the condition that the total sample size is fixed and that the stratum sizes have a multinomial distribution, the criterion changes for achieving a reduction in variance. The relationship between the stratified and unstratified variances is established and shown to be approximately the same for prestratified and post-stratified trials. It is demonstrated why stratification may actually increase the variance compared with no stratification even when the mean square error is reduced on account of stratification. Data from a real clinical trial will be used for illustration. The benefit attributed to stratification needs to be re-examined in light of the findings presented, particularly given its widespread use.

Safety and immunogenicity of an intramuscular Helicobacter pylori vaccine in noninfected volunteers: a phase I study.

INTRODUCTION: Helicobacter pylori infection is among the most common human infections and the major risk factor for peptic disease and gastric cancer. Immunization with vaccines containing the H pylori vacuolating cytotoxin A (VacA), cytotoxin-associated antigen (CagA), and neutrophil-activating protein (NAP), alone or in combination, have been shown to prevent experimental infection in animals. AIM: We sought to study the safety and immunogenicity of a vaccine consisting of recombinant VacA, CagA, and NAP given intramuscularly with aluminium hydroxide as an adjuvant to noninfected healthy subjects. METHODS: This controlled, single-blind Phase I study randomized 57 H pylori-negative volunteers into 7 study arms exploring 2 dosages (10 and 25 microg) of each antigen and 3 schedules (0, 1, 2 weeks; 0, 1, 2 months; and 0, 1, 4 months) versus alum controls. All participants were followed for 5 months. Thirty-six subjects received a booster vaccination 18-24 months after the completion of the primary vaccination. RESULTS: Local and systemic adverse reactions were mild and similar in placebo and vaccine recipients on the monthly schedules. All subjects responded to 1 or 2 of the antigens and 86% of all vaccines mounted immunoglobulin G antibody responses to all 3 antigens. Vaccinees exhibited an antigen-specific cellular response. Vaccination 18-24 months later elicited anamnestic antibody and cellular responses. CONCLUSIONS: This intramuscular H pylori vaccine demonstrated satisfactory safety and immunogenicity, produced antigen-specific T-cell memory, and, therefore, warrants further clinical study.