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Biomedical subjects

Ivan S F Chan

Publications and source records attributed to Ivan S F Chan.

16 recordsLinked to original sources
PubMed2006

Safety and tolerability of a high-potency zoster vaccine in adults >/= 50 or years of age.

BACKGROUND: Herpes zoster (HZ) incidence rises with age, especially after 50 years of age, probably due to waning varicella-zoster virus (VZV)-specific immunity. The Shingles Prevention Study [Oxman MN, Levin MJ, Johnson GR, Schmader KE, Straus SE, Gelb LD, et al. A vaccine to prevent herpes zoster and postherpetic neuralgia in older adults, N Engl J Med 2005;352:2271-84], enrolled people >/= 60 or years of age and showed that zoster vaccine prevents HZ and postherpetic neuralgia (PHN), presumably through boosting VZV-specific immunity. This study of people >/= 50 or years of age compared the safety and tolerability of two zoster vaccine potencies. METHODS: Adults >/= 50 or years old enrolled in a randomized, double-blind, multicenter study to compare the safety and tolerability of one dose of two zoster vaccine potencies, approximately 58,000 and approximately 207,000 plaque-forming units/dose. Adverse experiences (AEs) were recorded on a standardized Vaccination Report Card for 42 days postvaccination. For assessment of injection-site AEs, clinically acceptable tolerability was predefined based on experience with PNEUMOVAX 23, a licensed vaccine recommended for use in older people. RESULTS: Six hundred and ninety-eight subjects (age 50-90 years, median 64 years) were enrolled. No serious vaccine-related AEs were reported. Similar AE rates were observed in the higher and lower potency groups (overall systemic AEs: 37.5 and 39.3%, vaccine-related systemic AEs: 10.9 and 13.2%, injection-site AEs: 63.0 and 59.8%). Rates for a combined endpoint of moderate or severe injection-site pain/tenderness/soreness and swelling were 17.2% (95% CI 13.9, 21.0) and 9.0% (95% CI 5.6, 13.4), respectively. Most combined endpoint events were reported as moderate in intensity. CONCLUSIONS: Both vaccine potencies were generally well tolerated in this study of people > or years of age. Although rates of some moderate or severe injection-site AEs were greater in the higher potency group, all rates met the prespecified criteria for clinically acceptable tolerability.

Aged↗
PubMed2006

Safety, tolerability, and immunogenicity of a two-dose regimen of high-titer varicella vaccine in subjects > or =13 years of age.

A new manufacturing process, known as process upgrade varicella vaccine (PUVV) was developed for a refrigerated formulation of varicella vaccine and for an investigational zoster vaccine. Safety and tolerability of a two-dose regimen of high-titered (approximately 50,000 PFU) PUVV were compared to a lower-titer formulation (approximately 5400 PFU) of VARIVAX; in 1366 healthy subjects > or =13 years old. Only one vaccine-related clinical serious adverse experience (pruritus; no hospitalization) was reported, in the VARIVAX group. Injection-site adverse experiences following any dose were higher in the PUVV group, 70.0%, than in the VARIVAX group, 56.2%, but generally were mild. Immunogenicity were similar in both groups in seronegative subjects. PUVV was generally well tolerated, and elicited an immune response similar to that induced by the marketed formulation of VARIVAX.

Adolescent↗
PubMed2006

Detecting qualitative interactions in clinical trials: an extension of range test.

To help interpret a treatment effect in clinical trials, investigators usually examine whether the observed treatment effect is the same in various subsets of patients. The qualitative interaction, which means that the treatment is beneficial in some subsets and harmful in others, is of major importance. In this paper, a new statistical test is developed for detecting such interactions. The new test is an extension of the well-known range test, but utilizes all observed treatment differences rather than only the maximum and the minimum values. Extensive simulations indicate that the proposed extended range test generally outperforms the range test and is even better than the likelihood ratio test in the sense that the extended range test is much more powerful than the likelihood test when one treatment is superior to the other in most subsets and yet does not lose much power otherwise. It is also illustrated through a real clinical trial example that the extended range test detects the qualitative interaction while the range test and likelihood ratio test do not.

Algorithms↗
PubMed2005

Confidence interval construction for proportion difference in small-sample paired studies.

Paired dichotomous data may arise in clinical trials such as pre-/post-test comparison studies and equivalence trials. Reporting parameter estimates (e.g. odds ratio, rate difference and rate ratio) along with their associated confidence interval estimates becomes a necessity in many medical journals. Various asymptotic confidence interval estimators have long been developed for differences in correlated binary proportions. Nevertheless, the performance of these asymptotic methods may have poor coverage properties in small samples. In this article, we investigate several alternative confidence interval estimators for the difference between binomial proportions based on small-sample paired data. Specifically, we consider exact and approximate unconditional confidence intervals for rate difference via inverting a score test. The exact unconditional confidence interval guarantees the coverage probability, and it is recommended if strict control of coverage probability is required. However, the exact method tends to be overly conservative and computationally demanding. Our empirical results show that the approximate unconditional score confidence interval estimators based on inverting the score test demonstrate reasonably good coverage properties even in small-sample designs, and yet they are relatively easy to implement computationally. We illustrate the methods using real examples from a pain management study and a cancer study.

Biometry↗
PubMed2005

Correlation coefficient inference on censored bioassay data.

In vaccine clinical trials, immunologic responses sometimes can not be accurately measured by bioassays. For example, a serial dilution assay usually reports the range of the response instead of the exact value. In some other assays, the measurement is not available if the response is lower than the assay's detection limit. In both cases, the measurements are censored. We are interested in computing the confidence interval for the correlation coefficient of two assay measurements that are subject to censoring. We propose using the maximum likelihood method to estimate the correlation coefficient, and constructing its confidence interval based on the second-order Taylor's expansion of the Fisher Z transformation. The method can be viewed as an extension of the Fisher Z transformation to the case of censored data. Extensive simulations show that the proposed method provides satisfactory coverage probabilities under finite sample sizes. The proposed method performs well compared with existing methods, but it is computationally much simpler. In addition, the proposed method works with many types of censored data in a similar way. Furthermore, we proposed an Monte Carlo exact test to assess the goodness-of-fit of the model.

Algorithms↗
PubMed2004

Development of a measure of the burden of pain due to herpes zoster and postherpetic neuralgia for prevention trials: adaptation of the brief pain inventory.

UNLABELLED: In preparation for clinical trials of a vaccine against herpes zoster (HZ), we conducted a prospective, observational study to evaluate (1) the Zoster Brief Pain Inventory (ZBPI), an HZ-specific questionnaire to quantify HZ pain and discomfort, (2) an operational definition of postherpetic neuralgia (PHN), and (3) a severity-duration measure of the burden of illness caused by HZ. HZ patients aged 60 years or older (n = 121) were enrolled within 14 days of rash onset and completed ZBPI, McGill Pain Questionnaire Present Pain Intensity (PPI), quality of life (QoL), and activities of daily living (ADL) questionnaires on a predetermined schedule. Reliability, measured by intraclass correlation coefficients within 14 days of rash onset, ranged between 0.63 and 0.78. ZBPI pain scores were strongly correlated with other pain measures, interference with ADL, and worsening QoL. The operational definition of PHN, a ZBPI pain score of 3 or greater occurring 90 or more days after rash onset, had high agreement with pain worse than mild on the PPI (kappa = 0.72). The ZBPI pain severity-duration measure had high correlations with severity-duration measures of ADL interference, worsening QoL, and other pain scales. These findings support the validity and utility of the ZBPI, the definition of PHN, and the severity-duration measure of the burden of HZ illness. PERSPECTIVE: Herpes zoster pain, as measured by the ZBPI severity-duration measure, is associated with impairment in daily living activities and quality of life. The ZBPI measure appears useful for quantifying herpes zoster pain, postherpetic neuralgia, and impairment in daily living activities for clinical trials of herpes zoster prevention.

Ambulatory Care↗
PubMed2003

Decline in varicella-zoster virus (VZV)-specific cell-mediated immunity with increasing age and boosting with a high-dose VZV vaccine.

The safety and immunogenecity of a booster dose of live attenuated varicella-zoster virus (VZV) vaccine was evaluated in 196 healthy subjects, >or=60 years old, who had already received a VZV vaccine >5 years before. This repeat booster dose was well tolerated. Cell-mediated immunity (CMI) to VZV was measured by an interferon-gamma (IFN-gamma) enzyme-linked immunosorbent spot-forming cell (ELISPOT) assay and a limiting dilution responder cell frequency (RCF) assay. Prevaccination responses decreased as a function of increasing age but were detectable in all subjects by use of the IFN-gamma ELISPOT assay. In most subjects, VZV-specific CMI was increased at 6 weeks postvaccination. The magnitude of the vaccine-induced IFN-gamma ELISPOT response was inversely related to prevaccination values. Although there was a significant correlation between the IFN-gamma ELISPOT and RCF assays, the ELISPOT assay had greater sensitivity and a wider dynamic range. A live attenuated VZV vaccine is safe and immunogenic in an elderly population, and the vaccine-induced immunity may be monitored by the IFN-gamma ELISPOT assay.

Age Factors↗
PubMed2003

Measurement of cell-mediated immunity with a Varicella-Zoster Virus-specific interferon-gamma ELISPOT assay: responses in an elderly population receiving a booster immunization.

An interferon-gamma ELISPOT assay has been developed for assessment of cellular immune responses to Varicella-Zoster Virus (VZV) in large, multi-center clinical vaccine trials. We show that the assay performed best when testing peripheral blood mononuclear cells (PBMCs) that had been isolated and then frozen on the same day as blood was drawn, and that freezing PBMCs from blood that was stored overnight before processing resulted in dramatically reduced responses. This assay was used to monitor cell-mediated immunity (CMI) in response to a booster immunization with an investigational live, attenuated VZV vaccine in an elderly population that had been vaccinated 8-10 years previously. The booster vaccine elicited a 1.6- to 1.7-fold rise in the VZV-specific cellular immune response as measured by the ELISPOT assay. The increase from pre to post booster vaccination response was more pronounced (approximately 2.2-fold rise) in a subset of subjects who had received two prior immunizations with a live, attenuated vaccine.

Aged↗
PubMed2003

A cautionary note on exact unconditional inference for a difference between two independent binomial proportions.

Fisher's exact test for comparing response proportions in a randomized experiment can be overly conservative when the group sizes are small or when the response proportions are close to zero or one. This is primarily because the null distribution of the test statistic becomes too discrete, a partial consequence of the inference being conditional on the total number of responders. Accordingly, exact unconditional procedures have gained in popularity, on the premise that power will increase because the null distribution of the test statistic will presumably be less discrete. However, we caution researchers that a poor choice of test statistic for exact unconditional inference can actually result in a substantially less powerful analysis than Fisher's conditional test. To illustrate, we study a real example and provide exact test size and power results for several competing tests, for both balanced and unbalanced designs. Our results reveal that Fisher's test generally outperforms exact unconditional tests based on using as the test statistic either the observed difference in proportions, or the observed difference divided by its estimated standard error under the alternative hypothesis, the latter for unbalanced designs only. On the other hand, the exact unconditional test based on the observed difference divided by its estimated standard error under the null hypothesis (score statistic) outperforms Fisher's test, and is recommended. Boschloo's test, in which the p-value from Fisher's test is used as the test statistic in an exact unconditional test, is uniformly more powerful than Fisher's test, and is also recommended.

Humans↗
PubMed2003

Statistical analysis of noninferiority trials with a rate ratio in small-sample matched-pair designs.

Testing of noninferiority has become increasingly important in modern medicine as a means of comparing a new test procedure to a currently available test procedure. Asymptotic methods have recently been developed for analyzing noninferiority trials using rate ratios under the matched-pair design. In small samples, however, the performance of these asymptotic methods may not be reliable, and they are not recommended. In this article, we investigate alternative methods that are desirable for assessing noninferiority trials, using the rate ratio measure under small-sample matched-pair designs. In particular, we propose an exact and an approximate exact unconditional test, along with the corresponding confidence intervals based on the score statistic. The exact unconditional method guarantees the type I error rate will not exceed the nominal level. It is recommended for when strict control of type I error (protection against any inflated risk of accepting inferior treatments) is required. However, the exact method tends to be overly conservative (thus, less powerful) and computationally demanding. Via empirical studies, we demonstrate that the approximate exact score method, which is computationally simple to implement, controls the type I error rate reasonably well and has high power for hypothesis testing. On balance, the approximate exact method offers a very good alternative for analyzing correlated binary data from matched-pair designs with small sample sizes. We illustrate these methods using two real examples taken from a crossover study of soft lenses and a Pneumocystis carinii pneumonia study. We contrast the methods with a hypothetical example.

Biometry↗
PubMed2002

Use of statistical models for evaluating antibody response as a correlate of protection against varicella.

In vaccine clinical trials, humoral antibody responses are often used to measure the effect of a vaccine because they correlate with a vaccine's protective efficacy against the target disease. While the concept of a correlate of protection usually refers to establishing a protective level of antibody titre, identifying a clear-cut value is often impossible because vaccine efficacy is not related solely to the antibody titre. We propose examining the relationship between disease protection and the whole distribution of antibody responses rather than a single cut-off level. In particular, we use failure-time models to estimate the relationship between long-term disease breakthroughs and primary antibody responses after vaccination. We apply these models to show that the varicella antibody response measured by glycoprotein enzyme-linked immunosorbent assay 6 weeks after vaccination strongly correlate with protection against varicella (chickenpox); we used 7-year follow-up data from children who received one dose of a live attenuated varicella (Oka/Merck) vaccine. In addition, we explore the potential use of these models to predict long-term disease breakthrough rates and to estimate the predicted vaccine efficacy of a similar varicella vaccine made with a modified manufacturing process.

Antibodies, Viral↗
PubMed2002

A comparison of safety, tolerability and immunogenicity of Oka/Merck varicella vaccine and VARILRIX in healthy children.

This study compared safety, tolerability, and immunogenicity of the Oka/Merck varicella vaccine and VARILRIX [Oka-RIT strain SmithKline Beecham Biologicals] in healthy children 12-24 months of age. Subjects were randomized in this double blind study to receive either a single dose of Oka/Merck varicella vaccine, (approximately 50,000 plaque forming units (PFU), Group A or approximately 16,000 PFU, Group B) or 1 dose of VARILRIX, (approximately 40,000 PFU/dose, Group C). Safety profiles in each treatment group were similar. The proportions of subjects achieving a 6-week postvaccination titer> or = 5 gpELISA units in Groups A, B or C were 97.1, 95.2 and 85.6%, respectively.

Antibodies, Viral↗
PubMed2002

Response to recombinant hepatitis B vaccine in children and adolescents with chronic renal failure.

BACKGROUND: Diminished antibody responses to the dosage of hepatitis B (HB) vaccine indicated for healthy adults has led to a greater dosage recommendation (40 microg of HB surface antigen [HBsAg]) for adults with chronic renal failure (CRF), but an appropriate dosage for children with CRF has not been established. METHODS: Seventy-eight children and adolescents with CRF (22 patients, predialysis; 42 patients, chronic dialysis therapy; 14 renal transplant recipients) aged 1 to 19 years (mean, 10.1 years) were enrolled onto a study to test a three 20-microg dose course of the HB vaccine Recombivax HB (Merck & Co, Inc, West Point, PA). RESULTS: The vaccine was well tolerated; no patient had a serious adverse event attributable to vaccine, and no patient withdrew from the study because of an adverse event. Overall, 91% of 66 patients administered three doses had a protective titer of 10 mIU/mL or greater for antibody against HBsAg (anti-HBs) and a geometric mean titer (GMT) of 733 mIU/mL, with seroprotection rates and GMTs among predialysis, dialysis, and renal transplant patients of 100% (4,140 mIU/mL), 94% (419 mIU/mL), and 64% (152 mIU/mL), respectively. All (100%) predialysis patients had a 10-mIU/mL or greater anti-HBs titer after only two doses of vaccine compared with 64% of dialysis patients and 50% of transplant recipients. Eighty-eight percent of 57 fully vaccinated patients tested 12 months after the first dose retained a 10-mIU/mL or greater anti-HBs titer. CONCLUSION: A regimen of three 20-microg doses of Recombivax HB is suitably immunogenic for children with CRF not administered immunosuppressive medication. When possible, at least two, and preferably all three, doses of vaccine should be administered before progression to end-stage renal disease.

Adolescent↗
PubMed2002

Power and sample size determination for noninferiority trials using an exact method.

Noninferiority studies are frequently conducted to justify the development of new drugs and vaccines that have been shown to offer better safety profiles, easier administration, or lower cost while maintaining similar efficacy as compared to the standard treatment. Recently, exact methods have been developed to address the concern that existing asymptotic methods for analyzing and planning noninferiority may fail because of small sample size or because of skewed or sparse data structure. In this paper, we explore the use of exact methods in determining sample size and power for noninferiority studies that focus on the difference of two proportions. The methodology for sample size and power calculations is developed based on an exact unconditional test of noninferiority. We illustrate this exact method using a clinical trial example in childhood nephroblastoma and briefly discuss the optimal sample-size allocation strategy. This exact unconditional method performs very well in various scenarios and compares favorably to its asymptotic counterpart in terms of sensitivity. Therefore, it is a very desirable tool for planning noninferiority trials, especially in situations where asymptotic methods are likely to fail.

Algorithms↗
PubMed2002

Inverse relationship between six week postvaccination varicella antibody response to vaccine and likelihood of long term breakthrough infection.

BACKGROUND: We used the large clinical database that supported the development of Oka/ Merck varicella vaccine to study the relationship between the primary varicella antibody response, as determined by gpELISA, an enzyme-linked immunosorbent assay that detects antibodies to varicella-zoster virus (VZV) glycoprotein, and the subsequent risk of postvaccination breakthrough varicella. METHODS: We vaccinated 1,164 healthy children with a single dose of varicella vaccine containing 2900 to 9000 plaque-forming units/dose. The primary immune response to vaccination was determined by gpELISA 6 weeks after vaccination. Subjects were followed annually for 7 years to ascertain cases of breakthrough varicella. RESULTS: The estimated vaccine efficacy among children with a 6-week postvaccination antibody titer of > or = 5 gpELISA units was 95.5% (95% confidence interval, 94.2%, 96.8%) compared with 83.5% (95% confidence interval, 76.9%, 89.5%) for subjects with a titer of <5 gpELISA units. Children with a 6-week postvaccination antibody titer of <5 gpELISA units were 3.5 times more likely than those with a titer of > or = 5 gpELISA units to develop breakthrough varicella. CONCLUSIONS: We identified a 6-week postvaccination antibody titer of > or = 5 gpELISA units as an approximate correlate of protection. In addition we established an accelerated failure time model based on log normal hazard that predicted varicella breakthrough rates based on the distribution of 6-week postvaccination varicella antibody titers.

Antibodies, Viral↗