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

Nonparametric estimation of the causal effect of a stochastic threshold-based intervention

Identifying a biomarker or treatment-dose threshold that marks a specified level of risk is an important problem, especially in clinical trials. In view of this goal, we consider a covariate-adjusted threshold-based interventional estimand, which happens to equal the binary treatment-specific mean estimand from the causal inference literature obtained by dichotomizing the continuous biomarker or treatment as above or below a threshold. The unadjusted version of this estimand was considered in Donovan et al.. Expanding upon Stitelman et al., we show that this estimand, under conditions, identifies the expected outcome of a stochastic intervention that sets the treatment dose of all participants above the threshold. We propose a novel nonparametric efficient estimator for the covariate-adjusted threshold-response function for the case of informative outcome missingness, which utilizes machine learning and targeted minimum-loss estimation (TMLE). We prove the estimator is efficient and characterize its asymptotic distribution and robustness properties. Construction of simultaneous 95% confidence bands for the threshold-specific estimand across a set of thresholds is discussed. In the Supporting Information, we discuss how to adjust our estimator when the biomarker is missing at random, as occurs in clinical trials with biased sampling designs, using inverse probability weighting. Efficiency and bias reduction of the proposed estimator are assessed in simulations. The methods are employed to estimate neutralizing antibody thresholds for virologically confirmed dengue risk in the CYD14 and CYD15 dengue vaccine trials.

Epidemiological Impact and Cost-Effectiveness of Varicella Vaccination Strategies in the United Kingdom

Background

Despite the burden of varicella, there is no universal varicella vaccination (UVV) program in the United Kingdom (UK) due to concerns that it could increase herpes zoster (HZ) incidence. We assessed the cost-utility of a first-dose monovalent (varicella [V]) or quadrivalent (measles-mumps-rubella-varicella [MMRV]) followed by a second-dose MMRV UVV program. GSK and MSD varicella-containing vaccines (VCVs) were considered.

Methods

Dynamic transmission and cost-effectiveness models were adapted to the UK. Outcomes measured included varicella and HZ incidences and the incremental cost-utility ratio (ICURs) over a lifetime horizon. Payer and societal perspectives were evaluated.

Results

The impact of V-MMRV and MMRV-MMRV UVV programs on varicella incidence was comparable between both VCVs at equilibrium. HZ incidence increased by 1.6%–1.7% over 7 years after UVV start, regardless of the strategies, then decreased by >95% at equilibrium. ICURs ranged from £5665 (100 years) to £18 513 (20 years) per quality-adjusted life-year (QALY) gained with V-MMRV and from £9220 to £27 101 per QALY gained with MMRV-MMRV (payer perspective). MMRV-MMRV was cost-effective in the medium- and long-terms with GSK VCV and only cost-effective in the long term with MSD VCV at a £20 000 per QALY gained threshold. Without the exogenous boosting hypothesis, HZ incidence decreased through UVV implementation. ICURs were most sensitive to discount rates and MMRV price.

Conclusions

A 2-dose UVV was demonstrated to be a cost-effective alternative to no vaccination. With comparable effectiveness as MSD VCV at lower costs, GSK VCV may offer higher value for the money.

A method to estimate probability of disease and vaccine efficacy from clinical trial immunogenicity data

Vaccine efficacy is often assessed by counting disease cases in a clinical trial. A new quantitative framework proposed here ("PoDBAY," Probability of Disease Bayesian Analysis), estimates vaccine efficacy (and confidence interval) using immune response biomarker data collected shortly after vaccination. Given a biomarker associated with protection, PoDBAY describes the relationship between biomarker and probability of disease as a sigmoid probability of disease ("PoD") curve. The PoDBAY framework is illustrated using clinical trial simulations and with data for influenza, zoster, and dengue virus vaccines. The simulations demonstrate that PoDBAY efficacy estimation (which integrates the PoD and biomarker data), can be accurate and more precise than the standard (case-count) estimation, contributing to more sensitive and specific decisions than threshold-based correlate of protection or case-count-based methods. For all three vaccine examples, the PoD fit indicates a substantial association between the biomarkers and protection, and efficacy estimated by PoDBAY from relatively little immunogenicity data is predictive of the standard estimate of efficacy, demonstrating how PoDBAY can provide early assessments of vaccine efficacy. Methods like PoDBAY can help accelerate and economize vaccine development using an immunological predictor of protection. For example, in the current effort against the COVID-19 pandemic it might provide information to help prioritize (rank) candidates both earlier in a trial and earlier in development.

Assurance in vaccine efficacy clinical trial design based on immunological responses

The assurance of a future clinical trial is a key quantitative tool for decision-making in drug development. It is derived from prior knowledge (Bayesian approach) about the clinical endpoint of interest, typically from previous clinical trials. In this paper, we examine assurance in the specific context of vaccine development, where early development (Phase 2) is often based on immunological endpoints (e.g., antibody levels), while the confirmatory trial (Phase 3) is based on the clinical endpoint (very large sample sizes and long follow-up). Our proposal is to use the Phase 2 vaccine efficacy predicted by the immunological endpoint (using a model estimated from epidemiological studies) as prior information for the calculation of the assurance.

Correlation of protection against varicella in a randomized Phase III varicella-containing vaccine efficacy trial in healthy infants

BACKGROUND

Varicella vaccination confers high and long-lasting protection against chickenpox and induces robust immune responses, but an absolute correlate of protection (CoP) against varicella has not been established. This study models the relationship between varicella humoral response and protection against varicella.

METHODS

This was a post-hoc analysis of data from a Phase IIIb, multicenter, randomized trial (NCT00226499) conducted in ten varicella-endemic European countries. Healthy children aged 12-22 months were randomized 3:3:1 to receive one dose of measles-mumps-rubella and one dose of varicella vaccine (one-dose group) or two doses of measles-mumps-rubella-varicella vaccine (two-dose group) or two doses of measles-mumps-rubella vaccine (control group) six weeks apart. The study remained observer-blind until completion, except in countries with obligatory additional immunizations. The objective was to correlate varicella-specific antibody concentrations with protection against varicella and probability of varicella breakthrough, using Cox proportional hazards and Dunning and accelerated failure time statistical models. The analysis was guided by the Prentice framework to explore a CoP against varicella.

RESULTS

The trial included 5803 participants, 5289 in the efficacy (2266: one-dose group, 2279: two-dose group and 744: control group) and 5235 (2248, 2245 and 742 in the same groups) in the immunogenicity cohort. The trial ended in 2016 with a median follow-up time of 9.8 years. Six weeks after vaccination with one- or two-dose varicella-containing vaccine, more than 93.0% of vaccinees were seropositive for varicella-specific antibodies. Estimated vaccine efficacy correlated positively with antibody concentrations. The fourth Prentice CoP criterion was not met, due to predicted positive vaccine efficacy in seronegative participants. Further modelling showed decreased probability of moderate to severe varicella breakthrough with increasing varicella-specific antibody concentrations (ten-year probability <0.1 for antibody concentrations ≥2-fold above the seropositivity cut-off).

CONCLUSIONS

Varicella-specific antibody concentrations are a good predictor of protection, given their inverse correlation with varicella occurrence.

CLINICAL TRIAL

NCT00226499.

NONPARAMETRIC INFERENCE FOR IMMUNE RESPONSE THRESHOLDS OF RISK IN VACCINE STUDIES

An important objective in vaccine studies entails identifying an immune response which is predictive of disease risk. Nonparametric methods are developed for inference on immune response thresholds that are associated with specified levels of disease risk, including where the risk level is zero. This threshold is defined as the minimum immune response value above which disease risk is less than or equal to the desired level. The proposed nonparametric methods are compared to previously developed parametric methods in simulation studies. The methods are extended for use in studies that only measure the immune response in a subset of participants, such as case-cohort or case-control studies, and with right censored time to disease outcomes. Finally, these methods are used to estimate neutralizing antibody thresholds for virologically confirmed dengue risk using data from two recent dengue vaccine trials.

Assessing correlates of protection in vaccine trials: statistical solutions in the context of high vaccine efficacy

Background

The use of correlates of protection (CoPs) in vaccination trials offers significant advantages as useful clinical endpoint substitutes. Vaccines with very high vaccine efficacy (VE) are documented in the literature (VE ≥95%). The rare events (number of infections) observed in the vaccinated groups of these trials posed challenges when applying conventionally-used statistical methods for CoP assessment. In this paper, we describe the nature of these challenges, and propose easy-to-implement and uniquely-tailored statistical solutions for the assessment of CoPs in the specific context of high VE.

Methods

The Prentice criteria and meta-analytic frameworks are standard statistical methods for assessing vaccine CoPs, but can be problematic in high VE cases due to the rare events data available. As a result, lack of fit and the problem of infinite estimates may arise, in the former and latter methods respectively. The use of flexible models within the Prentice framework, and penalized-likelihood methods to solve the issue of infinite estimates can improve the performance of both methods in high VE settings.

Results

We have 1) devised flexible non-linear models to counteract the Prentice framework lack of fit, providing sufficient statistical power to the method, and 2) proposed the use of penalised likelihood approaches to make the meta-analytic framework applicable on randomized subgroups, such as regions. The performance of the proposed methods for high VE cases was evaluated by running simulations.

Conclusions

As vaccines with high efficacy are documented in the literature, there is a need to identify effective statistical solutions to assess CoPs. Our proposed adaptations are straight-forward and improve the performance of conventional statistical methods for high VE data, leading to more reliable CoP assessments in the context of high VE settings.

Correlates of protection for rotavirus vaccines: Possible alternative trial endpoints, opportunities, and challenges

Rotavirus (RV) is a major vaccine-preventable killer of young children worldwide. Two RV vaccines are globally commercially available and other vaccines are in different stages of development. Due to the absence of a suitable correlate of protection (CoP), all RV vaccine efficacy trials have had clinical endpoints. These trials represent an important challenge since RV vaccines have to be introduced in many different settings, placebo-controlled studies are unethical due to the availability of licensed vaccines, and comparator assessments for new vaccines with clinical endpoints are very large, complex, and expensive to conduct. A CoP as a surrogate endpoint would allow predictions of vaccine efficacy for new RV vaccines and enable a regulatory pathway, contributing to the more rapid development of new RV vaccines. The goal of this review is to summarize experiences from RV natural infection and vaccine studies to evaluate potential CoP for use as surrogate endpoints for assessment of new RV vaccines, and to explore challenges and opportunities in the field.

Surrogate Endpoint Evaluation: Principal Stratification Criteria and the Prentice Definition

A common problem of interest within a randomized clinical trial is the evaluation of an inexpensive response endpoint as a valid surrogate endpoint for a clinical endpoint, where a chief purpose of a valid surrogate is to provide a way to make correct inferences on clinical treatment effects in future studies without needing to collect the clinical endpoint data. Within the principal stratification framework for addressing this problem based on data from a single randomized clinical efficacy trial, a variety of definitions and criteria for a good surrogate endpoint have been proposed, all based on or closely related to the "principal effects" or "causal effect predictiveness (CEP)" surface. We discuss CEP-based criteria for a useful surrogate endpoint, including (1) the meaning and relative importance of proposed criteria including average causal necessity (ACN), average causal sufficiency (ACS), and large clinical effect modification; (2) the relationship between these criteria and the Prentice definition of a valid surrogate endpoint; and (3) the relationship between these criteria and the consistency criterion (i.e., assurance against the "surrogate paradox"). This includes the result that ACN plus a strong version of ACS generally do not imply the Prentice definition nor the consistency criterion, but they do have these implications in special cases. Moreover, the converse does not hold except in a special case with a binary candidate surrogate. The results highlight that assumptions about the treatment effect on the clinical endpoint before the candidate surrogate is measured are influential for the ability to draw conclusions about the Prentice definition or consistency. In addition, we emphasize that in some scenarios that occur commonly in practice, the principal strata sub-populations for inference are identifiable from the observable data, in which cases the principal stratification framework has relatively high utility for the purpose of effect modification analysis, and is closely connected to the treatment marker selection problem. The results are illustrated with application to a vaccine efficacy trial, where ACN and ACS for an antibody marker are found to be consistent with the data and hence support the Prentice definition and consistency.

Determining the cutoff based on a continuous variable to define two populations with application to vaccines

In clinical research, it is sometimes desirable to dichotomize a continuous variable so that the information expressed using a dichotomous variable is more straightforward for clinicians to interpret and communicate with patients. The distribution of the continuous variable can differ between two populations defined by a disease case status. Under such a scenario, the dichotomization process can be based on distributions of the continuous variable in two distinct populations. The resulting dichotomous variable can be used as an endpoint in future studies. Even though dichotomization has not been extensively studied, dichotomization has been commonly carried out in clinical trials. We developed a methodology for determining the optimal cutoff point based on maximizing the correlation between the two populations and the dichotomous variable. In some real-world scenarios where outcome status in samples from two populations is not completely identified, we recommend using EM method to first estimate the parameters associated with the two populations before applying the proposed method to find the optimal cutoff point. In addition, we have investigated the performance of the proposed method for several common distributions (e.g., normal, log-normal and exponential distribution) of the continuous variable. Finally, we applied the proposed methods to a varicella vaccine example.

A threshold method for immunological correlates of protection

Background

Immunological correlates of protection are biological markers such as disease-specific antibodies which correlate with protection against disease and which are measurable with immunological assays. It is common in vaccine research and in setting immunization policy to rely on threshold values for the correlate where the accepted threshold differentiates between individuals who are considered to be protected against disease and those who are susceptible. Examples where thresholds are used include development of a new generation 13-valent pneumococcal conjugate vaccine which was required in clinical trials to meet accepted thresholds for the older 7-valent vaccine, and public health decision making on vaccination policy based on long-term maintenance of protective thresholds for Hepatitis A, rubella, measles, Japanese encephalitis and others. Despite widespread use of such thresholds in vaccine policy and research, few statistical approaches have been formally developed which specifically incorporate a threshold parameter in order to estimate the value of the protective threshold from data.

Methods

We propose a 3-parameter statistical model called the a:b model which incorporates parameters for a threshold and constant but different infection probabilities below and above the threshold estimated using profile likelihood or least squares methods. Evaluation of the estimated threshold can be performed by a significance test for the existence of a threshold using a modified likelihood ratio test which follows a chi-squared distribution with 3 degrees of freedom, and confidence intervals for the threshold can be obtained by bootstrapping. The model also permits assessment of relative risk of infection in patients achieving the threshold or not. Goodness-of-fit of the a:b model may be assessed using the Hosmer-Lemeshow approach. The model is applied to 15 datasets from published clinical trials on pertussis, respiratory syncytial virus and varicella.

Results

Highly significant thresholds with p-values less than 0.01 were found for 13 of the 15 datasets. Considerable variability was seen in the widths of confidence intervals. Relative risks indicated around 70% or better protection in 11 datasets and relevance of the estimated threshold to imply strong protection. Goodness-of-fit was generally acceptable.

Conclusions

The a:b model offers a formal statistical method of estimation of thresholds differentiating susceptible from protected individuals which has previously depended on putative statements based on visual inspection of data.

Anthrax vaccine-induced antibodies provide cross-species prediction of survival to aerosol challenge

Because clinical trials to assess the efficacy of vaccines against anthrax are not ethical or feasible, licensure for new anthrax vaccines will likely involve the Food and Drug Administration's "Animal Rule," a set of regulations that allow approval of products based on efficacy data only in animals combined with immunogenicity and safety data in animals and humans. U.S. government-sponsored animal studies have shown anthrax vaccine efficacy in a variety of settings. We examined data from 21 of those studies to determine whether an immunological bridge based on lethal toxin neutralization activity assay (TNA) can predict survival against an inhalation anthrax challenge within and across species and genera. The 21 studies were classified into 11 different settings, each of which had the same animal species, vaccine type and formulation, vaccination schedule, time of TNA measurement, and challenge time. Logistic regression models determined the contribution of vaccine dilution dose and TNA on prediction of survival. For most settings, logistic models using only TNA explained more than 75% of the survival effect of the models with dose additionally included. Cross-species survival predictions using TNA were compared to the actual survival and shown to have good agreement (Cohen's κ ranged from 0.55 to 0.78). In one study design, cynomolgus macaque data predicted 78.6% survival in rhesus macaques (actual survival, 83.0%) and 72.6% in rabbits (actual survival, 64.6%). These data add support for the use of TNA as an immunological bridge between species to extrapolate data in animals to predict anthrax vaccine effectiveness in humans.

Using proportion of similar response to evaluate correlates of protection for vaccine efficacy

A question of interest in many vaccine clinical development programmes is whether vaccine-induced serum antibody level can be used as a correlate of vaccine efficacy; that is, whether serum antibody levels induced by a candidate vaccine can reliably predict the risk of breakthrough disease. Traditionally, analyses to answer this question have been based on modelling the incidence of breakthrough disease as a function of antibody level, among vaccinated subjects in clinical trials. The Proportion of Similar Response (PSR) method will be described and explored, and compared to the Receive Operator Characteristics (ROC) curve as a graphical tool and the area under the ROC (AUROC) as a summary measure in the context of evaluating correlates of protection. A way to use PSR analysis as complementary to Youden's index as a simple and elegant method to determine the discriminatory ability of a test and to set an optimal threshold value will be presented. In addition, the relationships among PSR and other measures of overlap and discrimination will be described. An example based on a clinical trial from a development programme for a vaccine against human papillomavirus (HPV) will be presented.

Handling missing data in vaccine clinical trials for immunogenicity and safety evaluation

In clinical trials, study subjects are usually followed for a period of time after treatment, and the missing data issue is almost inevitable due to various reasons, including early dropout or lost-to-follow-up. It is important to take the missing data into consideration at the study design stage to minimize its occurrence throughout the study and to prospectively account for it in the analyses. There are many methods available in the literature that are designed to handle the missing data issue under various settings. Vaccines are biological products that are primarily designed to prevent infectious diseases, and are different from pharmaceutical products, which traditionally have been chemical products designed to treat or cure diseases. While a lot of similarities exist between clinical trials for vaccines and those for pharmaceutical products, there are some unique issues in vaccine trials, including how to handle the missing data, which calls for special considerations. In this report we present a variety of statistical approaches for analyses of vaccine immunogenicity and safety trials in the presence of missing data. The methods are illustrated with numerical simulations and vaccine trial examples.

Evaluating a surrogate endpoint at three levels, with application to vaccine development

Identification of an immune response to vaccination that reliably predicts protection from clinically significant infection, i.e. an immunological surrogate endpoint, is a primary goal of vaccine research. Using this problem of evaluating an immunological surrogate as an illustration, we describe a hierarchy of three criteria for a valid surrogate endpoint and statistical analysis frameworks for evaluating them. Based on a placebo-controlled vaccine efficacy trial, the first level entails assessing the correlation of an immune response with a study endpoint in the study groups, and the second level entails evaluating an immune response as a surrogate for the study endpoint that can be used for predicting vaccine efficacy for a setting similar to that of the vaccine trial. We show that baseline covariates, innovative study design, and a potential outcomes formulation can be helpful for this assessment. The third level entails validation of a surrogate endpoint via meta-analysis, where the goal is to evaluate how well the immune response can be used to predict vaccine efficacy for new settings (building bridges). A simulated vaccine trial and two example vaccine trials are presented, one supporting that certain anti-influenza antibody levels are an excellent surrogate for influenza illness and another supporting that certain anti-HIV antibody levels are not useful as a surrogate for HIV infection.

Sample size for equivalence trials: a case study from a vaccine lot consistency trial

For some trials, simple but subtle assumptions can have a profound impact on the size of the trial. A case in point is a vaccine lot consistency (or equivalence) trial. Standard sample size formulas used for designing lot consistency trials rely on only one component of variation, namely, the variation in antibody titers within lots. The other component, the variation in the means of titers between lots, is assumed to be equal to zero. In reality, some amount of variation between lots, however small, will be present even under the best manufacturing practices. Using data from a published lot consistency trial, we demonstrate that when the between-lot variation is only 0.5 per cent of the total variation, the increase in the sample size is nearly 300 per cent when compared with the size assuming that the lots are identical. The increase in the sample size is so pronounced that in order to maintain power one is led to consider a less stringent criterion for demonstration of lot consistency. The appropriate sample size formula that is a function of both components of variation is provided. We also discuss the increase in the sample size due to correlated comparisons arising from three pairs of lots as a function of the between-lot variance.

Varicella prevention in the United States: a review of successes and challenges

OBJECTIVE

In 1995, the United States was the first country to introduce a universal 1-dose childhood varicella vaccination program. In 2006, the US varicella vaccine policy was changed to a routine 2-dose childhood program, with catchup vaccination for older children. The objective of this review was to summarize the US experience with the 1-dose varicella vaccination program, present the evidence considered for the policy change, and outline future challenges of the program.

METHODS

We conducted a review of publications identified by searching PubMed for the terms "varicella," "varicella vaccine," and "herpes zoster." The search was limited to US publications except for herpes zoster; we reviewed all published literature on herpes zoster incidence.

RESULTS

A single dose of varicella vaccine was 80% to 85% effective in preventing disease of any severity and >95% effective in preventing severe varicella and had an excellent safety profile. The vaccination program reduced disease incidence by 57% to 90%, hospitalizations by 75% to 88%, deaths by >74%, and direct inpatient and outpatient medical expenditures by 74%. The decline of cases plateaued between 2003 and 2006, and outbreaks continued to occur, even among highly vaccinated school populations. Compared with children who received 1 dose, in 1 clinical trial, 2-dose vaccine recipients developed in a larger proportion antibody titers that were more likely to protect against breakthrough disease and had a 3.3-fold lower risk for breakthrough disease and higher vaccine efficacy. Two studies showed no increase in overall herpes zoster incidence, whereas 2 others showed an increase.

CONCLUSIONS

A decade of varicella prevention in the United States has resulted in a dramatic decline in disease; however, even with high vaccination coverage, the effectiveness of 1 dose of vaccine did not generate sufficient population immunity to prevent community transmission. A 2-dose varicella vaccine schedule, therefore, was recommended for children in 2006. Data are inconclusive regarding an effect of the varicella vaccination program on herpes zoster epidemiology.

Humoral and cell-mediated immune responses in children and adults after 1 and 2 doses of varicella vaccine

Humoral and cell-mediated immune responses to varicella-zoster virus (VZV) have been evaluated after 1 and 2 doses of live attenuated varicella vaccine, Oka strain, in several studies. One dose of varicella vaccine, however, elicits detectable immune responses that are low and, in some cases, may be insufficient for complete protection against the virus after the normal decline in humoral and cell-mediated immunity with time. In contrast, immune responses after 2 doses are significantly higher and approximate the levels seen after natural disease. These investigations of vaccine-induced immunity suggest that 2 doses of VZV vaccine will better achieve the goals of the VZV vaccination program, by reducing the VZV burden of disease in childhood and preventing accumulation of young adults who are susceptible to or only partially protected from varicella.

Prediction of pertussis vaccine efficacy using a correlates of protection model

A previously developed statistical model relates vaccine immune responses to protection against pertussis disease in a household contact setting. Before this model can be used to predict the risk of disease based on immune responses, it must be validated to demonstrate reliable predictions. The model is shown here to be validated in terms of statistical criteria (Prentice surrogacy measures) as well as predictive capability in an independent efficacy trial (meta-analysis). Additionally, the model is used to predict efficacy from two recent immunogenicity trials comparing a 5-component acellular pertussis pentavalent combination vaccine with separate administration of its vaccine components. The model predicted similar protective efficacy rates: 82% after three doses and 83% after four doses. Follow-up of these subjects the model also predicted sustained and comparable efficacy for the combination and separate vaccines (75%) up to the pre-school booster age.

Evaluating candidate principal surrogate endpoints

SUMMARY

Frangakis and Rubin (2002, Biometrics 58, 21-29) proposed a new definition of a surrogate endpoint (a "principal" surrogate) based on causal effects. We introduce an estimand for evaluating a principal surrogate, the causal effect predictiveness (CEP) surface, which quantifies how well causal treatment effects on the biomarker predict causal treatment effects on the clinical endpoint. Although the CEP surface is not identifiable due to missing potential outcomes, it can be identified by incorporating a baseline covariate(s) that predicts the biomarker. Given case-cohort sampling of such a baseline predictor and the biomarker in a large blinded randomized clinical trial, we develop an estimated likelihood method for estimating the CEP surface. This estimation assesses the "surrogate value" of the biomarker for reliably predicting clinical treatment effects for the same or similar setting as the trial. A CEP surface plot provides a way to compare the surrogate value of multiple biomarkers. The approach is illustrated by the problem of assessing an immune response to a vaccine as a surrogate endpoint for infection.

Prevention of varicella: recommendations for use of varicella vaccines in children, including a recommendation for a routine 2-dose varicella immunization schedule

National varicella immunization coverage using the current 1-dose immunization strategy has increased among vaccine-eligible children 19 through 35 months of age from 27% in 1997 to 88% by 2005. These high immunization rates have resulted in a 71% to 84% decrease in the reported number of varicella cases, an 88% decrease in varicella-related hospitalizations, a 59% decrease in varicella-related ambulatory care visits, and a 92% decrease in varicella-related deaths in 1- to 4-year-old children when compared with data from the prevaccine era. Despite this significant decrease, the number of reported cases of varicella has remained relatively constant during the past 5 to 6 years. Since vaccine effectiveness for prevention of disease of any severity has been 80% to 85%, a large number of cases of varicella continue to occur among people who already have received the vaccine (breakthrough varicella), and outbreaks of varicella have been reported among highly immunized populations of schoolchildren. The peak age-specific incidence has shifted from 3- to 6-year-old children in the prevaccine era to 9- to 11-year-old children in the postvaccine era for cases in both immunized and unimmunized children during these outbreaks. Outbreaks of varicella are likely to continue with the current 1-dose immunization strategy. After administration of 2 doses of varicella vaccine in children, the immune response is markedly enhanced, with > 99% of children achieving an antibody concentration (determined by glycoprotein enzyme-linked immunosorbent assay) of > or = 5 U/mL (an approximate correlate of protection) and a marked increase in geometric mean antibody titers after the second vaccine dose. The estimated vaccine efficacy over a 10-year observation period of 2 doses for prevention of any varicella disease is 98% (compared with 94% for 1 dose), with 100% efficacy for prevention of severe disease. Recipients of 2 doses of varicella vaccine are 3.3-fold less likely to have breakthrough varicella, compared with those who are given 1 dose, during the first 10 years after immunization. To achieve greater levels of immunity with fewer serosusceptible people, greater protection against breakthrough varicella disease, and reduction in the number of outbreaks that occur nationwide among school-aged populations, a 2-dose varicella immunization strategy is now recommended for children > or = 12 months of age.

Immunogenicity of Oka/Merck varicella vaccine in children vaccinated at 12-14 months of age versus 15-23 months of age

OBJECTIVE

Recent reports suggest that breakthrough varicella may be more common in children when the Oka/Merck varicella vaccine is given at 12-14 months of age than when it is given at older ages. An analysis of 5 postlicensure clinical trials with this vaccine was conducted to evaluate immune response relative to the age of the vaccine recipient.

METHODS

In 5 clinical trials, 3771 children, 12 through 23 months of age with no history of varicella, received an injection of varicella vaccine. Varicella-zoster virus (VZV) antibody was measured 6 weeks postvaccination by glycoprotein enzyme-linked immunosorbent assay (gpELISA), an assay that correlates with neutralizing antibody. Endpoints evaluated were the response rate (percent of subjects with VZV antibody > or =5 gpELISA units/mL, a titer shown to correlate with protection) and geometric mean titer (GMT) of VZV antibody. Each endpoint was compared across 3 age groups (12-14, 15-17, and 18-23 months of age). Response rates by initial VZV serostatus were evaluated for children vaccinated at 12-14 months of age to assess whether maternal antibody had an impact on the immune response.

RESULTS

The response rates were similar among 12-14, 15-17, and 18-23 month olds (93.8, 90.8, and 93.1%, respectively); GMTs were significantly higher among the 12-14 month olds (15.1, 13.5, and 13.7 gpELISA units/mL, respectively). Among children 12-14 months of age, response rates and GMTs were similar regardless of their prevaccination VZV serostatus.

CONCLUSIONS

Oka/Merck varicella vaccine is highly immunogenic when given to children 12-14 months of age. The immunogenicity profile is similar to that of children 15-17 and 18-23 months of age. The presence of low titers of VZV antibody before vaccination did not influence vaccine response in 12-14 month olds. These results support current recommendations for universal varicella vaccination beginning at 12 months of age.

Statistical methods for active extension trials

This paper develops methods of analysis for active extension clinical trials. Under this design, patients are randomized to treatment or placebo for a period of time (period 1), and then all patients receive treatment for an additional period of time (period 2). We assume a continuous outcome is measured at baseline and at the end of the two consecutive periods. If only period 1 data is available, classic estimators of the treatment effect include the change score, analysis of covariance, and maximum likelihood (ML). We show how to extend these estimators by incorporating period 2 data which we refer to as the period 2 estimators. Under the assumption that the mean responses for treatment and placebo arms are the same at the end of period 2, the new estimators are unbiased and more efficient than estimators that ignore period 2 data. If this assumption is not met, the period 2 tests may be more powerful than period 1 tests, but the estimators are biased downward (upward) if the treatment effect during period 2 is larger (smaller) in treatment arm than the placebo arm. In general, the proposed period 2 procedure can provide an efficient way to supplement but not supplant the usual period 1 analysis.

Augmented designs to assess immune response in vaccine trials

This article introduces methods for use in vaccine clinical trials to help determine whether the immune response to a vaccine is actually causing a reduction in the infection rate. This is not easy because immune response to the (say HIV) vaccine is only observed in the HIV vaccine arm. If we knew what the HIV-specific immune response in placebo recipients would have been, had they been vaccinated, this immune response could be treated essentially like a baseline covariate and an interaction with treatment could be evaluated. Relatedly, the rate of infection by this baseline covariate could be compared between the two groups and a causative role of immune response would be supported if infection risk decreased with increasing HIV immune response only in the vaccine group. We introduce two methods for inferring this HIV-specific immune response. The first involves vaccinating everyone before baseline with an irrelevant vaccine, for example, rabies. Randomization ensures that the relationship between the immune responses to the rabies and HIV vaccines observed in the vaccine group is the same as what would have been seen in the placebo group. We infer a placebo volunteer's response to the HIV vaccine using their rabies response and a prediction model from the vaccine group. The second method entails vaccinating all uninfected placebo patients at the closeout of the trial with the HIV vaccine and recording immune response. We pretend this immune response at closeout is what they would have had at baseline. We can then infer what the distribution of immune response among placebo infecteds would have been. Such designs may help elucidate the role of immune response in preventing infections. More pointedly, they could be helpful in the decision to improve or abandon an HIV vaccine with mediocre performance in a phase III trial.

Lot Consistency as an Equivalence Problem

One requirement for licensure of a vaccine in the United States is demonstration by the manufacturer of consistently produced lots of vaccine. Demonstration of consistency of manufacturing can be viewed as a multigroup equivalence problem. The standard statistical procedures for evaluating equivalence assume normally distributed data and define equivalence margins with respect to group means. As an alternative approach, we define a measure of the similarity among group distributions and their nonparametric estimators. Through computer simulations we explore the statistical properties of an equivalence test based on this estimator and compare them to the standard methods. Preliminary work suggests that a test of similarity can be useful in demonstrating equivalence when distributions are not normal or when there are differences in scale or shape. It appears to detect departures from equivalence that are not reflected by differences among group means.

Varicella

Varicella-zoster virus, a herpesvirus, causes varicella (chickenpox) and, after endogenous reactivation, herpes zoster (shingles). Varicella, which is recognised by a characteristic vesicular rash, arises mainly in young children, although older individuals can be affected. In immunocompetent patients, symptoms are usually mild to moderate, but an uncomplicated severe case can have more than 1000 lesions and severe constitutional symptoms. Serious complications--including central nervous system involvement, pneumonia, secondary bacterial infections, and death--are sometimes seen. Varicella can be prevented by vaccination. Vaccine is about 80-85% effective against all disease and highly (more than 95%) effective in prevention of severe disease. In the USA, a routine childhood immunisation programme has reduced disease incidence, complications, hospital admissions, and deaths in children and in the general population, indicating strong herd immunity. Similar immunisation programmes have been adopted by some other countries, including Uruguay, Germany, Taiwan, Canada, and Australia, and are expected to be implemented more widely in future.

A model for immunological correlates of protection

Immunological assays measure characteristics of the immune system, such as antibody levels, specific to certain diseases. High assay values are often associated with protection from disease. A question of interest is how the relationship between assay values and subsequent development of disease should be quantitatively modelled. Existing approaches successfully model the relationship for high assay values, where the probability of developing disease is low. However at low assay values, the probability of developing disease is more closely associated with factors such as disease prevalence rates and an individual's chance of exposure to infection; these are less well captured by existing models. This paper presents a model that accommodates both assay values and factors independent of assay values, enabling protection from disease to be modelled over the whole range of assay values and proposing a method for predicting the efficacy of a vaccine from the assays of vaccinees and non-vaccinees.

[Immunisation against varicella]

Two vaccines against varicella are now being licensed in France, both deriving from the Oka strain. Seroconversion has been obtained in almost 100% of the cases after one dose in toddlers and children, and two doses in adolescents and adults. Efficacy has been mainly established from the US experience, where a universal immunisation programme of children aged > 12 months with a catch-up for susceptible adolescents and adults was begun in 1995. The incidence of varicella has decreased by about 85% over all age groups. The safety of the vaccine is good, and most adverse events are represented by fever, reactions at the injection site and varicella-like rashes. For the time being, France has adopted restrictive recommendations for the use of this vaccine because of uncertainties with respect to the duration of protection, a shift of the disease towards older age and the potential increase of the incidence of herpes-zoster.

Statistical considerations for vaccine immunogenicity trials

Part 2 of this series investigates the statistical considerations of vaccine evaluation in an active-control trial. In particular, the strengths and weaknesses of the noninferiority methodology will be explored and contrasted for T-cell independent (does not elicit a memory response) and T-cell dependent (elicits a memory response) vaccines. At present, the noninferiority model is widely accepted as the primary tool for comparing the immunogenicity of a new or reformulated vaccine to an already existing licensed product. However, conclusions drawn from statistical hypothesis testing are dependent on the bioassay endpoint (e.g., antibody concentration) and the metric analyzed (e.g., geometric mean concentration, proportion fold-response, etc.). Competing vaccines may be highly immunogenic and still be judged inferior to licensed vaccines. T-cell dependent vaccines introduce new issues into the evaluation process regarding the analysis of short- and long-term immune response. Also, the kinetics of vaccine response is increasingly being recognized as an important variable in quantifying peak antibody levels after an immunization. This report will also illustrate a method for using multiple immunogenicity endpoints to measure vaccine effectiveness and protection through the use of statistical models and indicate the strengths and weaknesses of using these techniques.

The impact of varicella vaccination in the United States

The addition of varicella vaccine to the universal childhood immunization schedule in the United States in 1995 can be seen as a bold step. Shown to be safe and efficacious against varicella in extensive prelicensure studies, it is nonetheless the first vaccine against a herpesvirus and, furthermore, it is a live, attenuated vaccine. Both wild-type and vaccine strain varicella zoster virus (VZV) are noteworthy for their ability to establish latent infection within the host, with the subsequent possibility of reactivation. Therefore, at the population level, a successful vaccination program could result in the eventual displacement of wild-type VZV by the attenuated vaccine virus. The immediate objective of universal vaccination, however, was to reduce the significant morbidity and mortality associated with primary VZV infection. Data now accumulating suggest that the varicella vaccine as used in the United States has so far been highly effective. The challenge for the future is to predict how the resulting substantial reduction in circulation of VZV will affect immunity among both vaccinees and the unvaccinated. Vaccination strategies likely will need to be adjusted as the epidemiology of VZV in the United States continues to evolve.

Preventing varicella-zoster disease

Varicella-zoster virus (VZV), the cause of chickenpox and shingles, is a pathogen in retreat following the introduction of mass vaccination in the United States in 1995. The live attenuated Oka vaccine, which is safe and immunogenic, gives good protection against both varicella and zoster in the short to medium term. It has undoubtedly been highly effective to date in reducing all forms of varicella, especially severe disease. However, the huge pool of latent wild-type virus in the population represents a continuing threat. Both the biology and the epidemiology of VZV disease suggest that new vaccination strategies will be required over time.

Endpoints in vaccine trials

In this paper we discuss statistical considerations regarding endpoints in preventive vaccine trials. Brief discussion is given to preclinical, Phase I, and Phase II trials, with the bulk of attention paid to endpoint choice and analysis in Phase III efficacy trials. In addition to traditional efficacy measures of vaccine effects for immunized individuals, consideration is given to waning, strain specific efficacy, correlates of protective immunity, postinfection endpoints, and cluster randomized trials.

Ten year follow-up of healthy children who received one or two injections of varicella vaccine

BACKGROUND

The rate of varicella and persistence of varicella antibody after a one dose vs. a two dose regimen of varicella virus vaccine live Oka/Merck (VARIVAX; Merck & Co., Inc., West Point, PA) in approximately 2000 children were compared during a 9- to 10-year follow-up period.

METHODS

Children 12 months to 12 years of age with a negative history of varicella were randomized in late 1991 to early 1993 to receive either one or two injections of varicella vaccine given 3 months apart. Subjects were actively followed for varicella, any varicella-like illness or zoster and any exposures to varicella or zoster on a yearly basis for 10 years after vaccination. Persistence of varicella antibody was measured yearly for 9 years.

RESULTS

Most cases of varicella reported in recipients of one or two injections of vaccine were mild. The risk of developing varicella >42 days postvaccination during the 10-year observation period was 3.3-fold lower (P < 0.001) in children who received two injections than in those who received one injection (2.2% vs. 7.3%, respectively). The estimated vaccine efficacy for the 10-year observation period was 94.4% for one injection and 98.3% for two injections (P < 0.001). Measurable serum antibody persisted for 9 years in all subjects.

CONCLUSIONS

Administration of either one or two injections of varicella vaccine to healthy children results in long term protection against most varicella disease. The two dose regimen was significantly more effective than a single injection.