I conduct biostatistics research with application to design and analysis of clinical
trials, with emphasis on vaccine trials. Methodological research areas include analysis
of pathogen genetics, evaluation of surrogate endpoints (especially vaccine-induced
immune responses), survival analysis, and causal inference. 

General Statistical Methods in Vaccine Trials

Increasing the Efficiency of Prevention Trials by Incorporating Baseline Covariates (with Min Zhang), Statistical Communications in Infectious Diseases (2010)

Most randomized efficacy trials of interventions to prevent HIV or other infectious diseases have assessed...



Some design issues in Phase 2b prevention trials for testing efficacy of products or concepts, Statistics in Medicine (2010)

After one or more Phase 2 trials show that a candidate preventive vaccine induces immune...



Efficient and robust method for comparing the immunogenicity of candidate vaccines in randomized clinical trials. (with Devan Mehrotra, X Sun, and Alicia Sato), Vaccine (2009)

In randomized clinical trials designed to compare the magnitude of vaccine-induced immune responses between vaccination...



A Bayesian framework for estimating vaccine efficacy per infectious contact (with Yang Yang, Ira Longini Jr, and Elizabeth Halloran), Annals of Applied Statistics (2008)

In vaccine studies for infectious diseases such as human immunodeficiency virus (HIV), the frequency and...


Surrogate Endpoint Evaluation


Statistical identifiability and the surrogate endpoint problem, with application to vaccine trials. (with Julian Wolfson), Biometrics (2010)

Given a randomized treatment Z, a clinical outcome Y , and a biomarker S measured...



Assessing surrogate endpoints in vaccine trials with case-cohort sampling and the Cox model (with Li Qin, Dean Follmann, and D Li), Annals of Applied Statistics (2008)

Assessing immune responses to study vaccines as surrogates of protection plays a central role in...



Evaluating a surrogate endpoint at three levels, with application to vaccine development. (with Li Qin and Steve Self), Statistics in Medicine (2008)

Identification of an immune response to vaccination that reliably predicts protection from clinically significant infection,...



Evaluating candidate principal surrogate endpoints. (with Michael G. Hudgens), Biometrics (2008)

Frangakis and Rubin (2002, Biometrics 58, 21–29) proposed a new definition of a surrogate endpoint...


Competing Risks/Sieve Analysis (Dependency of Vaccine Efficacy on Pathogen Genetics)


Proportional hazards models with continuous marks. (with Yanqing Sun and Ian McKeague), Annals of Statistics (2009)

For time-to-event data with finitely many competing risks, the proportional hazards model has been a...



Genome scanning methods for comparing sequences between groups, with application to HIV vaccine trials (with Chunyuan Wu and David Jobes), Biometrics (2008)

Consider a placebo-controlled preventive HIV vaccine efficacy trial. An HIV amino acid sequence is measured...



Testing and estimation of time-varying cause-specific hazard ratios with covariate adjustment. (with Yanqing Sun and S Hyun), Biometrics (2008)

In the evaluation of efficacy of a vaccine to protect against disease caused by a...



The two-sample problem for failure rates depending on a continuous mark: an application to vaccine efficacy (with Ian McKeague and Yanqing Sun), Biostatistics (2008)

The efficacy of an HIV vaccine to prevent infection is likely to depend on the...



Two-sample tests for comparing intra-individual genetic sequence diversity between populations (with Anthony Rossini and Raj Shankarappa), Biometrics (2005)

Consider a study of two groups of individuals infected with a population of a genetically...


Causal Vaccine Effects Conditional on a Post-Randomization Event


Semiparametric estimation of the average causal effect of treatment on an outcome measured after a post-randomization event, with missing outcome data. (with Yuying Jin), Biostatistics (2009)

In the past decade, several principal stratification–based statistical methods have been developed for testing and...



Semiparametric estimation of treatment effects given base-line covariates on an outcome measured after a post-randomization event occurs (with Andrea Rotnitzky, Bryan Shepherd, and Yannis Jemiai), Journal of the Royal Statistical Society Series B (2007)

We consider estimation, from a double-blind randomized trial, of treatment effect within levels of base-line...



Sensitivity analyses comparing time-to-event outcomes only existing in a subset selected post-randomization, conditional on covariates, with application to HIV vaccine trials (with Bryan Shepherd and Thomas Lumley), Journal of the American Statistical Association (2007)

In some randomized studies, researchers are interested in determining the effect of treatment assignment on...



Sensitivity analyses comparing outcomes only existing in a subset selected post-randomization, conditional on covariates, with application to HIV vaccine trials (with Bryan Shepherd, Yannis Jemiai, and Andrea Rotnitzky), Biometrics (2006)

In many experiments, researchers would like to compare between treatments and outcome that only exists...



Sensitivity analysis for assessing vaccine effects on viral load in HIV-1 vaccine trials (with Ronald Bosch and Michael Hudgens), Biometrics (2003)

Vaccines with limited ability to prevent HIV infection may positively impact the HIV/AIDS pandemic by...


Repeated-Low Dose Non-Human Primate Challenge Trials


Assessing vaccine effects in repeated low-dose challenge experiments. (with Michael G. Hudgens), Biometrics (2009)

Evaluation of HIV vaccine candidates in nonhuman primates (NHPs) is a critical step toward developing...



Power to detect the effects of HIV vaccination in repeated low-dose challenge experiments. (with Michael G. Hudgens, C Wu, Dan Barouch, John Mascola, and Steve Self), Journal of Infectious Diseases (2009)

Simulation studies were conducted to estimate the statistical power of repeated low-dose challenge experiments performed...