Statistical methodology for evaluating a cell mediated immunity based hiv vaccine
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Statistical Methodology for Evaluating a Cell Mediated Immunity-Based HIV Vaccine. Devan V. Mehrotra* and Xiaoming Li Merck Research Laboratories, Blue Bell, PA *e-mail: [email protected] Biostat 578A Lecture 4

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Statistical Methodology for Evaluating a Cell Mediated Immunity-Based HIV Vaccine

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Statistical Methodology for Evaluating a Cell Mediated Immunity-Based HIV Vaccine

Devan V. Mehrotra* and Xiaoming Li

Merck Research Laboratories, Blue Bell, PA

*e-mail: [email protected]

Biostat 578A Lecture 4

Adapted from Devan’s presentation at the ASA/Northeastern Illinois Chapter Meeting

October 14, 2004


  • Science behind the numbers

  • Merck’s HIV vaccine project

  • Proof of concept (POC) efficacy study

  • Statistical methods

  • Simulation study

  • Concluding remarks

Worldwide Distribution of HIV-1 Clades (Subtypes)*

A, B, AB, Other

B, Other





B, Other




B, G, Other

G, Other

B, O



B, Other


A, Other


AE, B, Other

B, AE, Other



G, Other


B, F, Other





B dominant + Another

C, Other


B, F




B, C



Note:*Dominant clades are bolded above; All regions have multiple clades in their populations

T Cell Recognition of Infected Cells

HIV Infection: CD4 cell count and Viral Load

Merck’s HIV Vaccine Project

  • Lead vaccine is an Adenovirus type 5 (Ad5) vector encoding HIV-1 gag, pol and nef genes

  • Goal: to induce broad cell mediated immune (CMI) responses against HIV that provide at least one of the following:

    Protection from HIV infection: acquisition or sterilizing immunity.

    Protection from disease: if infected, low HIV RNA “set point”, preservation of CD4 cells, long term non-progressor (LTNP)-like clinical state.

Proof of Concept (POC) Efficacy Study

  • Design

    -Randomized, double-blind, placebo-controlled

    - Subjects at high risk of acquiring HIV infection

    - HIV diagnostic test every 6 mos. (~ 3 yrs. f/up)

  • Co-Primary Endpoints

    -HIV infection status (infected/uninfected)

    -Viral load set-point (vRNA at ~ 3 months after diagnosis of HIV infection)

  • Secondary/exploratory endpoints: vRNA at 6-18 months, rate of CD4 decline, time to initiation of antiretroviral therapy, etc., for infected subjects

POC Efficacy Study (continued)

  • Vaccine Efficacy (VE) =

  • Null Hypothesis: Vaccine is same as Placebo

    Same HIV infection rates (VE = 0) and

    Same distribution of viral load among infected subjs.

  • Alternative Hypothesis: Vaccine is better than Placebo

    Lower HIV infection rate (VE > 0) and/or

    Lower viral load for infected subjects who got vaccine

  • Proof of Concept: reject above composite null hypothesis with at least 95% confidence

Notation for Statistical Methodology

Notation (cont’d)

Notation (cont’d)

Competing Methods for Establishing POC

Optimal Weights for Viral Load Component of Composite Test (w2) under Different Scenarios

Methods for Establishing POC (cont’d)

Methods for Establishing POC (cont’d)

Illustration of Simes, Weighted-Simes, Fisher’s, Weighted-Fisher’s Methods (Hypothetical Examples)

Note: w1 = .14, w2 = .86 for weighted-Simes’ and weighted-Fisher’s methods

Critical Boundaries: Simes, Weighted-Simes, Fisher’s, Weighted-Fisher’s

Note: w1 = .14, w2 = .86 for weighted Fisher’s method. Boundaries are shown assuming p2  p1

Additional Notation for Two Other MethodsBasic Idea: Plug in viral load = 0 for uninfected subjects

Additional Notation for Two Other Methods (cont’d)

Methods for Establishing POC (cont’d)

Methods for Establishing POC (cont’d)

Illustrative Example: Hypothetical Data

Illustrative Example: Hypothetical Data (cont’d)

Simulation Study

Assumed Distributions for log10(viral laod)

SD = 0.75



SD = 0.91


μ - δ

Note: Assumed VL distribution for vaccine is asymmetric and more variable (mixture of vaccine “non-responders” and “responders”)

Simulation Study (cont’d)

Simulation Results: Type-I Error Rate (=5%)

Simulation Results: Type-I Error (nominal =5%)

Simulation Results: Power ( = 5%, 1-tailed)



Simulation Results: Power ( = 5%, 1-tailed)



Simulation Results: Power ( = 5%, 1-tailed)



Number of Infections Required for Establishing POC*Simes’, Fisher’s, Weighted-Fisher’smethods80% power, =5% (1-tailed)

Challenge for the Merck Vaccine

  • Pre-existing immunity to Adenovirus Type 5 may prevent or dampen the T cell response to the HIV proteins

  • In the U.S., ~30-50% of people have neutralizing antibodies to Ad-5 virus

  • In Southern Africa, ~75-95% of people neutralize Ad-5

  • Summary of data from Phase I-II trials

    • Ad-5 Neut Titers < 18: ~80% vaccinees have a CD8+ ELISpot response

    • Ad-5 Neut Titers > 1000: ~40% have a response

    • In responders, geometric mean titer ~200 for vaccinees with Ad-5 Neut Titers < 18; ~100 for vaccinees with Ad-5 Neut Titers > 1000

Concluding Remarks

  • For a POC trial of a CMI-based HIV vaccine, Fisher’s (and Simes’) methods are good choices.

  • If the composite null hypothesis is rejected at the 5% level, the p-values for the two endpoints can each be assessed separately at the 5% level.

  • Challenges for the viral load analysis:

    -Initiation of antiretroviral therapy < 3 monthsafter HIV+ diagnosis (“missing” vRNA data)

    -Important to add “sensitivity analyses” to safeguard against potential selection bias (e.g., Gilbert et al, 2003).

    -Estimating causal effect of vaccine on post-infection viral load (ongoing research)



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