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Randomization and baseline transmission in vaccine field trials

Published online by Cambridge University Press:  26 June 2006

C. J. STRUCHINER  and
M. E. HALLORAN
C. J. STRUCHINER
Affiliation:
IMS/UERJ and Escola Nacional de Saúde Pública, Fundação Oswaldo Cruz, Rio de Janeiro, Brazil
M. E. HALLORAN
Affiliation:
Fred Hutchinson Cancer Research Center and University of Washington, Seattle, WA, USA
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Abstract

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In randomized trials, the treatment assignment mechanism is independent of the outcome of interest and other covariates thought to be relevant in determining this outcome. It also allows, on average, for a balanced distribution of these covariates in the vaccine and placebo groups. Randomization, however, does not guarantee that the estimated effect is an unbiased estimate of the biological effect of interest. We show how exposure to infection can be a confounder even in randomized vaccine field trials. Based on a simple model of the biological efficacy of interest, we extend the arguments on comparability and collapsibility to examine the limits of randomization to control for unmeasured covariates. Estimates from randomized, placebo-controlled Phase III vaccine field trials that differ in baseline transmission are not comparable unless explicit control for baseline transmission is taken into account.

Type
Research Article
Information
Epidemiology & Infection , Volume 135 , Issue 2 , February 2007 , pp. 181 - 194
Copyright
2007 Cambridge University Press