title: Components of the indirect effect in vaccine trials: identification of contagion and infectiousness effects.
abstract: Vaccination of one person may prevent the infection of another either because the vaccine prevents the first from being infected and from infecting the second, or because, even if the first person is infected, the vaccine may render the infection less infectious. We might refer to the first of these mechanisms as a contagion effect and the second as an infectiousness effect. In the simple setting of a randomized vaccine trial with households of size two, we use counterfactual theory under interference to provide formal definitions of a contagion effect and an unconditional infectiousness effect. Using ideas analogous to mediation analysis, we show that the indirect effect (the effect of one person's vaccine on another's outcome) can be decomposed into a contagion effect and an unconditional infectiousness effect on the risk difference, risk ratio, odds ratio, and vaccine efficacy scales. We provide identification assumptions for such contagion and unconditional infectiousness effects and describe a simple statistical technique to estimate these effects when they are identified. We also give a sensitivity analysis technique to assess how inferences would change under violations of the identification assumptions. The concepts and results of this paper are illustrated with hypothetical vaccine trial data.
authors: Department of Epidemiology, Harvard School of Public Health, Boston, MA02115, USA. tvanderw@hsph.harvard.edu
authors: Tyler J
authors: TJ
authors: Vanderweele
authors: Eric J
authors: EJ
authors: Tchetgen Tchetgen
authors: M Elizabeth
authors: ME
authors: Halloran
mesh_terms: D015984:Causality; D003627:Data Interpretation, Statistical; D018562:Disease Transmission, Infectious / Q000517:prevention & control*; D015987:Effect Modifier, Epidemiologic; D006801:Humans; D016014:Linear Models; D016015:Logistic Models; D017063:Outcome Assessment, Health Care / Q000379:methods / Q000706:statistics & numerical data; D016032:Randomized Controlled Trials as Topic / Q000706:statistics & numerical data*; D014612:Vaccines
chemical_list: D014612:Vaccines