TITLE:
A Simple Method of Measuring Vaccine Effects on Infectiousness and Contagion
AUTHORS:
Yasutaka Chiba
KEYWORDS:
Indirect Effect; Interference; Mediation Analysis; Potential Outcome; Principal Stratification
JOURNAL NAME:
Open Journal of Statistics,
Vol.3 No.4A,
August
23,
2013
ABSTRACT:
The vaccination of one person may prevent another from becoming
infected, either because the vaccine may prevent the first person from
acquiring the infection and thereby reduce the probability of transmission to
the second, or because, if the first person is infected, the vaccine may impair
the ability of the infectious agent to initiate new infections. The former
mechanism is referred as a contagion effect and the latter is referred as an
infectiousness effect. By applying a principal stratification approach, the
conditional infectiousness effect has been defined, but the contagion effect is
not defined using this approach. Recently, new definitions of unconditional
infectiousness and contagion effects were provided by applying a mediation
analysis approach. In addition, a simple relationship between conditional and
unconditional infectiousness effects was found under a number of assumptions.
These two infectiousness effects can be assessed by very simple estimation and
sensitivity analysis methods under the assumptions. Nevertheless, such simple
methods to assess the contagion effect have not been discussed. In this paper,
we review the methods of assessing infectiousness effects, and apply them to
the inference of the contagion effect. The methods provided here are
illustrated with hypothetical vaccine trial data.