In a recent publication (Benza et al "Contemporary Risk Scores Predict Clinical Worsening in Pulmonary Arterial Hypertension - An Analysis of FREEDOM-EV"), we conducted an analysis called 'Mediation analysis'. In the statistical analysis section, the 'mediation analysis' was stated as the following:
"To determine whether the change in Week 12 REVEAL Lite 2 risk score ‘mediated’ the treatment effect in delaying clinical worsening, we used SAS (v14.3) CAUSALMED procedure which operationalizes the work of Valeri and VanderWeele.
This analysis attempts to determine what fraction of the total treatment effect appears to be attributable to the treatment effect on the REVEAL Lite 2 score. The analysis was adjusted for baseline REVEAL Lite 2 score; we did the analysis both with and without assuming that there is a treatment and mediator (REVEAL Lite 2 score) interaction on the outcome model (clinical worsening). The definition ‘net clinical benefit’ has been previously proposed as the achievement of all three French non-invasive low risk factors without a clinical worsening event; we retrospectively used the present database to model the performance of this definition."
According to Wikipedia, the mediation model and mediation analysis are defined as the following:
In statistics, a mediation model seeks to identify and explain the mechanism or process that underlies an observed relationship between an independent variable and a dependent variable via the inclusion of a third hypothetical variable, known as a mediator variable (also a mediating variable, intermediary variable, or intervening variable). Rather than a direct causal relationship between the independent variable and the dependent variable, a mediation model proposes that the independent variable influences the mediator variable, which in turn influences the dependent variable. Thus, the mediator variable serves to clarify the nature of the relationship between the independent and dependent variables.
Mediation analyses are employed to understand a known relationship by exploring the underlying mechanism or process by which one variable influences another variable through a mediator variable. In particular, mediation analysis can contribute to better understanding the relationship between an independent variable and a dependent variable when these variables do not have an obvious direct connection.
A mediator variable can either account for all or some of the observed relationship between two variables.
Full Mediation
Maximum evidence for mediation, also called full mediation, would occur if the inclusion of the mediation variable drops the relationship between the independent variable and dependent variable to zero. In other words, the effect of the independent variable on the dependent variable is all through the mediator variable. 
Partial mediation
Partial mediation maintains that the mediating variable accounts for some, but not all, of the relationship between the independent variable and dependent variable. Partial mediation implies that there is no only a significant relationship between the mediator and the dependent variable, but also some direct relationship between the independent and dependent variable - the line from independent variable to dependent variable is solid and c is not equal to zero.
The mediation analysis has been used in the data analysis for observational data, clinical trial data, survey data, and epidemiology study data.
In an article by Eyre et al "Effect of Covid-19 Vaccination on Transmission of Alpha and Delta Variants", the mediation analysis was used to assess whether
the effect of the vaccination status of the index
patient was explained by Ct values at diagnosis. Ct values are cycle-threshold values (indicative of viral load21) in the index
patient.
In an article by Reaven et al "Intensive Glucose Control in Patients with Type 2 Diabetes — 15-Year Follow-up", mediation analyses were performed:
In prespecified mediation analyses, Cox
proportional-hazards models were used to examine the effects of the glycated hemoglobin level
on the primary cardiovascular disease outcome
and on the observed treatment effects. Specifically, the log-linear association of the cumulative glycated hemoglobin level (modeled as a
time-varying covariate) with the primary cardiovascular disease outcome was assessed during
the period of separation of the glycated hemoglobin curves and after convergence.
Models examined the effect of treatment group
(intensive therapy or standard therapy) on the
primary outcome in an unadjusted analysis
(model 1) or while accounting for baseline, most
recent, or cumulative mean glycated hemoglobin
level (models 2, 3, and 4, respectively).
A paper by Vo et al summarized "the conduct and reporting of mediation analysis in recently published randomized controlled trials: results from a methodological systematic review"
Mediation analysis can be performed using SAS procedure CAUSALMED. CAUSALMED procedure was developed for estimating causal mediation effects from observational data, but can definitely be used for estimating mediation effects from the randomized controlled clinical trial data. Please see the references below:
Mediation analysis can be performed using other software. This is very well summarized in a paper by Valente et al "
Causal Mediation Programs in R, Mplus, SAS, SPSS, and Stata".
