Symmetric Vaccine Efficacy

TL;DR

This paper introduces symmetric vaccine efficacy (SVE), a bounded and interpretable measure that addresses the asymmetry and interpretability issues of traditional VE, with practical tools for analysis.

Contribution

It proposes SVE as a new symmetric measure of vaccine efficacy, detailing its statistical properties, relationship to VE, and providing open-source R tools for its computation.

Findings

SVE ranges within [-1, 1], offering a symmetric scale for vaccine effects.

Reanalysis of HIV vaccine trial data demonstrates SVE's practical utility.

Open-source R package 'sve' facilitates SVE estimation and inference.

Abstract

Traditional measures of vaccine efficacy (VE) are inherently asymmetric, constrained above by $1$ but unbounded below. As a result, VE estimates and corresponding confidence intervals can extend far below zero, making interpretation difficult and potentially obscuring whether the apparent effect reflects true harm or simply statistical uncertainty. The proposed symmetric vaccine efficacy (SVE) is a bounded and interpretable alternative to VE that maintains desirable statistical properties while resolving these asymmetries. SVE is defined as a symmetric transformation of infection risks, with possible values within $[-1, 1]$, providing a common scale for both beneficial and harmful vaccine effects. This paper describes the relationship between SVE and traditional VE, considers inference about SVE, and illustrates the utility of the proposed measure by reanalyzing data from a randomized trial of a candidate HIV vaccine. Open-source tools for computing estimates of SVE and corresponding confidence intervals are available in R through the sve package.