Retrospective, Observational Studies for Estimating Vaccine Effects on the Secondary Attack Rate of SARS-CoV-2

TL;DR

This paper reviews the use of retrospective observational studies to estimate COVID-19 vaccine effects on SARS-CoV-2 transmission, highlighting limitations of existing data and advocating for prospective study designs.

Contribution

It identifies challenges and biases in using existing healthcare databases for transmission studies and offers design considerations for future prospective research.

Findings

Retrospective databases have limitations in accurately identifying transmission events.

Common testing strategies introduce biases in vaccine efficacy estimates.

Prospective studies are recommended for more accurate assessment.

Abstract

COVID-19 vaccines are highly efficacious at preventing symptomatic infection, severe disease, and death. Most of the evidence that COVID-19 vaccines also reduce transmission of SARS-CoV-2 is based on retrospective, observational studies. Specifically, an increasing number of studies are evaluating vaccine efficacy against the secondary attack rate of SARS-CoV-2 using data available in existing healthcare databases or contact tracing databases. Since these types of databases were designed for clinical diagnosis or management of COVID-19, they are limited in their ability to provide accurate information on infection, infection timing, and transmission events. In this manuscript, we highlight challenges with using existing databases to identify transmission units and confirm potential SARS-CoV-2 transmission events. We discuss the impact of common diagnostic testing strategies including event-prompted and infrequent testing and illustrate their potential biases in estimating vaccine efficacy against the secondary attack rate of SARS-CoV-2. We articulate the need for prospective observational studies of vaccine efficacy against the SARS-CoV-2 SAR, and we provide design and reporting considerations for studies using retrospective databases.