Are some COVID vaccines better than others? Interpreting and comparing estimates of efficacy in trials of COVID-19 vaccines

COVID-19 vaccine trials provide valuable insight into the safety and efficacy of vaccines, with individually-randomized, placebo-controlled trials being the gold standard in trial design. However, a myriad of variables must be considered as clinical trial data are interpreted and used to guide policy decisions. These variables include factors such as the characteristics of the study population and circulating SARS-CoV-2 strains, the force of infection, the definition and ascertainment of endpoints, the timing of vaccine efficacy assessment, and the potential for performance bias. In this Viewpoint, we discuss critical variables to consider when comparing efficacy measurements across current and future COVID-19 vaccine trials.