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The COVID-19 pandemic due to the novel coronavirus SARS CoV-2 has inspiredremarkable breakthroughs in development of vaccines against the virus and thelaunch of several phase 3 vaccine trials in Summer 2020 to evaluate vaccineefficacy (VE). Trials of vaccine candidates using mRNA delivery systemsdeveloped by Pfizer-BioNTech and Moderna have shown substantial VEs of 94-95%,leading the US Food and Drug Administration to issue Emergency UseAuthorizations and subsequent widespread administration of the vaccines. As thetrials continue, a key issue is the possibility that VE may wane over time.Ethical considerations dictate that all trial participants be unblinded andthose randomized to placebo be offered vaccine, leading to trial protocolamendments specifying unblinding strategies. Crossover of placebo subjects tovaccine complicates inference on waning of VE. We focus on the particularfeatures of the Moderna trial and propose a statistical framework based on apotential outcomes formulation within which we develop methods for inference onwhether or not VE wanes over time and estimation of VE at any post-vaccinationtime. The framework clarifies assumptions made regarding individual- andpopulation-level phenomena and acknowledges the possibility that subjects whoare more or less likely to become infected may be crossed over to vaccinedifferentially over time. The principles of the framework can be adaptedstraightforwardly to other trials.

Estimating Vaccine Efficacy Over Time After a Randomized Study is Unblinded