Assessing Vaccine Durability in Randomized Trials Following Placebo Crossover

TL;DR

This paper develops methods to accurately assess long-term COVID-19 vaccine efficacy in randomized trials where placebo participants switch to the vaccine, ensuring efficacy estimates remain valid despite crossover.

Contribution

It introduces novel analysis techniques that recover vaccine efficacy profiles post-crossover without assumptions on efficacy shape or timing, applicable to long-term durability assessment.

Findings

Efficacy profiles can be recovered after placebo crossover.

Methods perform well under various efficacy dynamics.

Application to simulated COVID-19 trials demonstrates practical utility.

Abstract

Randomized vaccine trials are used to assess vaccine efficacy and to characterize the durability of vaccine induced protection. If efficacy is demonstrated, the treatment of placebo volunteers becomes an issue. For COVID-19 vaccine trials, there is broad consensus that placebo volunteers should be offered a vaccine once efficacy has been established. This will likely lead to most placebo volunteers crossing over to the vaccine arm, thus complicating the assessment of long term durability. We show how to analyze durability following placebo crossover and demonstrate that the vaccine efficacy profile that would be observed in a placebo controlled trial is recoverable in a trial with placebo crossover. This result holds no matter when the crossover occurs and with no assumptions about the form of the efficacy profile. We only require that the vaccine efficacy profile applies to the newly vaccinated irrespective of the timing of vaccination. We develop different methods to estimate efficacy within the context of a proportional hazards regression model and explore via simulation the implications of placebo crossover for estimation of vaccine efficacy under different efficacy dynamics and study designs. We apply our methods to simulated COVID-19 vaccine trials with durable and waning vaccine efficacy and a total follow-up of two years.