The Annals of Applied Statistics

Design of vaccine trials during outbreaks with and without a delayed vaccination comparator

Natalie E. Dean, M. Elizabeth Halloran, and Ira M. Longini

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Conducting vaccine efficacy trials during outbreaks of emerging pathogens poses particular challenges. The “Ebola ça suffit” trial in Guinea used a novel ring vaccination cluster randomized design to target populations at highest risk of infection. Another key feature of the trial was the use of a delayed vaccination arm as a comparator, in which clusters were randomized to immediate vaccination or vaccination 21 days later. This approach, chosen to improve ethical acceptability of the trial, complicates the statistical analysis as participants in the comparison arm are eventually protected by vaccine. Furthermore, for infectious diseases, we observe time of illness onset and not time of infection, and we may not know the time required for the vaccinee to develop a protective immune response. As a result, including events observed shortly after vaccination may bias the per protocol estimate of vaccine efficacy. We provide a framework for approximating the bias and power of any given analysis period as functions of the background infection hazard rate, disease incubation period, and vaccine immune response. We use this framework to provide recommendations for designing standard vaccine efficacy trials and trials with a delayed vaccination comparator. Briefly, narrower analysis periods within the correct window can minimize or eliminate bias but may suffer from reduced power. Designs should be reasonably robust to misspecification of the incubation period and time to develop a vaccine immune response.

Article information

Ann. Appl. Stat. Volume 12, Number 1 (2018), 330-347.

Received: September 2016
Revised: August 2017
First available in Project Euclid: 9 March 2018

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Vaccine trial infectious diseases per protocol


Dean, Natalie E.; Halloran, M. Elizabeth; Longini, Ira M. Design of vaccine trials during outbreaks with and without a delayed vaccination comparator. Ann. Appl. Stat. 12 (2018), no. 1, 330--347. doi:10.1214/17-AOAS1095.

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Supplemental materials

  • Supplement to “Design of vaccine trials during outbreaks with and without a delayed vaccination comparator”. The supplementary materials contains additional plots and tables under a range of scenarios. Supporting R code is also provided to produce estimates and plots.