Vaccine mega-trials: Rare behemoths in the vaccine trial landscape

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The vast majority of vaccines are prophylactic in nature. As a result, the demonstration of their efficacy paradoxically requires the infectious disease to occur among non-diseased study participants randomized between investigational vaccine and appropriate control groups. The statistics of vaccine efficacy (VE) calculation are nearly entirely and solely based on the number of observed incident disease cases during follow-up. For certain diseases, the sample sizes needed to achieve the required number of observations may reach 5,000 subjects or many more, requiring vaccine mega-trials to be conducted. In this article, the author provides an overview of mega vaccine trials conducted over the last 20 years.


Prophylactic vaccines generally are intended for healthy or medically stable populations. Administration to hundreds of thousands or even millions of people in a few yea…

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