The Delta variant, which was first detected in India in October and has recently been spreading very quickly in many regions of the world, is widely believed to be more than twice as transmissible as the original strain of the virus. In this post, I start by explaining what people mean when they say that a variant is more transmissible than another, which leads me to make a distinction between a transmissibility advantage and a transmission advantage. In fact, by looking at French data beyond Delta’s initial expansion, I show that, as it became the dominant strain in France, Delta’s transmission advantage collapsed rapidly. Finally, I propose a theory that can explain why Delta’s transmission advantage was initially very high before collapsing, just as Alpha’s before it.