Is the Delta variant really more than twice as transmissible as the original strain of the virus?

Summary

• 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. This belief has generated widespread anxiety and led public health agencies in several countries to revise some of their recommendations.

• 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. While this distinction is rarely made explicitly, it is absolutely crucial to interpret the evidence correctly, as the rest of the post shows.

• I then present the evidence used to support the claim that Delta is more than twice as transmissible as the other variants and argue that, while it clearly shows that Delta had a substantial transmission advantage during its initial expansion in many places, this doesn't show that it has a transmissibility advantage, let alone that estimates of its transmission advantage during its initial expansion accurately estimate any transmissibility advantage it might have.

• 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. This is the exact same thing that already happened a few months ago with Alpha and something that is hard to square with the hypothesis that it's more than twice as transmissible as the original strain of the virus. I also show that Delta's transmission advantage varies wildly across regions, which suggests that other factors besides whatever transmissibility advantage it might have explain why it initially had such a large transmission advantage.

• I explain why, if they continue to do the same thing, epidemiologists will eventually conclude that Omega or whatever they call the next variant of concern has a basic reproduction number of 125, at which point one hopes they will recognize the unreliability of their methods. Unfortunately, while the literature on Delta's transmissibility advantage is full of caveats that show they understand why the evidence must be interpreted carefully, most of them naively plug the estimates in that literature into the models they use to make projections, which has recently led to some spectacular failures.

• 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. This theory crucially rests on the assumption that, unlike what most epidemiological models used during the pandemic assume, the population is highly structured. The effect of complex population structure on transmission has far-reaching implications beyond the debate about Delta's transmissibility advantage, which I will explore in a forthcoming blog post where I will present modeling work I have done on this question.

The Delta variant of SARS-CoV-2, first detected in India last October, has recently been spreading rapidly in many countries and is now the dominant variant of the virus in most of them. According to the American Society for Microbiology, it’s more than twice as transmissible as the original strain of the virus, while the CDC claimed it was as transmissible as chickenpox. As a result, the agency recently published new guidelines on masking, recommending that even vaccinated people wear masks indoors in communities with high transmission of the virus. While there is compelling evidence that vaccines work fine against Delta, such as this study based on data from England, many are concerned that Delta’s high transmissibility means that vaccination will not be sufficient to contain the virus and that non-pharmaceutical interventions such as masking or even lockdowns and curfews will be necessary again. But I think the consensus on Delta’s transmissibility is deeply flawed and I will explain why in this post. I will start by explaining how epidemiologists have reached the conclusion that Delta is more than twice as transmissible as the original strain of the virus and why the inference they’re making could easily be misleading. In doing so, I will clarify some conceptual issues that I think are important to interpret the evidence correctly, but haven’t received enough attention in this debate. I will then present evidence showing that in fact Delta isn’t as transmissible as epidemiologists and public health officials claim. Finally, I will argue that, by not taking into account that evidence and assuming that Delta is more than twice as transmissible as the original strain in the models they used to make projections, epidemiologists are providing misleading guidance to decision-makers that might lead them to implement suboptimal policies.

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