Technically, Delta is not as contagious as chickenpox

(News)
– The delta variant is “probably the most contagious respiratory virus that we know of”, according to a Belgian biostatistician and professor of evolutionary biologist at NPR. It was a statement made as part of a larger question: Is this “most contagious respiratory virus” really as contagious as chickenpox? This was the claim made in a leaked CDC slideshow presentation and repeated title after title (including by Newser), but it is not entirely correct, at least according to Tom Wenseleers of the University of Louvain in Belgium. NPR describes him as “one of the first scientists to formally calculate the transmission advantage of alpha and delta variants over the original versions of SARS-CoV-2,” and he teaches a lesson in virus transmissibility.

The measure used is “R0”, and it represents the average number of people who will be infected by a sick person, assuming that each person encountered is vulnerable to it. Chickenpox is extremely contagious, with an R0 of 9 or 10, meaning that an infected person will end up infecting around 10 more. The flu’s R0 is much lower, at 2. So where does the delta fall? According to Wenseleers’ calculation, that’s about 6 or 7. That’s much higher than the 2 to 3 of the original COVID-19 strain, but not quite where the chickenpox is. As to how the slide went wrong, NPR says none of the R0s provided were correct: the CDC included preliminary data on the delta that had a higher R0 than was calculated, and it reports that R0 was placed on a graph showing other R0s that had been created by the New York Times. He placed the chickenpox slightly lower, more in the 8-9 range. (Read Wenseleers’ take on what an R0 of 6 means in terms of reducing delta spread.)