The delta variant is "probably the most contagious respiratory virus that we know" of, or so a Belgian biostatistician and professor of evolutionary biologist tells NPR. It was a statement made as part of a larger question: Is that "most contagious respiratory virus" really as contagious as chickenpox? That was the claim made in a leaked slideshow presentation from the CDC and repeated in headline after headline (including by Newser), but it's not quite accurate, at least according to Tom Wenseleers of the University of Leuven in Belgium. NPR describes him as "one of the first scientists to formally calculate the transmission advantage of the alpha and delta variants over the original versions of SARS-CoV-2," and he provides a lesson in the transmissibility of viruses.
The measure used is "R0," and it stands for the average number of people who will be infected by a sick person, assuming every person they encounter is vulnerable to it. Chickenpox is extremely contagious, with an R0 of 9 or 10, meaning one infected person will end up infecting about 10 others. The flu's R0 is much lower, at 2. So where does delta fall? By Wenseleers' calculation, it's about a 6 or a 7. That's much higher than the 2 to 3 of the initial COVID-19 strain but not quite where the chickenpox is. As for how the slide got it wrong, NPR says neither R0 provided was correct: The CDC included preliminary data on delta that had a higher R0 than has now been calculated, and it reports that R0 was placed on a graphic showing other R0s that had been created by the New York Times. It placed chickenpox slightly lower, more in the 8-9 range. (Read Wenseleers' take on what an R0 of 6 means in terms of tamping down the spread of delta.)