Tomas Pueyo in his Substack Newsletter:
The original Coronavirus variant has an R0 of ~2.71. Alpha—the “English variant” that caused a spike around the world around Christmas—is about 60% more infectious. Now it appears that Delta is about 60% more transmissible yet again. Depending on which figureyou use, it would put Delta’s R0 between 4 and 9, which could make it more contagious than smallpox. Just to give you a sense of the dramatic consequence of such an increase in R, this is what two months of growth get you with the previous transmission rate of 2.7 vs. with an R of 6 [see figure].
This is why so many graphs of cases look like rockets these days. Delta is very contagious.
Apparently, somebody in Australia was infected by the Delta variant just by walking past an infected person, in a 5- to 10- second encounter. Although this is probably an outlier, and we shouldn’t be scared of walking past other people as a rule of thumb, it illustrates how much more transmissible Delta is.