Experts think the COVID-19 Delta variant is much more contagious than the flu and chickenpox, and on par with the measles. See:
We have found that Ro for the Delta variant is between 9 and 12.7. See:
Values of Ro and herd immunity thresholds (HITs) of well-known infectious diseases prior to intervention. See:
In epidemiology, the basic reproduction number, or basic reproductive number (sometimes called basic reproduction ratio or incorrectly basic reproductive rate), denoted Ro (pronounced R nought or R zero), of an infection is the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection. The definition assumes that no other individuals are infected or immunized (naturally or through vaccination).
We see in the above table Ro is 12 – 18.
We have found that Ro for the Delta variant is between 9 and 12.7.
Clearly the ranges overlap.
Therefore the Delta variant may be as infectious as measles and is likely to be more infectious as the other diseases in the above table.