I really a lot on the UK data, right now it is very comprehensive data on all the variants, including Delta. Contact tracing is the best way to determine how transmissible a variant may be. The UK data shows only a small advantage for Delta. I believe that will disappear over time. You may recall that Alpha, B117, was also supposed to be way more transmissible. Once it was dominant and the data was in, it turned out not to be. But I also look at case curves and even leaving aside lots of confounders, like vax or prior infection status, more asymptomatic infections, changing in testing patterns; it isn’t apparent to me that Delta is inherently more transmissible. And here is a chart from Twitter, on cases in Ontario, Canada. You see that Delta largely took over, but cases stayed low. Now it is possible that seasonality trumps variants, but if it were really that much more transmissible, you would see a lot more infections and a lot faster case growth. I firmly believe that if you put the same uninfected person in a room with a person infected and spewing virus of the Alpha variant and then in a room with a person infected with Delta, there would be no greater or different risk of transmission between the two.
And as usual, illogic abounds as people spout opinions on the matter. For example, we hear that Delta leads to higher viral loads, another dubious proposition. But if it does, than any difference in transmissibility might come from the viral dose expelled, not because the virus is inherently more transmissible. At the end of day, just like with Alpha, viral load and transmissibility will turn out to be very comparable and fortunately, the severity of disease much lower.