They reviewed a good paper on TWIV the other day. Tons of data looking at a comparison of live Alpha and Delta variants in cell cultures. So they do now have actual data about Delta invading cells quicker and replicating faster to produce more virus. Of course, this is in cell cultures, and those cultures do not have an immune system! And those cells are from the kidney of an African green monkey, and are used because very few primate cell lines can be grown in cultures.

But still, this is the first confirmation by experiments that Delta probably spreads better than earlier variants. The idea that it is more virulent has been found to be false. If you are not immune and hospitalized for a Delta infection, it is about the same as being hospitalized for any other variant. There are a lot of people who have been infected previously or vaccinated now, and doctors know a lot more about treating Covid patients, so your chances of dying are much lower these days. We also have MABs, which can be VERY effective if you can get them soon enough.

Most deaths are from the unvaccinated, and these folks tend to deny they have Covid until MABS can't help.