Good explanation of Delta infectiousness.
https://twitter.com/angie_rasmussen/status/1415672461111271424
“... This study attempted to address this in quarantined close contacts of confirmed cases. These people were tested daily by PCR, which allowed the investigators to determine the course of infection: longitudinal viral loads and time from exposure to positivity and symptoms...”
There’s another study that goes further into the mechanics of why Delta is more virulent. It asserts that Delta’s L452R mutation not only increases binding, but escapes cellular immunity, thus the immune system takes longer to detect and attack the virus and infected cells.
https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(21)00284-5