“ If this is so different from Covid, why does the same test pick it up? ”
It’s different from the point of view of the antibodies. Since the spike has mutated significantly in the delta, the antibodies from the vaccine are not able to bind to the spike and the virus is able to escape. In that respect it’s different. Otherwise the rest of the virus is much the same.
Good explanation of Delta infectiousness.
https://twitter.com/angie_rasmussen/status/1415672461111271424
Basically, it’s not the difference in the S protein that makes it so transmissable but it’s much more rapid reproduction.
“-People infected with delta test positive more quickly following exposure, suggesting more rapid viral growth.
-The viral loads (by PCR Ct) are 1000 times higher in delta when first detected...”
Link to study in thread (linked to thread because the discussion helps us laymen undertand.)