The FDA application for boosters claims that efficacy against transmission for delta for persons vaccinated when the vaccine became generally available is now 42%. That isn't enough to drop Reff below 1 even with 100% vaccination-- in fact, it leaves delta with an Reff higher than the original L strain had absent vaccination.

"no spread" is R=0. Vaccinated people are spreading it to vaccinated people as well as unvaccinated. Assuming that vaccinated people don't know that they get it we have essentially created a big cesspool for the virus to spread undetected.

Luckily that's a bad assumption because the vaccines are so bad that they aren't even protecting people from getting very ill.