It could be that fully vaccinated people get sick and think it can’t be Covid so instead of staying in like the rest of us, they go out and spread it.
A remote coworker of mine got covid symptoms three days after traveling. I suggested he needed to stay in because he might have Covid, and he tells me he’s fully vaccinated so it can’t be Covid. He then told me all the places he had to go that day, including visiting his father in a nursing home rehab facility.
He was still sick with a cough that wouldn’t go away and a headache a week later and was having issues breathing, but still insisted it couldn’t be Covid.
Sounds just like the symptoms i had with the China virus.