[user104658]

Current daily infections graphs suggest that there's a very strong probability that vaccination is driving down infection rates but it's too early to say that conclusively, since it's not really possible to identify exactly who someone has caught a virus from (so you can't say for sure, "oh he got it from Bob but Bob is vaccinated").

They'd have to do broad-scale follow up random testing on asymptomatic vaccinated people, to see if any actually have undetected Covid, and then test to see if those people have a high enough viral load to actually be contagious (since its not as simple as saying "if you have it you can spread it" either)