We actually at the moment still don't know if having some people vaccinated would "prevent cases among their contacts." The published results don't show that at all. What's published is only if the vaccines prevent the occurrence of the illness (as in "did somebody have symptoms"). Also: "Animal challenge data suggest that vaccinated animals may still be infected even if they don't experience symptoms." For more details (including the whole article from which is that citation) see my other comments here.

Unfortunately there is some pessimism about whether a coronavirus vaccine is likely to provide "sterilizing immunity" which would prevent spread, as opposed to just reducing the severity of the disease. https://www.statnews.com/2020/05/22/the-world-needs-covid-19...

I'm a complete layman here, but I would expect that the "prevent spread" is not binary, it's a spwetrum [0,1].

Thus, even if it does not completely prevent spread, if just a part, or even a smallish part, is prevented, that also helps others.

(Same thing in mask discussion. Lots of people dismiss masks because they may not completely prevent an infection; that is stupid, because masks still decrease the probability of infection (and more importantly, spreading to others).

Would this mean we are having a bunch of asymptomatic reinfections resulting in retransmission? Surely we would have seen a lot of examples of this already with test and trace?