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.