[taejo]

You're right that the recent efficacy studies of the COVID vaccines only demonstrated protection from the disease. That doesn't mean that they do not prevent spreading the disease, only that we don't know whether they do. Presumably the reason for this choice of endpoint is that we can detect sickness, but we can't reliably detect transmission.

If I may pick another nit, it's not asymptomatic but presymptomatic that seem to be the major concern, from what I've heard: that is, people who spread the disease will eventually show symptoms (usually within a day or two). The balance of evidence seems to suggest that people with viral loads that never get high enough to cause illness are probably not enough to cause a significant number of transmissions, but we will have to wait for widespread vaccination to confirm it.