True, but the fact that after millions of vaccinations worldwide there have been no (to my knowledge) reports of a vaccinated person carrying an infectious viral load is a pretty good sign, particularly because there are epidemiologists all over the place looking for just this thing. There will never be absolute certainty, of course, because things don't work that way. But if it doesn't emerge as a significant factor -- or doesn't appear at all -- that will be sufficient evidence for public health policy. We'll know pretty soon.
You appear to know your stuff, so I hesitate to comment, but I think that we're far from having an abundance of data to assess the degree to which vaccinated people who become infected may be capable of transmitting the virus.
Sure, millions of people have received vaccine doses, but in all but a fraction of those cases, there is no ongoing monitoring with PCR tests. We gather no data (about viral loads in asymptomatic or mildly symptomatic infections) from the vast majority of vaccinated people after their vaccinations. What's more, we don't yet know what the infectious viral load is.
During its clinical trials, Astra Zeneca did weekly monitoring of participants (PCR test w. nasal swab). Pfizer and Moderna did not do this kind of frequent monitoring. Post-approval, I understand that Pfizer has been doing studies in Israel to get an idea of transmission in largely vaccinated populations. I have also read that the UK is seeking to determine what the infectious dose is, through "challenge trials." I am sure that there are other ongoing studies and that scientists will soon have a better picture, as you say. For the moment, though, I think there's a lack of data - not an abundance - to shed light on this point.
