Reported Phase 3 trials were over-conservative in their design. Their primary goal was to fly through FDA rigid acceptance rules. The risk of failure was – months to do another trial.
They passed it. Note the French fiasco. I understand that they did not achieve sufficient efficacy and now going into another Phase 3 (= several months).
All indications are that it will be soon (1-2 months?) clear what is effect of vaccine on transmission – by analyzing current data, updating trials, observing vaccinated population.
Another guess: possibly, recommended dose is too high (to guarantee passing the test). mRNA teams could do that because there is no risk of getting COVID. Traditional vaccines have to more careful. So, if the doses will be lowered, there will be more of them and less side effects.
A note for those taking it soon: today’s FDA panel discussed that taking more time between two doses gives more final protection, but it is hard to recommend more time as this increases time when a person is protected less by just one dose. So, if you are not an exposed medical worker, but a relatively isolated alter – you may want to ask your doctor about spacing doses a little longer.