In order to test transmission after vaccination, they would have to do a challenge study. That would mean exposing people who were both vaccinated and unvaccinated to the virus to see if they got ill. I seem to remember something done like that in the UK with covid and young healthy adults, but many consider that unethical so not many researchers want to go in that direction. It would probably be difficult to get a good number of people willing to sign up for such a study.
