In Provincetown, Mass., this summer, a lot of vaccinated people got infected with the coronavirus. And the assumption was that this was an example of vaccinated people with breakthrough infections giving their disease to other vaccinated people. But Ross Kedl says there's a problem with that conclusion. “In all these cases where you have these big breakthrough infections, there's always unvaccinated people in the room.” Kedl is an immunologist at the University of Colorado School of Medicine. He says it's hard to prove that an infected vaccinated person actually was responsible for transmitting their infection to someone else.