evidence: Hamer: The challenge is that Delta is so transmissible that I am starting to advise people to wear masks again in supermarkets and stores and public places. But Delta is causing outbreaks mostly in unvaccinated people and only in some vaccinated people. Until we get to the point where transmission slows to a trickle, Delta is just more easily spread, and we are quickly learning that it can easily lead to vaccine breakthroughs and even be spread from one vaccinated person to another.