They haven't so far. The delta variant first started infecting people 8 months ago and now make up something like 70% of new cases and in 8 months here the death rate has only gone down. Delta is more transmissible but it isn't statistically more deadly. With the delta variant, vaccinated people are catching a cold and getting over the cold. It sucks greatly if you're unvaccinated, and doubly so if you have comorbidities, so everyone should get vaccinated but for the vaccinated this isn't a health crisis. Requiring fully vaccinated people to wear unregulated cloth masks isn't about saving anyone's life.
There are over 330 million people in the US and a lot die every day. Per the CDC over 1,900 people died per day from accidents. More people right now are dying daily from Alzheimer's than SARS-CoV-2. I'm not saying it isn't a big deal and I don't understand why some are so resistant to getting vaccinated but I also am rolling my eyes pretty hard at government reinstituting mask mandates for vaccinated people.