Early evidence suggests that the highly contagious delta variant, which has prompted concern worldwide as it leads to new surges of Covid-19 across the globe, is spreading in undervaccinated pockets of the U.S. The genomics firm Helix analyzed about 20,000 samples from Covid-19 tests across more than 700 U.S. counties. It found that cases of the variant first documented in India appear to be growing far faster in counties with lower vaccination rates than in areas that have higher rates... In a new report issued by the CDC on Monday, the agency found that among those 18 to 39 years old, vaccine safety and effectiveness were both commonly cited barriers to vaccination. The agency said it is especially important to reduce hesitancy among younger Americans, who have been among the demographic groups least likely to take the vaccine.