As a renewed sense of optimism sweeps the US this summer with lower reported Covid-19 cases, the US Surgeon General has issued a warning for those not vaccinated: don’t let your guard down quite yet. “For those who are unvaccinated, they are increasingly at risk as more and more variants develop,” Surgeon General Vivek Murthy told CNN’s Chris Cuomo on Wednesday, specifically citing the B.1.617.2, or Delta variant, first identified in India. “The news about the Delta variant is evidence of really why it’s so important for us to get vaccinated as soon as possible,” he said, adding that the...