Other experts are also afraid the strain may further evolve into something more dangerous, since Delta's high transmissibility enables it to spread easily among unvaccinated people, and therefore to keep replicating and mutating. "The worst-case scenario is if Delta mutates into something completely different, a completely different animal, and then our current vaccines are even less effective or ineffective," said Vivek Cherian, an internal-medicine physician in Baltimore... Wachter worries that Delta could turn a mild breakthrough case into a more serious one. "It increases the risk that we're going to see more breakthrough infections and maybe more serious breakthrough infections than I would have worried about a few weeks ago," he said. The biggest risk may be for elderly or immunocompromised people, he added.