Delta has changed the course of the pandemic in the United States yet again. After spurring a summer surge in COVID-19 cases, the more transmissible coronavirus variant is now driving federal health officials’ decision to recommend that everyone, regardless of vaccination status, wear masks again in indoor public places. That’s especially important in areas where infection rates are high, Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, said during a news conference July 27 to announce revised masking guidelines. New evidence that vaccinated people with breakthrough infections of the delta variant produce just as much virus as infected, unvaccinated people prompted the new guidance. That suggests that these people may also easily spread the virus. “We have new science related to the delta variant that requires us to update the guidance regarding what you can do when you’re fully vaccinated,” Walensky said.