The U.S. Centers for Disease Control and Prevention warned that the "war has changed" against COVID-19 because the Delta variant is as contagious as chickenpox, can be passed on by vaccinated people and may cause more serious disease than earlier strains.That stark update came in an internal CDC document made public on Friday - which added that the fast-spreading variant required a new approach to help the public understand its danger. While vaccinated people are less likely to become infected – and still greatly protected against severe disease and death - those contracting rare "breakthrough infections" might be just as likely as unvaccinated people to pass the disease on to others, the document said. White House principal deputy press secretary Karine Jean-Pierre said the CDC’s findings underscored the need for more Americans to get the shot. "We need more people to get vaccinated. That's the answer. We need more people to get vaccinated, that's if we want to stop the spread of this variant and avoid mutations, we can." The CDC described the Delta variant as more transmissible than MERS, SARS, Ebola, smallpox, the common cold and seasonal flus. The agency recommended making vaccines mandatory for health care professionals and a return to the universal wearing of face masks. The CDC confirmed the authenticity of the document, which was first reported by the Washington Post. The agency on Friday also released data from a study of an outbreak in Massachusetts in which it said three quarters of those infected had been fully vaccinated. CDC director Rochelle Walensky in a statement said that study played a pivotal role in the agency’s decision earlier this week to again recommend that vaccinated people wear masks in some situations.