Lifestyle Health CDC Warns COVID Delta Variant as Contagious as Chickenpox and Causes More Severe Illness "It's one of the most transmissible viruses we know about. Measles, chickenpox, this — they're all up there," said CDC Director Dr. Rochelle Walensky By Julie Mazziotta Julie Mazziotta Twitter Associate Editor, PEOPLE Health People Editorial Guidelines Published on July 30, 2021 01:04 PM Share Tweet Pin Email Person in a hospital. Photo: Getty The delta variant of COVID-19 is likely as contagious as chickenpox and causes more severe illness than other strains of the virus, according to data found in an internal document from the Centers for Disease Control. Health officials at the CDC said in the document, obtained by the Washington Post, that it's time to "acknowledge the war has changed" as the U.S. struggles to deal with the delta variant with just under half of the country not vaccinated against COVID-19. The findings back up CDC Director Dr. Rochelle Walensky's decision this week to recommend that fully vaccinated people return to wearing masks indoors in areas with high rates of COVID-19, she told CNN after verifying the legitimacy of the document. "I think people need to understand that we're not crying wolf here. This is serious," she said. "It's one of the most transmissible viruses we know about. Measles, chickenpox, this — they're all up there." The CDC also announced this week fully vaccinated people with breakthrough cases may spread COVID-19 as easily as unvaccinated people. What to Know Now About COVID Breakthrough Cases and the Delta Variant As of July 30, new COVID-19 cases have jumped by 151% in the last 14 days, according to The New York Times, and the U.S. is now averaging around 71,600 new infections a day. "The number of cases we have now is higher than any number we had on any given day last summer," Walensky said. RELATED VIDEO: Nurse Whose Husband Died of COVID Is on a Door-to-Door Vaccine Crusade: 'I Can Help Save Others' Along with urging mask-wearing indoors for both vaccinated and unvaccinated people, the CDC has said that anyone in schools this fall, including students, staff and visitors, should wear masks, regardless of their vaccination status. In the document, the CDC also recognized that they need to change their messaging to get more Americans vaccinated. "The measures we need to get this under control — they're extreme. The measures you need are extreme," Walensky said. Four months since the COVID-19 vaccines became widely available, less than half — 49.4% — of the country is fully vaccinated against the virus, according to CDC data. Slightly more, 57.2%, have received at least one dose of a vaccine. Among those who are eligible for vaccination, people aged 12 and up, 66.9% have received at least one dose and 57.7% are fully vaccinated. Fauci Says the Delta Variant Is the 'Greatest Threat' to Ending the COVID Pandemic in the U.S. COVID-19 cases had been on the decline as Americans got vaccinated, dipping down to around 11,000 a day in early June. But as the delta variant became the dominant strain in the U.S., cases again soared back to levels not seen since February, when the vaccines were not readily available to all Americans. The biggest increase in cases have been in southern states like Louisiana, Missouri, Arkansas and Florida, all of which have some of the lowest vaccination rates in the country. On Thursday, President Joe Biden announced that all federal employees will be required to get vaccinated or be subjected to weekly COVID testing. He urged everyone to get vaccinated as soon as possible. "This is an American tragedy. People are dying — and will die — who don't have to die. If you're out there unvaccinated, you don't have to die," he said. "Read the news. You'll see stories of unvaccinated patients in hospitals, as they're lying in bed dying from COVID-19, they're asking, 'Doc, can I get the vaccine?' The doctors have to say, 'Sorry, it's too late.' " As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from the CDC, WHO and local public health departments. PEOPLE has partnered with GoFundMe to raise money for the COVID-19 Relief Fund, a GoFundMe.org fundraiser to support everything from frontline responders to families in need, as well as organizations helping communities. For more information or to donate, click here.