The CDC warned House lawmakers Thursday that the new Delta variant, which is sweeping across the country, as contagious as chickenpox, has a longer transmission window than the original Covid-19 strain and can make the elderly sicker, even if it does they have been fully vaccinated on a copy of the document checked by CNBC and authenticated by the US authorities.

Delta, now in at least 132 countries and already the predominant form of the disease in the United States, is more communicable than the common cold, 1918 Spanish flu, smallpox, Ebola, and MERS and SARS, two diseases also caused by coronaviruses. according to the document. Only measles seem to spread faster than the variant.

“The war has changed,” wrote CDC officials.

Health officials said leaders should communicate the benefits of vaccination to the public.

Vaccines prevent more than 90% of serious illnesses, but they can be less effective at preventing infection, they said, making it more likely to spread to the community among those vaccinated. At the current rate, there are 35,000 symptomatic infections per week in 162 million vaccinated Americans, according to the document.

Separately, the CDC has announced that by July 19, 5,914 fully vaccinated people had been hospitalized or died of Covid infections, the latest data available. Breakthrough cases that occur in those who are fully vaccinated are more common in congregational settings and groups at risk of primary vaccination failure, according to the document.

Health officials also said leaders should consider vaccine mandates, especially for health workers, universal masking and other community containment strategies. President Joe Biden announced Thursday that his administration would require federal workers to provide evidence of vaccination status or to undergo a series of strict safety protocols.

The documents presented to lawmakers came two days after the Centers for Disease Control and Prevention reversed course from their previous guidelines and recommended face masks again to fully vaccinated Americans living in areas with high rates of Covid infection to wear indoors. According to a CNBC analysis, the guidelines cover about two-thirds of the US population.

People infected with the Delta variant carry up to 1,000 times more viruses in their nasal passages than other strains, which, according to the federal health authorities, leads to a higher degree of transmission even among those who have been vaccinated. The CDC found studies in Canada, Singapore, and Scotland that showed higher chances of hospitalization, ICU admission, oxygen needs, pneumonia, or death in people infected with the Delta variant.

While the Delta variant continues to hit unvaccinated people the hardest, some vaccinated people could carry higher amounts of the virus than previously thought and potentially transmit it to others, said CDC Director Dr. Rochelle Walensky on Tuesday. She added that the variant “behaves uniquely differently from previous virus strains”.

“This pandemic continues to pose a serious threat to the health of all Americans,” Walensky told reporters on a call.

Rep. James E. Clyburn, chairman of the Select Subcommittee on the Coronavirus Crisis, said Walensky and White House chief medical officer Dr. Anthony Fauci, informed the committee about the new dates on Thursday.

“I am deeply concerned about the rapidly increasing rates of coronavirus infection in the states of the country that are being powered by the Delta variant,” said Clyburn in a statement, noting that the Covid cases have been in the past two weeks and Hospital stays have increased by 145% and deaths are rising again, especially in areas with low vaccination rates. “This sudden turn of events threatens to undermine the significant strides we have made this year in overcoming the pandemic.”

– CNBC’s Rich Mendez, Robert Towey and Nate Rattner contributed to this report.