The U.S. Centers for Disease Control and Prevention (CDC) has described the Delta variant of the coronavirus as being as transmissible as chickenpox and cautioned it could cause severe disease, the Washington Post said, citing an internal CDC document.
The variant was also more likely to break through protections afforded by the vaccines, but the health authority said such incidents were very rare, the CDC report posted on The Washington Post showed.
In its summary, the CDC report said Delta is highly contagious, likely more severe than other variants and breakthrough infections may be as transmissible as unvaccinated cases.
Still, the CDC's figures show that the vaccines are highly effective in preventing serious illness, hospitalization and death.
The findings are likely to stir debate about whether masks, social distancing and other measures may again be needed even in countries with broad vaccination campaigns and where lockdown restrictions have eased.
The CDC report said that universal mask wearing is still needed to reduce transmission in addition to vaccines.
The New York Times, which also reported the contents of the document, said the agency's reverse course this week on masking guidelines for fully vaccinated Americans was based on the report.
Israel recently reinstated mask-wearing requirements indoors and requires travelers to quarantine upon arrival.
"The main thing that does change (because of Delta) is that masks will still be used and that in countries where this requirement has been lifted, it will have to be re-introduced," said Carlo Federico Perno, Head of Microbiology and Immunology Diagnostics at Rome's Bambino Gesù Hospital.
New research showed the vaccinated people infected with the Delta variant carried tremendous amounts of the virus in the nose and throat, CDC Director Rochelle Walensky told the Times.
The Delta variant is more transmissible than the SARS-CoV-2 ancestral strain as well as the viruses that cause MERS, SARS, Ebola, the common cold, the seasonal flu and smallpox, the report said. It said it was as transmissible as chickenpox, a highly contagious infection common in children that causes itchy rashes.
But the variant is roughly as deadly as the ancestral strain, whereas SARS, Ebola and other diseases had far higher fatality rates, the report showed.
The immediate next step for the agency is to "acknowledge the war has changed" and improve the public's understanding of breakthrough infections as well as the big reduction in the risk of severe disease for vaccinated people, the document said.
CDC is expected to publish additional data on the variant on Friday, the NYT said.
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