The US 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 website 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, hospitalisation and death.
US tells vaccinated people in high Covid risk areas to mask again
The findings add to evidence of how the Delta variant, the fastest and most formidable version of the coronavirus that causes COVID-19, has upended assumptions among virologists and epidemiologists about the disease.
Delta has become the dominant variant globally, documented in 132 countries to date, according to the World Health Organization.
On Tuesday, the CDC reversed course on guidance for mask wearing, reinstituting them as a precaution against the possible transmission of the virus by fully vaccinated people.
The agency said it had seen evidence suggestive of this in outbreak investigations in the United States and overseas. The agency also cited high viral loads as a reason for changing guidance.
On Thursday, US President Joe Biden urged local governments to pay people to get vaccinated and set new rules requiring federal workers to provide proof of vaccination or face regular testing, mask mandates and travel restrictions.
Delta variant drives Eastern Mediterranean virus surge including Pakistan: WHO
The findings may 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.
Israel recently reinstated mask-wearing requirements indoors and requires travellers 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.
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.
U.S. will not lift travel restrictions, citing Delta variant, White House says
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 New York Times said.
Delta appears to be about 50% more transmissible than the three other "variants of concern", all of which are more transmissible than the original virus, spokesman WHO Tarik Jasarevic said on Friday.
"We continue to review the incoming evidence and will update our estimates going forward," he added.