The US Centres for Disease Control and Prevention (CDC) has described the Delta variant of the coronavirus to be as contagious as chickenpox and could cause severe illness, the New York Times has reported, citing an internal CDC document.

The variant was also more likely to break through protections afforded by the vaccines, the report said, adding that the agency's reverse course on masking guidelines for fully vaccinated Americans on Tuesday was based on this document.

However, CDC's figures show that the vaccines are highly effective in preventing serious illness, hospitalisation and death in vaccinated people, the report said, citing experts.

Read more from Reuters here.