The U.S. Centers for Disease Control (CDC) has concluded that the Delta variant of coronavirus which has now emerged as the dominant strain worldwide is as contagious as chickenpox and far more contagious than the common cold or flu.

The findings in the internal document were first reported in the Washington Post newspaper and the report has been confirmed by the CDC. The document, titled: "Improving communications around vaccine breakthrough and vaccine effectiveness", said the variant required a new approach to help the public understand the danger – including making clear that unvaccinated people were more than 10 times more likely than those who are vaccinated to become seriously ill or die.

It recommends mandatory vaccination for frontline health workers and a return to the universal wearing of masks.

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"Acknowledge the war has changed," it said. "Improve communications around individual risk among vaccinated."

The document states that Covid-19 can be passed on even by vaccinated people, and may cause more serious disease than earlier coronavirus strains.

"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," CDC chief Rochelle Walensky said in a statement.

On Friday the CDC released data from a study of an outbreak in Massachusetts in which it said three quarters of those infected had been fully vaccinated. That study was an important factor in leading to a CDC decision this week to again recommend that vaccinated people wear masks in some situations, Walensky said.

The CDC has said that as of July 26, 6,587 people have contracted COVID-19 even after being fully vaccinated and were hospitalized or have died.