The American Society for Infectious Diseases said at a briefing on TuesdayThe spread of the novel coronavirus variety of delta has raised the threshold of mass immunity to more than 80%, and may be close to 90%.。

Richard Franco, an assistant professor at the University of Alabama, said this was "much higher" than the previous estimate of 60 to 70 per cent because the transmission rate of delta was twice as high as the previous virus.

“Obviously, this is a very dangerous virus, much more dangerous than the original virus."Franco said.

Mass immunization is based on the idea that when a certain percentage of the population has been vaccinated or previously infected, it will help protect the wider population and reduce the spread of the virus.

Nearly 60 per cent of the US population has received at least one dose of COVID-19 vaccine, and about 50 per cent have completed the vaccination, equivalent to about 165 million, according to CDC. Meanwhile, about 35 million people in the United States tested positive for the virus during the outbreak.