US to soon see explosive outbreak of Delta variant: Epidemiologist

"The US has just one month to act before it becomes full blown Delta variant dominant. One month to slow it down. One month to fully vaccinate," Feigl-Ding said in a series of tweets.

He urged the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) to take urgent steps to vaccinate people in the US.

"But we likely have even less time than that if CDC doesn't act soon," Feigl-Ding said.

"If the CDC doesn't issue new guidelines to stem the growth of #DeltaVariant, which can transmit via merely 5-10 seconds of passive breathing exposure, it will be extremely derelict," he said.

"The Delta variant is leaps and bounds faster than any other variant known to date. It represents a clear and present danger to the world. This Delta variant surge will not end well for low vaccination areas," he noted.

Feigl-Ding emphasised on the need for two doses of vaccines and said "1 dose is definitely not enough".

Concerns are rising about the spread of the Delta variant of Covid-19 in the US as President Joe Biden falls short of his July 4 vaccine rollout goal and cases explode in the UK. The highly contagious Delta variant, first identified in India, has spread to at least 92 countries.

According to the CDC, the variant now represents at least 20 per cent of Covid cases in the US, and is expected to become dominant in a matter of weeks.

Besides vaccinations, it is imperative to wear masks.

Feigl-Ding has urged the CDC for reversing its mask rule as nations like Australia and Israel are reimposing masks due to the threat of the Delta variant.

He cited a Queensland health official, who early last week had warned that "5-10 seconds" is all one needs to transmit the Delta variant. It can be a "fleeting moment" of seconds.

Meanwhile, the WHO has also said that being vaccinated may not be enough, and people must continue wearing masks, maintain social distancing and other Covid-appropriate behaviours.

"Israel and now the WHO are both coming out strongly for masks even if fully vaccinated. Israel has 60 per cent people who have received both doses of Pfizer vaccine, yet it knows it needs to reinstate masks because of the new Delta Variant outbreak. The WHO is on the right side of the science this time, better than CDC," Feigl-Ding said.

"Don't act slowly against the Delta Variant. Speed matters. You must move quickly or else once Delta hits a critical mass, you will be in trouble and lose control," he said.