The Omicron variant of Covid-19 has “extraordinary spreading capabilities”, the top US infectious diseases expert said on Sunday, and promises to bring a bleak winter as it continues “raging through the world”.

Dr Anthony Fauci’s warning came ahead of the busy holiday travel period, which he said would elevate the risk of infection even in vaccinated people.

In an interview with NBC’s Meet the Press, Fauci, Joe Biden’s chief medical adviser and head of the National Institute of Allergy and Infectious Diseases, urged Americans to get booster shots and wear masks.

He also appeared to attempt damage control over Vice-President Kamala Harris’s contention that the Biden administration “didn’t see” the Omicron or Delta variants coming.

Harris’s comments on Friday were “taken out of context”, Fauci insisted, and referred to the “extraordinary number of mutations” of Covid-19 rather than any lack of readiness.

“We were well prepared and expected that we were going to see variants,” he said. “There’s no doubt about that.”

Fauci looked ahead to a scheduled national address by Biden on Tuesday, in which he said the president would “upscale” elements of the White House Covid winter plan.

“He’s going to stress several things,” Fauci said. “… Getting people boosted who are vaccinated, getting children vaccinated, making testing more available, having surge teams out, because we know we’re going to need them because there will be an increased demand on hospitalisation.”

The White House reset comes at the end of a week in which the US surpassed 800,000 deaths from coronavirus and saw a 17% surge in cases and a 9% rise in deaths.

Medical experts have warned of an Omicron-fueled “viral blizzard” sweeping the country. Biden has spoken of a “winter of severe illness and death” among the unvaccinated.

Fauci repeated such dire predictions on CNN’s State of the Union.

“One thing that’s clear is [Omicron’s] extraordinary capability of spreading, its transmissibility capability,” he said. “It is just raging through the world.

“This virus is extraordinary. It has a doubling time of anywhere from two to three days in certain regions of the country, which means it’s going to take over. If you look at what it’s done in South Africa, what it’s doing in the UK, and what it’s starting to do right now, the president is correct.

“It is going to be tough. We can’t walk away from that because with the Omicron that we’re dealing with it is going to be a tough few weeks to months as we get deeper into the winter. We are going to see significant stress in some regions of the country, on the hospital system, particularly in those areas where you have a low level of vaccination.”

Many cases of Omicron are so-called “breakthrough” infections. Florida, one of the hardest-hit states throughout the pandemic, reported on Sunday that about 30% of new infections were in people vaccinated but yet to receive a booster.

Fauci and other experts have said immunisations alone will not prevent the spread of Omicron, but are confident that the risk of serious disease or death is vastly reduced in those who are vaccinated.

Dr Francis Collins, director of the National Institutes of Health, told CBS’s Face the Nation he was concerned about the effects of Omicron on those who are not vaccinated.

“It’s a brand new version and so different that it has the properties to potentially be evasive of the vaccines and other measures that we’ve taken,” he said.

“The big message for today is if you’ve had vaccines and a booster you’re very well protected against Omicron causing you severe disease. Anybody who’s in that 60% of Americans who are eligible for a booster but haven’t yet gotten one, this is the week to do it. Do not wait.”

In New York, authorities said 22,000 people tested positive for Covid-19 on Friday, eclipsing the previous record since testing became widely available.

Meanwhile, a study in South Africa this week suggested that the Pfizer vaccine has a weaker efficacy against Omicron in patients who have received two doses than it does against the Delta variant.

The research by Discovery Health, the country’s largest medical insurance administrator, calculated a 70% protection from hospitalization compared with the unvaccinated, and 33% protection against infection.The group said that represented a drop from 93% hospitalization protection and 80% infection prevention for Delta.