Top US pandemic adviser Anthony Fauci on Sunday warned of a bleak winter ahead as the Covid-19 Omicron variant spurs a new wave of infections globally, sparking restrictions and concerns over hospital capacity. “One thing that’s very clear … is (Omicron’s) extraordinary capability of spreading,” Fauci told NBC News. “It is just … raging through to the world.” Since it was first reported in South Africa in November, Omicron has been identified in dozens of countries, prompting many to reimpose travel restrictions and other measures. Despite indications it is not more severe than the Delta variant – currently still the dominant strain – the heavily mutated Omicron has been shown in early data to have a worrying resistance to vaccines and higher transmissibility. Fauci also cautioned against too much optimism over Omicron’s severity, noting that in South Africa, while the hospitalisation-to-case ratio is lower than with Delta, this could be due to underlying immunity from widespread previous infections. “No matter how you look at it,” he underscored, “when you have so many, many infections, even if it is less severe, that overcomes this slight to moderate diminution in severity.” Bracing for Omicron: masks back as US Covid-19 cases hit 50 million “Our hospitals, if things look like they’re looking now in the next week or two, are going to be very stressed,” particularly in areas of the country with low levels of vaccination, Fauci said. On Sunday, two US senators, Democrats Elizabeth Warren and Cory Booker, said they tested positive for Covid-19 but were experiencing only mild symptoms. Warren, 72, said on Twitter she was “grateful for the protection provided against serious illness that comes from being vaccinated and boosted”, and urged other Americans to do the same. I regularly test for COVID & while I tested negative earlier this week, today I tested positive with a breakthrough case. Thankfully, I am only experiencing mild symptoms & am grateful for the protection provided against serious illness that comes from being vaccinated & boosted. — Elizabeth Warren (@SenWarren) December 19, 2021 Fauci urged unvaccinated Americans to get a shot and the vaccinated to get boosters, which have been shown to re-up protection. The administration of President Joe Biden , who is due to address the nation on pandemic developments on Tuesday, has been campaigning hard for vaccination. While a little more than 70 per cent of the US population has had at least one shot, according to the Centres for Disease Control and Prevention, another 50 million eligible people remain unprotected, Fauci said. “With Omicron … it is going to be a tough few weeks to months as we get deeper into the winter,” he added. Omicron now accounts for around three per cent of cases in the United States, a figure that is expected to rise rapidly as has been seen in other countries. New York coronavirus cases top 20,000 for daily record as shutdowns spread On Saturday, New York state announced a record number of daily cases for the second day in a row, with almost 22,000 positive results. The United States is the nation hit hardest by the pandemic, crossing 800,000 known Covid-19 deaths on Tuesday, according to the Johns Hopkins University tracker.