Holiday travel headaches and safety worries swelled Sunday with thousands of flights cancelled, events scrapped and new Omicron cases soaring, as people wrap up Christmas celebrations bruised by a resurgent Covid-19 pandemic. Some 8,300 flights have been grounded and tens of thousands more delayed from Friday through Sunday — one of the busiest travel periods of the year — with multiple airlines acknowledging that Omicron spikes have prompted staffing shortages. Effects have rippled worldwide and the hurt was expected to bleed into the work week, with more than 1,100 flights already facing cancellations Monday and almost 300 more on Tuesday, according to flight tracker FlightAware. The highly transmissible Omicron strain has sent new cases skyrocketing across the globe, with countries reviving dreaded lockdowns, major sports leagues cancelling Boxing Day football and rugby fixtures, and cruise ships returning to port with Covid-infected passengers. In China’s Xi’an city, 13 million residents were confined to their homes Sunday amid tightened restrictions, as the country recorded its biggest Covid-19 infection numbers in 21 months. Desperate to keep a lid on outbreaks before February’s Beijing Winter Olympics, China has implemented a vigilant “zero-Covid” strategy, involving tight border restrictions, lengthy quarantines and targeted lockdowns. But there have been sporadic flare-ups. Governments worldwide are also scrambling to boost testing and vaccinations. In the United States, top pandemic advisor Anthony Fauci warned of a Covid “testing problem” and vowed to make more tests available next month, as the virus overwhelms the nation. “There are still some issues now of people having trouble getting tested,” Fauci told ABC News on Sunday. “But we’re addressing the testing problem,” he added, saying it should be corrected “very soon.” Still, cases have soared. New York’s health department said Covid pediatric hospitalizations have risen four-fold over the past two weeks as Omicron took hold. ‘Petri dish’ In Florida, health officials said new Covid cases spiked to 125,201 for the week ending December 23, four times its previous week’s total. The state is a global hub for the cruise industry and US health authorities said dozens of ships were being monitored for possible infection after at least two vessels recorded Covid outbreaks in recent days, despite taking comprehensive measures regarding vaccinations. The Washington Post reported that several ships including the Carnival Freedom — whose parent company confirmed to AFP Sunday that a “small number” of passengers tested positive onboard — were denied port in the Caribbean. Brenda Hammer, who was set to board the Odyssey of the Seas, a Royal Caribbean cruise ship, told AFP on Sunday: “I’m a little nervous about it. I wasn’t sure I still wanted to come.” Earlier this week, 55 people on the ship tested positive for Covid, which spread among passengers and crew members despite 95 percent of people on board being vaccinated, the company said. However, air travel remained the major global headache. Aircrew and ground staff have fallen sick or gone into quarantine after exposure to Covid, multiple airlines said. Lufthansa, Delta, United Airlines, British Airways, Alaska Airlines and several others have been forced to cancel flights. JetBlue said it entered the holiday season with “the highest staffing levels we’ve had since the pandemic began.” But “despite our best efforts, we’ve had to cancel a number of flights, and additional flight cancellations and other delays remain a possibility as we see more Omicron community spread,” the airline said in a statement. Frustrations Chinese carriers, notably China Eastern and Air China, scrubbed several thousand flights over the long weekend, including many going in and out of Xi’an Xianyang International airport, according to tracker data. Flightaware.com reports nearly 2,900 flights were cancelled worldwide on Saturday, and 3,100 and counting on Sunday, with more than 1,300 of those taking off or landing at US airports. More than 14,000 Sunday flights have been delayed. The cancellations added to the frustration for many people eager to reunite with their families over the holidays after last year’s Christmas was severely curtailed. The American Automobile Association estimated more than 109 million Americans would travel by plane, train or automobile between December 23 and January 2, a 34 percent increase over the last year. But most of those plans were made before the detection of Omicron, which has become the dominant strain in the United States, overwhelming hospitals and healthcare workers.