US airlines cancelled more than 1,000 flights on Monday <a href="https://www.thenationalnews.com/world/us-news/2021/12/24/lufthansa-united-delta-cancel-flights-over-christmas/" target="_blank">after nixing thousands during the Christmas holiday weekend</a>, as Omicron cases soared across the country, forcing crews to isolate and travellers to seek other forms of transport. Eleven hundred flights were cancelled within, into or out of the US by Monday morning, a tally on flight-tracking website <i>FlightAware</i> showed. An additional 4,20 flights were delayed. The flight cancellations on Monday were on top of more than 3,000 cancellations during the Christmas holiday weekend, typically a peak time for travel for Americans. Top US infectious disease expert Anthony Fauci said the country should consider a vaccine mandate for domestic air travel, suggesting it could boost the US's lagging vaccination rate. “When you make vaccination a requirement, that’s another incentive to get more people vaccinated,” Dr Fauci told MSNBC. “If you want to do that with domestic flights, I think that’s something that seriously should be considered.” The Transportation Security Administration reported that more than 7.4 million travellers were screened from December 23-26. Delta cancelled 81 flights on Monday, compared to 189 on Sunday. American Airlines cancelled 86 and Alaska Airlines cancelled 143, <i>FlightAware</i> reported. United said it cancelled 115 flights on Monday, out of more than 4,000 scheduled, due to crews <a href="https://www.thenationalnews.com/travel/2021/12/17/is-it-safe-to-travel-for-the-holidays-amid-omicron-surge/" target="_blank">out with Covid-19</a>. Rising infections have forced airlines to cancel flights, with pilots and cabin crew needing to quarantine, while poor weather in some areas added to travellers' woes. On Monday, the US Centres for Disease Control and Prevention halved the isolation restrictions for Americans with Covid-19 -- as long as they don't have symptoms -- from 10 days to five, and also shortened the quarantine time for those who have come into close contact with a positive person. In announcing the decision, CDC Director Rochelle Walensky said the US can expect to see a lot of Omicron cases, but noted that "many are going to be asymptomatic". The loosened restrictions should be a boost to airlines that are facing staffing crises. Last week, a trade group representing US airlines urged the <a href="https://www.thenationalnews.com/world/us-news/2021/12/21/omicron-variant-now-73-of-us-covid-19-cases/" target="_blank">CDC</a> to shorten the quarantine period for those who test positive for Covid-19. “With the rapid spread of the Omicron variant, the 10-day isolation for those who are fully vaccinated may significantly impact our workforce and operations,” Delta chief executive Ed Bastian wrote Ms Walensky. Mr Bastian noted that the current guidance issued by the CDC was instituted in 2020, when vaccines and treatments were not readily available. More than 90 per cent of Delta staff are vaccinated, he said. The union for flight attendants pushed back against that, saying the isolation period should remain 10 days. Delta and United have both offered crew extra pay to work during the peak holiday shifts, NBC News reported. Winter weather in the Pacific North-West led to nearly 250 flight cancellations to or from Seattle on Sunday, said Alaska Airlines, and the company expects more than 100 flight cancellations on Monday. But it says that crew calling out sick because of Covid-19 is no longer a factor. <i>Agencies contributed to this report</i>