Airlines canceled nearly 1,000 flights on Sunday as the omicron variant of Covid-19 spread rapidly, increasing the number of sick calls from flight crews.

Airlines have canceled around 1,500 flights in the United States since Friday, delaying travel over the busy holiday weekend, which carriers expected to be among the busiest days of the pandemic. The spread of omicron among crews was stated as a reason for the cancellations by United Airlines, Delta Air Lines, American Airlines, and JetBlue Airways.

In a note to pilots seen by CNBC on Sunday, Bryan Quigley, United's senior vice president of flight operations, wrote that the current pilot Covid-19 case count is on the rise. A large number of pilots are on the sick list, and those who have exhibited symptoms are also in quarantine.

According to data from flight-tracking website FlightAware, Delta canceled 6% of its schedule on Sunday, or 164 flights, after canceling 310 the day before. United has canceled 113 flights, or 5% of its mainline schedule, compared to 240 flights the day before. On Saturday and Sunday, JetBlue canceled 252 flights. Over the weekend, American Airlines canceled 178 flights.

Last week, airline executives in the United States petitioned the Centers for Disease Control and Prevention to reduce the quarantine period for fully vaccinated people who test positive for Covid from 10 days to five days, citing potential staffing shortages and flight interruptions. Last Monday, the Centers for Disease Control and Prevention (CDC) relaxed its rules for health-care workers with breakthrough Covid infections.

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