JetBlue Airways is cutting close to 1,300 flights from its schedule from Thursday through mid-January in anticipation of more Covid-19 infections among pilots and flight attendants.

New York-based JetBlue and other carriers, including United Airlines, Delta Air Lines and American Airlines, have cancelled more than 4,000 flights since Christmas Eve, struggling with bad weather and a surge in sick calls from crews.

“This past week has been one of our most difficult operating periods during the pandemic,” three JetBlue department leaders wrote Tuesday in a note to staff, which was seen by CNBC. “The exponential growth in Omicron cases over just a couple of days is at a level that no one could reasonably prepare for.”

They said the airline would cancel flights “to get even further ahead of the expected increase in Omicron cases.” The planned flight cuts are slightly below 10% of JetBlue’s daily schedules.

JetBlue cancelled 104 flights, or 10% of its schedule on Wednesday.

The Centers for Disease Control and Prevention on Monday cut its recommended isolation time to five days from 10 for individuals who have tested positive for Covid but are asymptomatic.

JetBlue had followed Delta last week in urging the CDC to halve its guidance for isolation for breakthrough Covid cases to five days, warning about staffing shortages and flight disruptions as omicron was rapidly spreading. Other airlines followed suit.

JetBlue updated its leave policies after the CDC’s announcement to allow staff to return to work if they are symptom-free after five days.

The airline department heads said in the staff note that they are hopeful that the new guidelines would help bring back staff faster but added: “we know that Omicron cases have yet to peak in the Northeast (and won’t for at least another week or two) where the vast majority of our Crewmembers are based.”