At JetBlue Airways Corp., executives felt confident coming into the Christmas season. The airline had flown through Thanksgiving week with hardly any hiccups.

Then Covid-19’s Omicron variant arrived. With its headquarters in New York, which leads the country in new case counts, JetBlue was quickly overwhelmed. Daily sick calls more than quadrupled. On Dec. 21, JetBlue canceled no flights. Four days later, on Christmas Day, it scrapped 12% of its schedule.