Originally Published: 27 DEC 21 11:09 ET

By Jordan Valinsky, CNN Business

(CNN) -- Over the Christmas weekend, flying was a miserable experience for millions of travelers, as airlines canceled or delayed thousands of flights.

The problems continued Monday, with nearly 900 flights canceled within, into or out of the United States, according to FlightAware. More than 1,600 flights are delayed.

The troubles come at the busiest time of year for air travel: The US Transportation Security Administration said it screened millions of people each day over the holiday weekend, peaking at 2.19 million travelers on Thursday, December 23. Seven of the last 10 days have seen more than 2 million screenings.

A nasty brew of issues has complicated air travel, including the rapid spread of the Omicron variant, the labor shortage and a surge in travelers crowding airports and the skies.

Omicron variant

Just when airlines thought they were on the verge of normalcy and profitability again, along came the Omicron variant to put those hopes in doubt. The variant has sparked a sharp uptick in cases -- New York broke a single-day record with 49,708 new Covid-19 cases reported on Christmas Eve.

United Airlines said last week it had to cancel hundreds of flights because it lacked enough crew members to safely fly all of its scheduled routes.

"The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation," said a United memo obtained by CNN.

Delta Air Lines also said the cancellations are due to multiple issues including the Omicron variant.

"We apologize to our customers for the delay in their holiday travel plans," Delta said in a statement. "Delta people are working hard to get them to where they need to be as quickly and as safely as possible on the next available flight."

Airline travel is surging again

Leisure travel is back to near pre-Covid levels. On Friday and Saturday, TSA screened about two-thirds of the passengers it did on those days in 2019 (when Christmas fell on a Wednesday).

AAA recently estimated that more than 109 million Americans will travel over the long Christmas and New Year's week -- a number approaching the pre-pandemic record 119 million travelers of Christmas 2019.

Airlines are projected to carry 6.4 million of those passengers, AAA said. That's about triple the number from last year when the pandemic significantly curtailed holiday travel.

The labor shortage

Airlines were already having trouble finding enough crew to meet the surge in demand for travel. Omicron is making that labor shortage even worse.

Staffing shortages are leading to overworked flight crews and most of the canceled flights. Less choice in flights has led to higher ticket prices. And altercations over masks have been the cherry on the top of a miserable year for travel.

American Airlines and Southwest blamed service meltdowns in October and November on lacking enough pilots and flight attendants to adjust for weather-related cancellations.