Airlines have continued to cancel and delay flights amid staffing shortages and concerns about COVID-19.

A handful of airlines either delayed or canceled flights due to a shortage of flight crew for the sixth day in a row amid an increased spread of the omicron variant of COVID-19.

An estimated 2,400 flights have been canceled as of early Wednesday, with more than 780 of them occurring within, flying into, or departing the United States, according to FlightAware.

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There have also been an estimated 5,100 delays on Wednesday, with more than 1,000 of them occurring in the U.S., the flight tracking data company reported.

Airlines have blamed the cancelations, which have plagued fliers since Friday, on a "massive rise" of sick leave requests amid the spread of the omicron variant.

Inclement weather across the U.S. also factored into the reduced number of flights, with Alaska Airlines telling NBC News it intends to thin the number of Seattle departures by 20% to allow departing aircraft time to de-ice amid the winter storms in western states.

Despite surging reports of the omicron variant among flight crews, flight travel has grown significantly this holiday since the start of the pandemic. The Transportation Security Administration said it screened 1,709,601 people on Christmas Eve, compared with the 846,520 people the agency screened on Christmas Eve 2020.

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This year still marks a decrease from pre-pandemic levels of 2,552,194 on Christmas Eve in 2019, according to the TSA.

Omicron is now the dominant strain of COVID-19 in the U.S., though it is not as pervasive as originally believed, officials say. The new variant, combined with winter conditions, has made the recent spread of infections the second-largest wave since the pandemic began, according to Our World In Data.