DALLAS (AP) — Air travel in the U.S. is hitting new pandemic-era highs, and airlines are scrambling to keep up with the summer-vacation crowds.

Despite rising numbers of coronavirus infections fueled by the delta variant, the U.S. set another recent high mark for air travel Sunday, with more than 2.2 million people going through airport checkpoints, according to the Transportation Security Administration.

That is nearly 11,000 more people screened than July 18, and the highest number since Feb. 28, 2020, before the U.S. felt the full brunt of the pandemic. However, air travel was still down 17% Sunday from the same Sunday in 2019.

The resurgence of leisure travel, coupled with some bad weather, has led to delays and flight cancellations at airlines struggling to ramp up after being crushed by the pandemic. At times, airlines have been caught short-staffed even though they received $54 billion in taxpayer money to keep employees on the payroll.

On Monday, Florida-based discount carrier Spirit Airlines — citing weather and "operational challenges" — canceled about one-third of its flights. The airline is "working around the clock to get back on track," spokesman Field Sutton said.

About 7,400 flights arrived at least 15 minutes behind schedule on Sunday — the government's definition of late — and more than 900 were canceled, according to tracking service FlightAware,

Nearly half the cancellations were at Dallas/Fort Worth International Airport, which was hit with afternoon and evening thunderstorms. The airport is American Airlines' largest hub.

There have been at least 5,000 delayed flights on most days since early July, according to FlightAware figures. American, Southwest and Spirit are among airlines with the biggest problems.

At least 40% of Southwest and Spirit flights were delayed and 19% of Spirit flights were canceled Sunday. On Monday, Spirit canceled 32% of its flights by midday, which created long lines at ticket counters at Orlando International Airport in Florida.

A key senator is quizzing several airlines to explain the high numbers of flight delays and cancellations. Sen. Maria Cantwell, D-Wash., chair of the Senate Commerce Committee, said airlines did a poor job of managing their workforces and might have failed to live up to the purpose of the taxpayer funding.

The travel recovery faces a renewed public-health threat, as the number of new cases of COVID-19 continues to rise. The seven-day rolling average of new U.S. infections is around 80,000 a day, up nearly 150% from two weeks ago, although the increase in deaths is far smaller.

Airline officials say they haven't seen bookings suffer because of the delta variant, although some have said it could delay the return of business travel, which airlines were hoping would gain speed this fall.

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David Koenig can be reached at www.twitter.com/airlinewriter