The United States set another record for the number of air travelers since the pandemic set in, although passenger numbers remain far below 2019 levels.

Nearly 1.67 million people were screened at U.S. airport checkpoints on Sunday, according to the Transportation Security Administration. That was the highest number screened since March 12 of last year when air travel began to plummet.

However, it was still 35% below the number of airport travelers reported on the comparable Sunday in 2019, according to TSA figures.

Airlines started to see an increase in bookings around mid-February, and the TSA has screened at least 1 million people every day since March 11. Vacation destinations have been most popular — business travel, which is more profitable for airlines, remains drastically diminished.

Airline executives are cautiously optimistic about summer travel as more Americans get vaccinated against COVID-19. The Centers for Disease Control and Prevention says it is safe for vaccinated people to travel within the United States, although it continues to warn against travel to most foreign countries.

Last week, the TSA extended its requirement that airline passengers wear face masks. That rule was set to expire May 11, but TSA extended it through mid-September.

Travelers should expect planes to be more full. Over the weekend, Delta Air Lines stopped blocking off middle seats. It was the last U.S. airline to limit seating as a safety precaution.