More travelers were screened at airport security checkpoints on Sunday than on any day since the pandemic took hold in March, a worrying sign that people flying to visit their families for Thanksgiving could increase the spread of the coronavirus. A little more than one million people were screened by the Transportation Security Administration on Sunday, according to federal data published on Monday. That number is about half of what it was in 2019, but it represents a big increase from the spring, when less than a half a million people flew on any given day.