Sunday was the busiest day for U.S. airports since mid-March, underscoring concerns that a spike in holiday travel may contribute to the spread of the coronavirus, even as new cases remain alarmingly high and officials plead with Americans to avoid taking unnecessary risks. The Transportation Security Administration screened nearly 1.3 million people on Sunday, the most since March 15, when airline passenger numbers were in free-fall as the pandemic began to take hold within the United States. Since then, the number of travelers screened at airports has exceeded one million fewer than a dozen times, including around Thanksgiving and Christmas. About 3.8 million people passed through T.S.A. travel checkpoints between Dec. 23 and Dec. 26, compared with 9.5 million on those days last year. The surge in holiday travel compared with previous […]