The increase in travel comes at a point when a record 125,544 Americans are hospitalized with Covid-19 infections, with about 200,000 new cases reported every day. Meanwhile, a few states including New York have reported their first cases of a new, potentially more transmissible variant of the virus. On Jan. 3, surging cases in the United Kingdom—where the variant was first reported—led the country to implement a full lockdown.

Travel has still not returned to pre-pandemic levels: The record on Jan. 3 is still 260,000 fewer flyers than the slowest day of 2019. Back then, the daily average was 2.4 million; since the pandemic hit, it has mostly hovered between 500,000 and 1 million (the middle of the week tends to be busier than weekends). But the steep increase in holiday-time travel has the potential to accelerate the spread of the virus at a moment when hospitals in many parts of the country are already at capacity.

The drop-off in travel has been catastrophic for airline bottom lines; the industry is expected to see a record loss of $157 billion in 2020 and 2021. And it will still likely be years before air travel is fully back to pre-pandemic levels. A Nov. 24 analysis by the International Air Transport Association, a trade group, projects that global passenger air traffic won’t rebound to 2019 levels until 2024. The pace of vaccination and easing of travel restrictions are a bigger impediment to air traffic than the broader economic recovery, according to the analysis—air cargo traffic is almost back to pre-pandemic levels already.