(Reuters) – Delta Air Lines and Southwest Airlines on Thursday cautioned that the recent surge in COVID-19 cases may have a negative impact on travel over the winter holidays, a period the sector had hoped would see improved bookings.

The United States on Wednesday reported new COVID-19 infections reached an all-time daily high for a second day in a row and the number of people hospitalized also surged to the highest ever during the pandemic.

“With the U.S. hitting a grim milestone of 10 million positive cases and outbreaks in Europe and other parts of the world, all signs point to a challenging winter ahead,” Delta Chief Executive Ed Bastian said in a memo to employees on Thursday.

Earlier, low-cost carrier Southwest said an improvement in revenues in the past few months was losing steam in recent weeks, prompting caution about December trends.

“While the company expected the election to impact trends, it is unclear whether the softness in booking trends is also a direct result of the recent rise in COVID-19 cases,” Southwest said.

“As such, the company remains cautious in this uncertain revenue environment.”

The COVID-19 pandemic brought travel to a near halt earlier in the year, forcing airlines to scale back operations and seek government aid.

(Reporting by Tracy Rucinski; Editing by Sonya Hepinstall)