DALLAS — Southwest Airlines cautioned Thursday that the tenuous recovery in air travel could be fading as coronavirus cases spike across the United States.

The nation’s fourth-biggest airline said after a modest rise in leisure-travel bookings from August through October, it now sees a slowdown in what were improving revenue trends for November and December.

Separately, JetBlue Airways announced it will stop blocking seats starting Jan. 8 - a bet that people feel more confident about traveling on full flights during a pandemic. The airline currently limits flights to 70% of capacity but will raise that to 85% on Dec. 2.

Airline stocks surged on Monday after Pfizer reported promising early results from a trial of a coronavirus vaccine. Delta CEO Ed Bastian called the vaccine news a “welcome glimmer of light in the darkness.” However, the stocks have retreated as new confirmed cases of COVID-19 soared this week, topping 140,000, to set a new record Wednesday.

The report from Dallas-based Southwest added to fears that the spreading virus cases will hurt travel demand heading into Thanksgiving, a key period for airlines.

Air travel remains deeply depressed — in the U.S., it’s down about 65% from a year ago. Although that is improvement over April’s 95% decline, Calio told reporters that U.S. airlines are still losing about $180 million a day.