Southwest Airlines (NYSE:LUV) has seen the gradual revenue improvement that it experienced this summer decelerate in recent weeks as the number of new coronavirus cases has spiked higher, the airline said Thursday in a regulatory filing.

Airline stocks were hit hard in the early days of the pandemic, as travel demand evaporated almost overnight. The entire industry has been bleeding cash since March, but it saw a modest uptick during the summer months as the number of new cases ebbed and lockdowns lifted.

A Southwest airplane in flight.

Image source: Southwest Airlines.

In its filing, Southwest said that following modest improvements in passenger demand in May and June and again from August to October, the trends are beginning to reverse.

"The company has experienced a deceleration in improving revenue trends for November and December 2020 in recent weeks," Southwest wrote. "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 it "remains cautious in this uncertain revenue environment."

Airline shares rallied earlier this week on promising news concerning the development of a COVID-19 vaccine, but Southwest's commentary is a reminder that the pandemic will continue to weigh on results for the foreseeable future.

Southwest expects revenue to be down 60% to 65% year over year in both November and December.

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