Passengers wait to board a Southwest Airlines flight in January at Dallas Love Field in Dallas, Texas. Southwest warned the fast-spreading delta variant of coronavirus will “make it difficult” to operate in the black during August and September.

There was no news conference or conference call. There was no press release or CEO interview.

Instead, it was a short regulatory filing with the Securities and Exchange Commission. On Wednesday, Southwest Airlines alerted investors that fewer people were buying airline tickets to travel soon. More people also were canceling trips on their calendars in the weeks ahead.

The airline thinks the slowdown is “driven by the recent rise in COVID-19 cases associated with the delta variant.” A week earlier Frontier Airlines blamed the delta variant for a similar slowdown in ticket sales.

Anyone traveling by air this summer knows how busy the airlines have been. Fliers have crowded back into planes so fast that some carriers have had trouble finding enough equipment and people to handle the demand.

Southwest returned to profitability in July, but it warned the fast-spreading delta variant of SARS-CoV-2, the virus that causes COVID-19, will “make it difficult” to operate in the black during August and September. The news did not hurt shareholders when it was released. Southwest’s stock price traded higher the same day it disclosed the delta ding.

Still, the disclosures are tacit acknowledgements and stark reminders that the economy still is dependent upon the virus.

Last month, Federal Reserve Chairman Jerome Powell observed throughout the pandemic, “successive waves” of the virus have tended “to be less economic” in their impact. He noted the vaccine has changed people’s attitudes, and “we’ve kind of learned to live with” the germ.

Delta is testing that.

Thus far during the delta outbreak, the stock market has remained convinced this variant will not have any meaningful economic or business impact. Infections and hospitalizations continue rising. Schools are restarting. Travel plans are changing. And economic growth forecasts have been reduced.

This week investors will be listening if more companies turn cautious as this coronavirus strain keeps spreading.

Financial journalist Tom Hudson hosts “The Sunshine Economy” on WLRN-FM in Miami, where he is the vice president of news. He is the former co-anchor and managing editor of “Nightly Business Report” on public television. Follow him on Twitter @HudsonsView.