By David Shepardson

WASHINGTON (Reuters) -Southwest Chief Executive Gary Kelly tested positive for COVID-19 after appearing at a U.S. Senate hearing on Wednesday, the airline confirmed on Friday.

Kelly appeared at the Senate Commerce hearing with the CEOs of United Airlines and American Airlines (NASDAQ:AAL) as well as a senior Delta Air Lines (NYSE:DAL) executive and the head of a flight attendants union. Kelly drew attention at the hearing for questioning the health benefits of masks on airplanes.

Delta chief of operations John Laughter, United CEO Scott Kirby (NYSE:KEX) and American Airlines Doug Parker have all tested negative, the airlines said.

Southwest said that although he tested negative multiple times prior to the hearing, "Kelly tested positive for COVID-19 after returning home, experiencing mild symptoms, and taking a PCR test."

The airline said Kelly, who is fully vaccinated and received a booster, "is doing well and currently resting at home." It added his "symptoms continue to be mild, and each day he is moving closer to a full recovery."

Kelly is set to step down next year as chief executive.

Sara Nelson, president of the Association of Flight Attendants-CWA, also tested negative, the union said late Friday. She said in a statement earlier Kelly advised her he tested positive "just as I was returning to work after getting the booster shot. I am following CDC protocols and will test several times within the 5-7 day recommended period, and before traveling with my family for the holidays."

Southwest confirmed Kelly's positive test after Reuters learned of it through other officials. Kelly tested positive Thursday and told other airlines of the positive test the same day, the officials said.

Kelly did not wear a mask during the hearing and questioned the health benefit of masks on airplanes.

"I think the case is very strong that masks don't add much if anything in the air cabin environment -- it's very safe, very high quality compared to any other indoor setting," Kelly said.

Late Friday, Southwest released an email Kelly sent to employees that sought to clarify his comments, saying the airline supports the current federal mask mandate at airports and on airplanes: "There is no effort underway to change it before it expires ... The majority of our Employees and Customers have felt it has been an important layer of protection, and I certainly agree with that."

Earlier this month, the Biden administration extended the federal mandate on masks on airplanes and other transit modes through March 18.

Senate Commerce chair Maria Cantwell said Kelly's COVID-19 diagnosis "underscores the importance of everyone getting vaccinated and following health and safety protocol." Cantwell plans to get a COVID-19 test on Saturday, her office said.