Delta Air Lines sent the country’s largest flight attendant union a cease-and-desist letter after its president criticized the company’s shortened sick leave policy for staff with Covid-19.

Last Thursday, Sara Nelson, president of the Association of Flight Attendants, tweeted that the union was getting “multiple reports” that Delta “is telling workers across work groups that they should come to work w/ symptoms even if someone in the household tested positive.” She also said that positive workers were told to “come to work after 5 days if the fever is below 100.9, even if still testing positive.”

A day later, Peter Carter, Delta’s chief legal officer sent AFA the letter.

“Not only is this information false, but it is actionable because it places Delta in a highly negative light by suggesting Delta was asking employees to work while they were ill,” said Carter’s letter. “Such irresponsible conduct is inappropriate, defamatory and must cease immediately.”

Nelson, whose union doesn’t represent Delta’s flight attendants but began an organizing drive there in November 2019, defended her comments and said Delta’s policies have confused flight crews.

“Delta’s policy now refers to being asymptomatic before returning to work, which was a serious concern as that CDC guidance was initially omitted from Delta’s policy announcement,” she wrote to Delta CEO Ed Bastian on Tuesday. “But we are still getting questions from Delta flight attendants about returning to work with a low grade fever and about the fact that Delta’s current policy only recommends to test before returning to work and does not require a test.”

Delta updated its Covid sick leave policy on Dec. 28 to five days off with pay protection — reduced from 10 days — that doesn’t require staff to use days in their sick banks. Staff can get an additional two days if they test positive again on the fifth day.

“Delta has always followed the science to form our policies regarding COVID-19,” a Delta spokesman said Tuesday. “We sent a cease and desist letter because we believe institutions and leaders must speak carefully, truthfully, and factually.”

The carrier had asked the Centers for Disease Control and Prevention to halve its recommended isolation time for breakthrough Covid infections to five days, warning about staff shortages and flight cancellations, which later materialized. JetBlue Airways and other carriers asked for the same change. CDC had updated its guidance on Dec. 27, after loosening recommendations for health care workers.

Cancellations from staff out sick from Covid and a series of winter storms surpassed 20,000 between Christmas and the first week of the year. United Airlines, which still has 10 days of pay protection in place for crews with Covid, said Tuesday that it would further trim its schedule, with 3,000 workers, about 4% of its U.S. staff, positive for the coronavirus.

Frontier Airlines and Spirit Airlines give employees 10 days of pay protection if they test positive for Covid.