In this Dec. 2, 2015 file photo, Kelby Krabbenhoft, president and CEO of Sanford Health, poses for a photo at the Sanford Pentagon in Sioux Falls, S.D. Krabbenhoft, the head of one of the largest regional health systems in the Midwest, has told his employees that he has recovered from COVID-19 and is back in the office — without a mask. Krabbenhoft, said in an email that he believes he's now immune to the disease for "at least seven months and perhaps years to come." (Joe Ahlquist/The Argus Leader via AP, File)