Couple of reasons. 1) Many of these folk made a firm individual decision not to be vaccinated. Many probably had contracted COVID and recovered, thus becoming immune. 2) Many were probably tired of their jobs, accepted being fired, collect unemployment, take a rest and then if they are competent good people find another job. There is a dearth of good talented people to hire. Evetually the Mayo Clinic like other woke institutions are finding that to remain profitable and efficent, they end up hiring these folk back if they can.
There is also a more than slight chance the fired employees will be part of lawsuit. As all of us amature attorneys know it is much better position to have actual damages than go to court and premptivly stop a employment action.
The 700 are looking at the chance to move to something without taxable income while still having a very good chance at a big check for not working. Duel Income households with new parents would be insane not to have one family member burn family leave and get fired instead of going to part time/sub work. Specialists doctors, a ton that just look at medical imaging 7+ hours a day , can move their same work effort to another corporation that is a services vendor to Mayo. There are probably also 500 lax employees who are on staff but work less than 200 hours a year....IT trainers, super specialists, semi retired management consultants saying to Mayo we will see you in the consulting role later.