If everyone accepts that then the company will probably accept it. The doctors I've spoken with say natural immunity lasts for 3-6 months, so part of this would also depend on what the company's medical experts AND the host nation's medical experts say. I get it that you'll probably throw out some argument of what you read online somewhere that fits the narrative you want, but what will really matter is what the host nation says. More than likely it'll be much easier to track a current vaccination status than sifting through everyone's medical records from 300 different countries to determine natural immunity as defined by the host country's medical experts. It's highly likely they'll just require proof of vaccination... When that happens you either have it and are useful as a pilot or you can't fly to those countries and are worthless to FedEx.