Earlier quoted context omitted.
The most likely explanation is here [1]. Pilots (and apparently some air traffic controllers) are doing a sickout to protest vaccine mandates, presumably to make the powers that be understand just how difficult they can make things for the country if they proceed with intended mass firings. But the sickout is technically illegal, so nobody will acknowledge it. That is why you won't see a clear explanation of this inc…
I don't find pilots refusing vaccines in significant numbers plausible in the slightest. Airline pilots have to comply with plenty of regulations and they're continuously trained to comply with safety regulations. Thus you don't become a pilot or don't remain one for long if you're allergic to safety regulations. If there's one occupation where expert recommendations are respected, it's aviation, and for good reasons…
Just make COVID-19 vaccination a requirement to pass your medical, and be done with all this hand-wringing.