I got the booster shot on December 4th .  I had a low-grade fever the next day (around 100, with ibuprofen) and headaches most of the next week.  I did not have headaches after the first two shots.  I also had my period 3 days early, which rightly or wrongly) I correlated with the vaccine and possibly the headaches.  I flew on the 23rd and drove up to another city, at 8,500 feet altitude.   I had a slight bronchial cough before coming out to Colorado.  It was so minimal, it didn't register as a worry, but I did have to clear my lungs each morning and would hack a bit while biking on my way to work.  By mid-day, it would clear.  I still have that bit of a cough.  I slept restlessly my first three nights.  On the 24th and the 25th I had a slight headache, manageable with ibuprofen.   I woke up on the 26th with a splitting headache, dry cough, and unable to walk up stairs without hyperventilating.  I felt a little better after taking acetaminophen and drinking a liter of water.  I felt a lot better at later in the morning when we descended (6,000 feet altitude).  I didn't immediately feel worse when we came back up, but about half way through lunch I felt my energy levels tank.  In retrospect, it was probably actually my oxygen levels.  I lay down for an hour nap.  When the alarm rang, I could barely wake up.  It took me at least 30 minutes to make it out of my bed to the basement couch, and another 15 to make it upstairs to the living room couch.  My dry cough was back, but a lot worse.  At some point, I forget when, I started experiencing rales (i.e., when I tried to breathe deeply, it felt like I was breathing through a brittle filter that would vibrate with the breath).  Going up or down stairs caused severe hyperventilation, worse than in the morning.  I did not experience any relief when we descended around 6:00.  We went to the ER around 8:00PM.  My oxygen level was 63, dropping to the 50s upon movement.  The ER put me on 4 liters of oxygen somewhere around 8:00PM.  By 2:00 AM, before any medical intervention other than oxygen, I was doing better and if I focused on breathing and sat still, I could keep my oxygen above 90 on my own.  The X-ray showed liquid in the lungs and white blood count indicated an infection.  The PCR test was negative for Covid.  They diagnosed me with atypical pneumonia, put me on antibiotics and steroids (prednisone) and sent me home.  My oxygen dropped to 84 Monday night, so we asked for supplemental oxygen at home which I used the next two nights.  By Friday, I was sleeping comfortably without supplemental oxygen.   I flew back on Monday, January 3rd, without incident.  I had a follow-up appointment with a pulmonologist on Wednesday, January 5th.  The X-ray showed that the lungs were perfectly clear.  The pulmonologist said that bacterial pneumonia, even with antibiotics, does not clean up that fast, so it was probably either viral pneumonia or high altitude pulmonary edema .    No doctor mentioned any possible relation between the illness and the vaccine.  I saw online that interstitial lung disease seems to be a possible, though exceedingly rare, response to the Covid vaccine (and the flu vaccine).  It seems unlikely that I experienced interstitial lung disease, but I am reporting just in case.