Generalized itching began about 30 min following vaccination, followed by acute airway issues (cough, shortness of breath, wheezing). At outset of symptoms, I took 1 chewable Zyrtec (10mg) and 1 hydroxyzine (25mg-allergy to benadryl. One dose of epinephrine was administered by clinic staff at about 45min or so after injection, followed by them calling EMS for ED transport. At ED, physician ordered 125 mg Solu-medrol and 3x dosing of Duo Neds. Duo nebs were repeated q. need or q.4h. My normal blood pressure (non-anaphylaxis) runs high, typically requiring repeat measurements to fall at or under 140/90. During ED management (approximately 25-25h of observation), my BP fell as low as 94/50 or low 100s/40s (examples). I was finally allowed to take my MCAS medications (Zyflo and ketotifen) (late evening on 2/24 and the morning of 2/25) that I routinely take prior to any vaccinations or medical procedures. For the week prior, this including Cromolyn, Zyflo, Ketotifen (dosing beginning 7d before the vaccine) and 4x Singulair dose (2x dose, 2x per day starting 2days prior). Along with my usual daily meds. When getting home from the hospital, my peak flows continued to be much lower that usual (after 2d on steroids - respiratory symptoms improved, my Peak Flows are still only 350, compared to now (3/20) when my PF is 460-470. My asthma was reasonably well controlled (no overt symptoms) the morning of my vaccine. My recent experience (since 2019) with Zyflo and ketotifen is that when I continue to to take these after a reaction, they help to reduce the number and severity of my rebound reactions.   Technically, I was in the Emergency Department for my entire stay after the latest vaccine reaction, which was about 25-26h. I was moved from the main ED to their holding tank/observation unit across from the ED, and still staffed by ED people (mostly RN/NP). I opted to be discharged after 26h rather than waiting an additional 11h for a hospitalist consult because the NP did not want to consult with my usual care team - PCCM. I opted to get out and call pulmonary directly for advice post-discharge, which included using a step down dosing in prednisone on the following days of 40mg, 20mg, then 10mg. The advice was to followup with Internal Medicine at the beginning of the week and let them evaluate for need to continue with more prednisone, which I did on Monday afternoon. Her advice was to wait and see versus taking more prednisone, which I also followed. It took about 2 weeks for my airways to return to "normal"function and I did not add in any additional prednisone. However, I did continue with my MCAS meds at least through Monday of the following week instead of stopping soon after the vaccine.  General comment: It is my understanding that Pfizer changed one ingredient (PSB-- > TRIS buffers) between doses 3 (booster #1) and 4 (booster #2). I am neither an anti-vaxxer nor someone reading social media to manufacture nonsense triggering vaccine concerns.  However, because of my MCAS and multiple triggers,  I DO look for possibly commonalities between reactions to different medications. Tris (tromethamine) is also in toradol and gadolinium. Toradol was my worst anaphylaxis ever. Since this is the only change in components, this seems to be a reasonable explanation for the reaction. Many of these reactions triggered by my MCAS are more likely to be anaphylactoid-mediate rather than classically-IgE mediated.  Since the actual vaccine in the Pfizer Covid vaccine is RNA-based, I am defaulting to assume the change in buffer is the more likely culprit than IgE-mediated reaction to viral RNA (just making some basic molecular biologist/virologist assumptions here).   Finally, while my personal medical history reads like its own horror story, I continue to be high functioning and still swim masters whenever the pandemic suggests my exposures at the pool are well managed, also considering when the threat of waning immunity and resurgence of Covid are taken into account.