COVID19 SARS Vaccinations: Systemic Allergic Reactions to SARS-CoV-2 Vaccinations

This is a multicenter, randomized, initially blinded (masked), phase 2 trial to assess SARS-CoV-2 vaccination reactions in two populations: * One population including individuals with a history of allergic reactions or Mast Cell Disorder (HA/MCD), and * One comparison population without severe allergies or mast cell disorders. Approximately 2040 HA/MCD and 1360 comparison participants will be enrolled across participating sites in the United States. Approximately two-thirds of participants enrolled in each of the 2 groups will be female. This is because the vast majority of cases of anaphylaxis to the COVID-19 vaccines have occurred in women. Enrollment of participants who qualify only on the basis of reactions to multiple unrelated drugs will be limited to approximately 300. Enrollment of the MCD group is anticipated to be at least 200 participants, and not more than 300 participants. Participants in each population will be randomized 2:2:1:1 to receive the Pfizer-BioNTech COVID-19 Vaccine, Moderna COVID-19 Vaccine, placebo + Pfizer-BioNTech COVID-19 Vaccine, or placebo + Moderna COVID-19 Vaccine, if enrolled in the initial study period under protocol versions 1.0 -4.0. Participants enrolled under protocol version 5.0 will be randomized 2:1 to receive the Pfizer-BioNTech COVID-19 Vaccine or placebo + Pfizer-BioNTech COVID-19 Vaccine. If enrollment under protocol version 5.0 is robust and sustained, the active vaccine randomization may be changed to the Moderna COVID-19 Vaccine. Participants randomized to one of the placebo groups will receive placebo as a first dose and will receive two doses of their assigned active vaccine at subsequent visits. During the first visit, all participants will be initially-blinded to whether they are receiving placebo or vaccine, and to which vaccine they are receiving (if enrolling under protocol versions 1.0 -4.0). Due to the different dosing schedules for the Pfizer-BioNTech COVID-19 Vaccine and Moderna COVID-19 Vaccine, it will become apparent to both the site staff and the study participant which vaccine has been assigned, once the second injection visit is scheduled. However, the blind over placebo versus vaccine will remain in effect until after the second visit. During a follow-up call, scheduled 3 days after the second injection, participants will be unblinded as to whether they received placebo or active vaccine. However under protocol version 5.0, participants will be told if they received placebo as the first injection during the following up conducted 3 days after. Participants in this phase of the study will also know which company's vaccine they are receiving, as the vaccines are anticipated to be used sequentially and will also be noted in the consent form. Study Duration: Randomized and vaccinated participants will complete study participation in approximately 29 days if vaccinated with the Pfizer-BioNTech COVID-19 Vaccine, 36 days if vaccinated with the Moderna COVID-19 Vaccine. Participants enrolled under protocol versions 1.0 -4.0 will complete study participation in 50 or 64 days if administered placebo before receiving two doses of either the Pfizer-BioNTech COVID-19 Vaccine or the Moderna COVID-19 Vaccine, respectively and participants enrolled under protocol version 5.0 will complete participation in approximately 36 of 43 days, respectively, if they receive placebo first.