inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: ability to provide informed consent. stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. aged 16-69 years. must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per modified cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. patients without documented hypoxia, hypotension, or evidence of end-organ damage who were treated with epinephrine infusion would be considered as cofar grade 3 reaction and may be eligible per investigator discretion. must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. have a primary care physician or other health care provider who will manage their medical care outside of this study.

inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: ability to provide informed consent. stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. aged 16-69 years. must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per modified cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. patients without documented hypoxia, hypotension, or evidence of end-organ damage who were treated with epinephrine infusion would be considered as cofar grade 3 reaction and may be eligible per investigator discretion. must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. have a primary care physician or other health care provider who will manage their medical care outside of this study.

inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: ability to provide informed consent. stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. aged 16-69 years. must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per modified cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. patients without documented hypoxia, hypotension, or evidence of end-organ damage who were treated with epinephrine infusion would be considered as cofar grade 3 reaction and may be eligible per investigator discretion. must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. have a primary care physician or other health care provider who will manage their medical care outside of this study. willing to allow storage of blood and other biological samples for future use in medical research.

inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: ability to provide informed consent. stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. aged 16-69 years. must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per modified cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. patients without documented hypoxia, hypotension, or evidence of end-organ damage who were treated with epinephrine infusion would be considered as cofar grade 3 reaction and may be eligible per investigator discretion. must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. have a primary care physician or other health care provider who will manage their medical care outside of this study. willing to allow storage of blood and other biological samples for future use in medical research.

- inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: - ability to provide informed consent. - stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. - aged 18-69 years. - must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. - must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. - have a primary care physician or other health care provider who will manage their medical care outside of this study. - willing to allow storage of blood and other biological samples for future use in medical research.

- inclusion criteria: in order to be eligible to participate in this study, an individual must meet all of the following criteria: - ability to provide informed consent. - stated willingness to comply with all study procedures (including discontinuing medications as needed and availability for the duration of the study. - aged 18-69 years. - must have experienced a systemic allergic reaction (cofar grade 2 or 3 reaction regardless of tryptase or grade 1 reaction with elevated tryptase [1.2 x baseline plus 2 ng/ml] per cofar grading scale for systemic allergic reactions version 3.0) to the first dose of the pfizer-biontech or moderna covid-19 vaccine. - must be at least 28 days out from their first dose of the moderna vaccine or 21 days from their first dose of the pfizer-biontech vaccine before proceeding with the placebo or vaccine challenge in this study. - have a primary care physician or other health care provider who will manage their medical care outside of this study. - willing to allow storage of blood and other biological samples for future use in medical research.