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The small size of the phase 3 clinical trials of the Moderna and Pfizer-BioNTech COVID-19 mRNA vaccines limited the ability to define the incidence and management of adverse reactions. Now, two reports provide additional data on anaphylactic reactions and implications of delayed local reactions.
In a prospective study of 64,900 medical employees receiving the first dose of either vaccine, researchers documented reactions within the first 3 days. Acute allergic reactions were reported by 1365 participants (2.10%), significantly more commonly with Moderna (2.20%) than Pfizer (1.95%). Among 16 cases of anaphylaxis (0.02%), 9 and 7 occurred following Moderna and Pfizer, respectively, with one individual requiring hospital admission and ICU care.
The same group reported outcomes in 12 participants with large local reactions to the first dose of Moderna vaccine occurring a median 8 days after immunization. Reactions were associated with variable development of local pain, pruritus, warmth, and erythema (size range, 7.0–19.5 cm). Most patients received treatment with antihistamines and ice, several received topical or oral glucocorticoids, and one received antibiotics for cellulitis. Symptoms resolved a median 6 days after onset. All participants received the second dose in either the same or opposite arm (8 with antihistamine prophylaxis), and only 6 had recurring local reactions (none worse than the first episode).
Blumenthal KG et al. Delayed large local reactions to mRNA-1273 vaccine against SARS-CoV-2. N Engl J Med 2021 Mar 3; [e-pub]. (https://doi.org/10.1056/NEJMc2102131)
Blumenthal KG et al. Acute allergic reactions to mRNA COVID-19 vaccines. JAMA 2021 Mar 8; [e-pub]. (https://doi.org/10.1001/jama.2021.3976)
Comment
These two reports reaffirm the low incidence of severe reactions to either mRNA vaccine — although we still need clarity about the mechanisms of anaphylactoid reactions and precipitating antigens. The case series on local delayed reactions suggests that these apparent T-cell mediated hypersensitivity reactions do not represent a contraindication to a second dose. More follow-up will help guide the management of such reactions (especially given the theoretical concern that more-intense anti-inflammatory therapy might blunt the immune response). I also hope we will soon learn the answer to one of the most frequent questions I'm asked: Does the development of allergic reactions to the vaccine mean the recipient will achieve a higher level of protective immunity?