Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Moderna COVID-19 Vaccine — United States, December 21, 2020–January 10, 2021 | MMWR

The anaphylaxis rate after the first dose of Moderna COVID-19 vaccine is 10 in ~4,000,000.

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Event Rate = 10 / 4,041,396 = 0.00025%

or 

1 in 404,139

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https://www.cdc.gov/mmwr/volumes/70/wr/mm7004e1.htm?s_cid=mm7004e1_w&utm_source=STAT+Newsletters&utm_campaign=356c4680e3-MR_COPY_13&utm_medium=email&utm_term=0_8cab1d7961-356c4680e3-149703525