Here’s another article from 5/18/2020 with some information on the reactions. If this is what they are talking about, this is typical of vaccines which stimulate an immune response
https://www.fiercebiotech.com/biotech/moderna-posts-positive-early-data-for-covid-19-vaccine
. . . Of the 30 patients who received the two lowest doses, only one reported a side effect considered to be grade 3: redness around the injection site. Three people who got the highest dose, 250 mcg, developed grade 3 “systemic symptoms,” including fever, muscle pain and headache, Moderna Chief Medical Officer Tal Zaks, M.D., Ph.D., said on a call discussing the data. Although the side effects were temporary and went away on their own, Moderna won’t carry the highest dose into future trials. . .
Thanks.