Received moderna booster 4 days ago. 3 days ago had arm soreness, tenderness underarm and low grade 100 degree temp. arm at injection still mildly sore, underarm still tender and no temp. how long can i except underarm to stay sore? is that normal?

Individual variation of symptoms in post vaccinations incident to Pfizer, Moderna and J&J booster shots will resolve usually within 3-5 days. There is some evidence which supports the severity of symptoms increasing with the time elapsed subsequent to the "fully vaccinated" state (second shot with Pfizer and Moderna).

The soreness should dissipate in a few days. You may take acetaminophen to relieve the pain. No more than 4 g in one day and no alcohol. It is good that you took the Covid Vaccine. Take flu vaccination also. Wish you good health!