What is the reason for the redness and swelling in a 2-year-old's arm the day after getting a vaccine?

Patient's question:

No

Doctor's answer:

It is necessary to apply pressure, elevate the affected limb appropriately, and apply warm compresses locally. If the effect of the warm compress is not significant, and if the redness and swelling worsen along with fever, it should be considered as an infection. Active anti-infection treatment should be initiated.