4 y/o vaccinated two days ago has large red swelling around the injection site (2-3 in). getting lighter now/fading. reaction or infection?

Reaction: The needle used and the vaccine injected are sterile and the skin is carefully cleaned before hand. Infection is rare and injection reactions are very common. The reaction is usually quite red, very round, sometimes raised & occasionally tender, but, as you've noticed, it goes away w/o treatment. Ice and acetaminophen can help with the symptoms.