My 6 month old son got his 6 months shot on thursday and is still running fever between 100 and 103.giving tylenol (acetaminophen) every four hours. urine test - ve.

48 hours of fever: Poor baby! This timeline is definitely in the realm of fever related to vaccines. I would decide according to how the baby is looking after a dose of motrin. If he perks up and gives you a smile with a dose of motrin, then you can watch him tonight. if he appears ill, is not feeding well, or is hard to console, then bring him to the ER now.

Fever: Is your little one perking up when you give him Tylenol (acetaminophen)? How is he drinking? Still making wet diapers? Alert and smiling or quiet and sleepy/cranky? The immune system definitely gets revved up when given vaccines and fever is part of the immune reaction that can occur. If anything seems off, get him checked.