Hi. my son went for his 9 month immunization and now his arm is hot and swollen where they imjected?

Time to go back: A big reaction means a return trip to the doctor to evaluation for an infection or a non-infectious reaction at that injection site. There is usually no other way to solve the issue with an examination of the injection site. The clinic staff can review the record to verify which injection was given, with the person who gave the injection. Usually, injection site reactions resolve without problems.