How do I know if my child is having an adverse reaction to a vaccine?

Call for questions. Common reactions after vaccines include mild fussiness, low grade fevers for a day or two, and maybe a mild pink rash. (certain vaccines, like measles or chicken pox, tend to cause their reactions after several days or so.) anything beyond these common signs deserves a call to your child's pediatrician.
Call your doctor. Low grade fever, fussiness, and a little redness at the site are common and cause no long term problems. A high fever of 103 or more, a seizure (very rare, ) screaming inconsolably, difficulty breathing, or the presence of a large, warm, reddened area at the site warrant a call to your doctor.