evidence: Common Questions About Vaccines [...] Babies have stronger immune systems than you might think, and they can handle far more germs than what they get from vaccines. In fact, the amount of germs in vaccines is just a small percentage of the germs babies' immune systems deal with every day. [...] In rare cases, vaccines can trigger a serious problem, like severe allergic reactions. If your child has a history of allergies to food or medicine, or has had a problem with a vaccine before, tell the doctor before your child gets any vaccines. Every year, millions of kids are safely vaccinated and very few have serious side effects.