What it prevents: Hepatitis B is a disease caused by the Hepatitis B Virus (HBV) that infects and damages the liver. While it’s most commonly transmitted through the exchange of bodily fluids or blood (such as through needle-sharing during drug use, unprotected sex, or a blood transfusion of contaminated blood), or passed from a mom to her baby during delivery, more than 30 percent of those affected have no known risk factors.

When it’s given: Three shots: at birth, 2 months, and between 6 and 18 months. It can be given alone, or as part of a combination shot along with the Hib vaccine.

What you may have heard: Because Hep B is most commonly transmitted through risk factors like drug use or unprotected sex, some parents wonder why this vaccine would be given to a child at all, let alone to a newborn. The reason: If you’re infected as an adult with Hepatitis B, you have a less than 10 percent risk of becoming a chronic Hep B carrier, at high risk for developing liver cancer or severe liver disease, both of which can be fatal. In contrast, more than 90 percent of babies who are affected with Hepatitis B will become chronic Hep B carriers. What’s more, more than half of those infected with Hep B do not have any of the common symptoms—jaundiced (yellowed) eyes and skin, skin rash, nausea, and joint pain—so it’s sometimes impossible to know who is affected. It’s estimated that up to 5,000 children in the United States are infected with Hepatitis B each year, and most of these children are born to mothers who are not infected with Hep B.

Risk of a reaction: More than 65 percent of children receiving the vaccine will have no side effects at all. The remaining children may have redness or soreness at the injection site or a slight fever. Very rarely (2 in 100,000) a child may have an allergic reaction (typically those also allergic to Baker’s Yeast), signs of which include difficulty breathing, hives, and a rapid heartbeat.