The Centers for Disease Control and Prevention's vaccine advisory panel voted Friday to change the recommendation for when children should get their first dose of the hepatitis B vaccine. Instead of a first dose within 24 hours of birth — as the CDC has advised for more than 30 years — the panel voted to recommend delaying it until a child is 2 months old for children born to mothers who test negative for the virus.

U.S. vaccine advisers' decision to scrap longstanding guidance on hepatitis B shots will expose more children to the harmful virus and may signal how other evidence-based vaccine policies can be under...