In the U.S., hepatitis B vaccination became routinely recommended for all infants in 1991; later guidance specified that the first dose should happen within 24 hours of birth. However, in Dec. 2025, the U.S. Centers for Disease Control and Prevention (CDC) under the Trump Administration moved to cease recommending this practice for babies born to mothers who test negative, instead advising a shared decision-making model where parents and clinicians decide together what is medically appropriate: a dose shortly after birth, or a first dose "no earlier than 2 months of age." (For babies born to mothers who test positive or whose status is unknown, the CDC continues to recommend a dose shortly after birth.)

A new study, published today in Annals of Internal Medicine, uses computer modeling to simulate the hepatitis B infection status of an entire year’s worth of children born in the U.S. under the new recommendations over their first 20 years of life. The researchers find that if only about 26% of children are vaccinated—an estimate of what would occur under shared decision-making—chronic hepatitis B cases would rise by 225%.

When should kids get the first dose of hepatitis B vaccine? 

“It's been challenged in many states and by professional societies, but we wanted to add some evidence to the conversation about what the potential impact of those changes could be,” says Dr. Amir Mohareb, an infectious-disease researcher at Harvard Medical School and Massachusetts General Hospital who is an author of the new study. 

The simulation modeled how many of 3.6 million children, born the same year in the U.S., would develop hepatitis B under a number of scenarios, including if the former CDC recommendations continued to be followed, if the initial dose (also called a “birth dose”) was delayed, and if a shared decision-making scenario was employed. 

The model suggests that if the first dose is delayed until 2 months old, and only 75% of infants get it, then there would be a 19% increase in chronic cases compared to if every child received their first dose at birth. With a 26% vaccination rate at 2 months, the number climbed to 225%. 

This study modeled only at a single year of births, notes Mahoreb. “Imagine that happening year after year after year after year. That could quickly become thousands and thousands of new infections, and even a small number of new infections could result in lifelong chronic infection that could be detrimental to families, lead to more cases of cancer, and [become] costly to the health care system,” Mohareb says.