Infants are particularly vulnerable to hepatitis B. Changes to federal hepatitis B vaccination recommendations could lead to more childhood infections and leave up to 1.8 million young adults vulnerable to the virus, a new study finds.

It comes as health experts raise concerns about the potential long-term impact of changing childhood vaccination recommendations.

In late 2025, the Centers for Disease Control and Prevention (CDC) removed the universal hepatitis B vaccination recommendation for newborns. That decision, as well as other vaccine changes by the CDC under the Trump administration, have since been placed on pause following legal action.

"These findings show why policy makers should re-implement recommendations for universal hepatitis B vaccination at birth," said corresponding author Dr. Amir Mohareb, an infectious disease physician and clinician researcher at the Center for Global Health at Mass General Brigham, in a press release Monday. What is hepatitis B?

Hepatitis B is a viral infection that attacks the liver and can cause either a short-term illness or a lifelong infection, according to the CDC.

Many people have no symptoms while others may experience nausea, abdominal pain or yellowing of the skin and eyes, known as jaundice. Hepatitis B spreads through contact with infected blood and other bodily fluids.

Infants are particularly vulnerable to hepatitis B. According to the American Academy of Pediatrics, approximately 90% of babies infected around the time of birth develop chronic hepatitis B. Chronic hepatitis B can lead to serious complications, including liver damage.

"The hepatitis B vaccine can prevent potentially life-threatening chronic liver disease, which can lead to liver cancer," Dr. Alok Patel, clinical associate professor of pediatrics at Stanford, told ABC News. "Since we started vaccinating at birth in 1991, infant infections have dropped by a reported 99%."

Thousands at risk without routine vaccination To understand how changes to hepatitis B vaccine recommendations could affect children, researchers from Mass General Brigham used a computer model to follow a hypothetical group of 3.6 million babies born in the United States in 2026 through their first 20 years of life.

The researchers compared three approaches to hepatitis B vaccination for babies born to moms who test negative for hepatitis B in pregnancy.

The first followed the longstanding recommendation that all newborns receive their first hepatitis B vaccine dose shortly after birth. The second delayed the first dose until babies were at least 2 months old.

The third used a strategy called "shared clinical decision-making," meaning parents and their child's healthcare provider would discuss whether and when to vaccinate rather than routinely recommending the vaccine at birth for every baby.

The study, published in the Annals of Internal Medicine, estimated that delaying vaccination until 2 months could increase chronic hepatitis B infections by 19%, while the shared decision-making approach could increase infections by 83% compared with routinely vaccinating all newborns at birth, in 20 years.

The model also predicted that approximately 232,000 children could remain vulnerable to hepatitis B infection when they reached age 20 under the universal birth dose approach. That number increased to 474,000 when vaccination was delayed and 1.8 million under the shared decision-making approach.

Importantly, these figures represent children who could still become infected because they lack protection against hepatitis B, not children who have already developed the disease.

"We found that, even under the most optimistic conditions, the new recommendations could increase childhood hepatitis B infections by more than 80% and would result in more than half of children being susceptible to hepatitis B infection as they enter adulthood," Mohareb said.

If high-risk babies who are born to mothers who test positive for the virus or whose hepatitis B status is unknown were less likely to get a hepatitis B vaccine, the model predicted perinatal infections could double or triple in 20 years compared to keeping the universal vaccine recommendation in place.

Researchers emphasized that these findings are estimates from a computer simulation, not actual infections.

Why hepatitis B vaccination at birth matters In December 2025, the CDC changed its longstanding recommendation that all newborns receive the hepatitis B vaccine at birth.

Under the federal guidance at the time, parents of babies born to mothers who test negative for hepatitis B can decide with their health care provider whether to vaccinate at birth or begin the series later.

Babies born to mothers who have hepatitis B or whose infection status is unknown should still receive the vaccine within 12 hours of birth, the guidance stated.

In March, a federal judge in Massachusetts judge stayed all votes taken by the members of the CDC's vaccine advisory committee, which included the removal of the universal recommendation for the hepatitis B shot at birth. Currently, the CDC's website states that all infants should receive the hepatitis B vaccine.

In response to a request for comment, the Department of Health and Human Services pointed ABC News to a Tuesday post on X from Secretary Robert F. Kennedy Jr.

"I reject the idea that respecting parents’ choices means abandoning children’s health," he wrote in part. "Under my leadership, HHS puts informed consent, individual medical circumstances, and conversations between parents and doctors at the center of health decisions."

The American Academy of Pediatrics continues to recommend that newborns receive their first hepatitis B vaccine dose within 24 hours of birth followed by additional doses to complete the series.

Some physicians worry that the federal changes created confusion among parents and lead to fewer children receiving the vaccine.

"When a recommendation shifts from 'every baby' to 'Let's discuss it,' a lot of families hear 'optional' or 'Maybe it was never that important,'" Patel said.

Doctors and public health specialists say the vaccine is important even for babies whose mothers test negative for hepatitis B because children may encounter the virus through infected caregivers.

Physicians also warn that delaying the first dose could make it more difficult for some children to complete the recommended vaccine series.

"When parents fall off schedule or we lose them to follow-up, vaccination rates are far more likely to decline," Patel said.

Another concern is that hepatitis B can remain undetected for years, with serious complications sometimes developing decades after the initial infection.

"[If] even a small percentage of new babies go without receiving the hepatitis B vaccine ... we could potentially have many more preventable cases of chronic liver disease and liver cancer," Patel said.

"Even a small increase in childhood [hepatitis B virus] infections could have large clinical, public health and economic effects," Mohareb added.

Betsy-Ann Assoumou, MD, MBA, is a family medicine resident at Cheshire Medical Center/Dartmouth Health in Keene, New Hampshire, and a member of the ABC News Medical Unit.