By Nancy Lapid

Dec 4 (Reuters) - Vaccine advisers to U.S. Health Secretary Robert F. Kennedy Jr. were expected to vote on Thursday on whether to scrap a broad recommendation for the hepatitis B vaccine for children, a move that would leave the choice to parents in most cases.

The U.S. vaccine panel ​has proposed that only infants born to mothers who test positive for hepatitis B would be recommended to get the vaccine. It would represent the most significant ‌change to date in national immunization policy under Kennedy, who has long promoted anti-vaccine views.

However, the committee voted 6 to 3 to delay the decision until Friday, citing the need for additional time to review the language ‌of the proposal before casting their votes.

Here's what you need to know about the hepatitis B virus (HBV) and the vaccines.

WHAT IS HEPATITIS B?

HBV attacks the liver and is the leading cause of liver cancer worldwide. Many infected people do not have symptoms and do not realize they are infected. In most adults who acquire the virus, the infection resolves on its own. But it becomes chronic in more than 90% of infants and in up to 50% of young children who become infected.

Decades after HBV infection, patients can develop liver failure and require a liver transplant. ⁠Because there is no cure for HBV infections, patients often ‌have recurrent liver disease after a transplant.

HOW COMMON ARE HBV INFECTIONS TODAY?

U.S. HBV infection rates dropped nearly 90%, from about 9.6 per 100,000 before vaccination became widespread to approximately one per 100,000 in 2018.

The World Health Organization estimates that 254 million people worldwide were living with chronic hepatitis ‍B infection in 2022, with 1.2 million new infections each year.

HOW IS THE VIRUS TRANSMITTED?

Hepatitis B is most often acquired through contact with body fluids from an infected person. It can also be transmitted via needlestick injury, tattooing and piercing.

WHY IS THE VACCINE GIVEN TO NEWBORNS?

Before widespread availability of the vaccine, transmission from mothers to babies during childbirth was the most common route of infection. In the ​U.S., before vaccines were available, roughly 85% of newborns became infected when mothers had active infections, as did approximately 30% of babies born to mothers with previous infections in ‌whom the virus was inactive.

Presently, 0.7% to 1.1% of infants born to such mothers develop infection after vaccination at birth, according to the CDC.

Similar declines in infant HBV infections have been seen elsewhere with widespread vaccination of newborns. Since 2020, the practice had been adopted by 190 of the 194 World Health Organization member countries, according to a 2022 CDC report. More than half of those countries provide it to all newborns immediately after birth.

WHAT IS THE CURRENT VACCINE RECOMMENDATION IN THE U.S.?

Since October 2016, the CDC's Advisory Committee on Immunization Practices has recommended that all medically stable newborns receive the first dose of the HBV vaccine within 24 hours of birth, followed by doses at 1-2 months and 6-18 months of ⁠age. This replaced the previous recommendation from 2005, which called for the initial dose to be given ​before newborns were discharged from the hospital.

If a mother is known to have an active HBV infection, the ​birth dose is given within 12 hours of birth along with special antibodies to fight the virus.

ACIP's previous recommendations call for three doses of the vaccine for unvaccinated children and adults under age 60, and for older adults with risk factors for hepatitis B or without risk factors but ‍seeking protection.

"Scientific evidence overwhelmingly supports the safety" of ⁠the six vaccines currently licensed for use in the United States, the CDC has said.

WHY TREAT ALL BABIES, NOT JUST THOSE WITH INFECTED MOTHERS?

Universal birth dosing protects infants whose parents' HBV status is unknown or was not tested during prenatal care. Also, maternal testing can miss recent infections, making universal newborn vaccination more reliable.

Even when ⁠mothers are not infected, newborns can acquire HBV from close contact with other infected individuals and caregivers.

WHAT'S KNOWN ABOUT ADVERSE REACTIONS?

The hepatitis B vaccine is widely regarded as safe. Side effects are typically mild, such as soreness ‌at the injection site or a low-grade fever, while serious allergic reactions, including anaphylaxis, are extremely rare, according to the World Health Organization.

(Reporting by Nancy ‌Lapid and Kamal Choudhury in Bengaluru; Editing by Michele Gershberg, Bill Berkrot and Maju Samuel)