The U.S. Food and Drug Administration on Thursday expanded the use of Moderna's respiratory syncytial virus vaccine to a lower group of adults aged 18 to 59 years at increased risk for disease.

Moderna's RSV shot, mRESVIA, was the first non-COVID-19 messenger RNA-based (mRNA) vaccine to be approved in the United States. The shot, the company's second product, is already approved for the prevention of RSV-associated lower respiratory tract disease in adults aged 60 or older.

While the FDA approval is a necessary step, the U.S. Centers for Disease Control and Prevention still has to recommend the shots before they are available for the age group.

The CDC currently recommends the vaccine for adults aged 75 and older, as well as for adults aged 60 to 74 who are at increased risk of infection.

In April, the CDC's panel of outside experts recommended the use of approved RSV vaccines in at-risk adults aged 50 to 59.

Health Secretary Robert F. Kennedy Jr. on Wednesday named eight new members to serve on the key panel of vaccine advisers, known as the Advisory Committee on Immunization Practices, after previously firing all 17 of its members, saying it would "re-establish public confidence in vaccine science."

However, public health experts warn that this could undermine public confidence in available vaccines.

Some of the new appointees have openly expressed anti-vaccine views, including against the mRNA vaccine technology.

The panel, which provides guidance to the CDC on which groups of people would most benefit from an already-approved vaccine, is scheduled to meet later this month.