A steadfast figure in the anti-vaccine movement who has helped shape Health Secretary Robert F. Kennedy Jr.’s thinking on a possible link to autism has joined his department to work on a study examining the long-debunked theory, according to people familiar with the matter.

The new analyst, David Geier, has published numerous articles in the medical literature attempting to tie mercury in vaccines to autism. In 2012, state authorities in Maryland found that he had been practicing medicine without a license alongside his father, Mark Geier, who was a doctor at the time.

Maryland authorities also suspended Mark Geier’s medical license following claims that he endangered children with autism and exploited their parents, according to state records.

Federal judges have rejected their research on autism and vaccines as too unreliable to stand up in court.

David Geier’s new government role has stunned public health experts, who had already expressed concerns about Kennedy’s decisions to cancel a long-held vaccine meeting and to cut grants focused on understanding vaccine hesitancy. Geier’s involvement in government research heightens their fears that vaccine confidence could be further eroded.

“If we increase vaccine hesitancy and immunization rates go down further, we will see more vaccine-preventable disease outbreaks,” said Dr. Christopher Beyrer, director of the Duke Global Health Institute. “That’s how it works.”

Geier is listed in the Department of Health and Human Services directory as a “senior data analyst.” Several experts said that appointing Geier to work on a study of vaccine safety preordains the outcome.

“You’d think you’d want a fresh eye,” said Edward L. Hunter, a former head of the Washington office of the Centers for Disease Control and Prevention. “This isn’t a fresh eye. They have already published their results, and spending all this time and money is not going to help anyone.”

An official with Kennedy’s Department of Health and Human Services declined to comment. Two White House spokespeople did not respond to a request for comment. David Geier did not reply to emails or calls requesting comment.

This month, federal officials announced plans for a large study to reexamine whether there was a connection between vaccines and autism. Trump has voiced support for HHS officials who wanted to revisit the issue, citing increases in autism diagnoses in children over the decades.