When President Trump named a new leadership team at the Centers for Disease Control and Prevention two weeks ago, public attention focused on Dr. Erica Schwartz, his nominee to be the agency’s director. Her public support of vaccines was interpreted by some as a sign that Health Secretary Robert F. Kennedy Jr.’s quest to limit childhood immunizations might be coming to an end.

But another senior official Mr. Trump named to the team shares many of Mr. Kennedy’s views, suggesting the potential for continuing tension at the public health agency.

Mr. Trump appointed Dr. Sara Brenner, a deputy commissioner at the Food and Drug Administration and a self-described “MAHA mom,” to be Mr. Kennedy’s senior counselor for public health, a post that, unlike the C.D.C. director, does not require Senate confirmation. A look at Dr. Brenner’s background suggests she is aligned with Mr. Kennedy on some of his signature issues, including skepticism about vaccines and a strong belief in the importance of fitness.

The public health counselor serves as the liaison between the health secretary and the C.D.C. (and on occasion the White House). As such, Dr. Brenner, who starts the job in the next couple of weeks, will be Mr. Kennedy’s eyes and ears at an agency he has been warring with through most of his tenure as the nation’s top health official.

She will be based in Washington in the office of the secretary, and will most likely meet with the top officials at the C.D.C. at least once every day. She will also be Mr. Kennedy’s liaison to the National Institutes of Health, an agency that has come under fire for making large cuts to medical research grants.

Dr. Nirav Shah, who was the C.D.C.’s principal deputy director from March 2023 through February 2025, said Dr. Brenner could wield a powerful influence on the nation’s public health policies through her role. (Dr. Shah, a Democrat, is currently running for governor in Maine.)

He cited the work of previous officials who held that role and acted as “a conduit and a sounding board for agencies to help them get where they wanted to go.”

“If Sara follows that lead,” he added, “then there’s a possibility that it will be constructive. But if she rather asserts herself as a stand-in for the secretary or the director, then what we will see is more political interference.”

The Department of Health and Human Services would not make Dr. Brenner available for an interview.

“Dr. Brenner was selected for this role because of her experience as a physician and her work on federal public health policy,” Andrew Nixon, a spokesman for the department, said in an emailed statement.

“She brings a strong understanding of both clinical care and research and will work with Secretary Kennedy to optimize coordination with the C.D.C. and N.I.H.,” he added.

At a Make America Healthy Again event last May, Dr. Brenner said she had been pregnant during the pandemic and chose not to receive the Covid vaccine because she was concerned about the vaccine’s “biodistribution patterns.”

She co-wrote a memo saying there was “no clear evidence” that the benefits of Covid vaccines for children under 18 outweighed the risk of harm. She also intervened in the F.D.A.’s review of Novavax’s Covid vaccine, asking for more data on the shot at the 11th hour — a highly unusual step for the agency’s deputy commissioner.

Sarah Despres, who served as the public health counselor during the Biden administration, would not comment on Dr. Brenner specifically. But she said a person in that role would receive enough notice of new initiatives and scientific reports, including in the agency’s flagship journal, for her or the secretary’s office to block them.

“If you’re interested in interfering, you will have a heads-up,” Ms. Despres said.

Dr. Brenner, who holds degrees in medicine and public health, is currently a principal deputy commissioner at the F.D.A. In interviews, four former senior officials who worked closely with her described her as ambitious and eager to please her bosses, even if that meant going against the interests of the F.D.A.’s rank-and-file employees. (They asked to remain anonymous because of fear of retaliation from the administration.)

Two former colleagues recalled Dr. Brenner saying that people should not reflexively believe in vaccines but should insist on facts as they do for other medical products — echoing comments made by Mr. Kennedy and others who question vaccines. Dr. Brenner seemed unaware of the large body of evidence on vaccine safety that already exists, the colleagues said.

Asked about Dr. Brenner’s views on vaccines, Mr. Nixon said, “Anonymous characterizations don’t reflect her record.”

Aaron Siri, a lawyer who for years joined with Mr. Kennedy to bring lawsuits over vaccine safety, said he believed Dr. Brenner would “at least critically consider concerns” from people with opposing views.

“Wherever she comes out at the end, at least you feel like the data and evidence were given a more objective overall review,” he said.

Dr. Brenner joined the F.D.A. in 2019 as a midlevel scientist in the agency’s Center for Devices and Radiological Health. Shortly after Mr. Trump’s inauguration, in January 2025, she was catapulted to the position of acting commissioner, a job she held until April when his nominee for the job, Dr. Martin Makary, became the agency’s leader.

Before her ascent, some top officials were unfamiliar with her. Dr. Janet Woodcock, who worked at the agency for decades and was its principal deputy commissioner from 2022 to 2024, said she had “never even heard her name.”

Dr. Robert Califf, who led the F.D.A. during parts of the Obama and Biden administrations, said Dr. Brenner “was part of some briefings and seemed very professional,” but he did not know her well enough to comment on her work.

Dr. Brenner may have come to the attention of Mr. Trump’s team when she worked at the White House Office of Science and Technology Policy during his first term. After being named acting commissioner, she fully embraced the MAHA movement, signing her introductory emails to the F.D.A. staff with the slogan.

She was the agency’s leader while widespread layoffs were decimating the ranks of staff scientists. In her communications with staff members, she amplified the messages of the administration and did not address the distress of the employees, the former colleagues said. At the same time, she made it clear that she did not want to be in the office five days a week and sometimes showed up to meetings in workout clothes, the colleagues said.

In a video posted by the F.D.A. to social media as a “spin on the Pete and Bobby challenge” — referring to fitness videos made by Defense Secretary Pete Hegseth and Mr. Kennedy — she knocks out 50 burpees, 50 forearm to full planks and 100 bicycle situps in seven minutes and one second.

Sounding much like Mr. Kennedy, she says, “I’ve been encouraging people my whole career to understand that taking care of their body is one of the most important things that you can do on a daily basis.”

Mr. Nixon, the health department spokesman, said that “Dr. Brenner exemplifies what it means to be a MAHA mom through her commitment to improving children’s health and well-being through nutrition and fitness.”

The other three officials Mr. Trump named to lead the C.D.C. have all been enthusiastically received by public health experts.

Dr. Schwartz, the White House’s nominee to be the agency’s director, has degrees in medicine and public health and has praised vaccines, prompting cautious optimism that she will steady the C.D.C. It is unclear when she might be confirmed by Senate. The White House has not yet filed papers to move her nomination forward.

The agency has been without an official director since Mr. Kennedy fired Susan Monarez after a dispute over vaccine policy 29 days after she began the job. The agency has also experienced widespread layoffs, resignations and a shooting since Mr. Trump returned to the White House.

In hearings before Congress over the past few days, Mr. Kennedy has contradicted himself, telling one group of lawmakers that he would not commit to carrying out Dr. Schwartz’s policies on vaccines, but later recanting that stance in front of another.

But Dr. Schwartz is also a longstanding member of the armed forces, used to following orders in that context, some public health experts noted.

Mr. Trump named two others, Sean Slovenski, a seasoned health care executive, and Dr. Jennifer Shuford, a highly respected epidemiologist and physician, as deputies to Dr. Schwartz. But more quietly, the administration has also appointed Stephen Sayle, a former tobacco executive, as deputy director for legislative affairs. And several other political appointees with known antipathy to vaccines, including Stuart Burns, remain at the agency.

Mr. Kennedy and his appointees have already skirted the normal procedures to try to alter the childhood vaccine schedule, although a federal judge has blocked those efforts. And Dr. Jay Bhattacharya, the C.D.C.’s acting head, recently canceled publication of a report showing the effectiveness of Covid shots last winter.

It is unclear where Dr. Brenner stands on other issues and whether she would go along with Mr. Kennedy’s agenda without question.

“It’s important for people who work in the Office of the Secretary to have their own red lines, and to be willing to push back if a request from their boss comes that is inappropriate,” Ms. Despres said.