By Sarah Owermohle, Adam Cancryn, CNN

(CNN) — Deputy Health and Human Services Secretary Jim O’Neill will serve as acting director of the US Centers for Disease Control and Prevention, US Health and Human Services Secretary Robert F. Kennedy said in a letter to CDC staff.

O’Neill joined HHS in June after years as a Silicon Valley-based technology and biotech investor, including stints with the Thiel Foundation and later the Thiel Fellowship, a grant program for young entrepreneurs started by billionaire Palantir founder Peter Thiel.

O’Neill also worked for HHS during the George W. Bush administration, when, as principal associate deputy secretary, he led changes to food regulation and emergency response.

The Washington Post first reported O’Neill’s new role.

He assumes the role amid turmoil for the CDC in the wake of Director Dr. Susan Monarez’s firing and a series of high-profile resignations this week.

Agency employees rallied outside the CDC’s Atlanta headquarters on Thursday to salute the departing Dr. Debra Houry, former chief medical officer; Dr. Demetre Daskalakis, former chief of vaccines and respiratory diseases; Dr. Daniel Jernigan, director of the National Center for Emerging and Zoonotic Infectious Diseases; and data chief Dr. Jennifer Layden.

O’Neill accompanied Monarez and Kennedy on a tour of the CDC campus this month after a gunman opened fire there, pocking the buildings with hundreds of bullet holes and killing DeKalb County Police Officer David Rose.

At his confirmation hearing in May, O’Neill told Sen. Bill Cassidy, a Louisiana Republican, that he’s “very strongly pro-vaccine. I’m an adviser to a vaccine company. I support the CDC vaccine schedule.”

Cassidy then asked O’Neill specifically about mandates. “Do you agree with the mandate, which currently exists, that if somebody wishes to become a legal resident, that they be up to date on their immunizations, as per recommended by CDC?”

O’Neill said, “I haven’t looked into the immigration law aspect of this, but I do support the CDC schedule, and that rule seems reasonable to me.”

He also told senators that he would help reform “outdated rules” and “pursue transparency” within federal health agencies.

“Our health care system is difficult to navigate, and it prioritizes pills over prevention. Providers spend too much time clicking through pop-up screens and fighting with insurance companies instead of looking patients in the eye. Federal policies can empower people to break this cycle,” O’Neill said. “If confirmed, I’ll help reform outdated rules, pursue transparency and gold standard science, and champion healthy lifestyles and prevention so Americans can thrive.”

CNN’s Jacqueline Howard contributed to this report.

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