FILE PHOTO: U.S. Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. looks on, during a press conference with Texas Governor Greg Abbott at the State Capitol in Austin, Texas, U.S., August 28, 2025. REUTERS/Kaylee Greenlee/ File Photo

By Ahmed Aboulenein and Trevor Hunnicutt

WASHINGTON (Reuters) -U.S. Secretary of Health and Human Services Robert F. Kennedy Jr. has named his deputy, Jim O'Neill, as acting director at the Centers for Disease Control and Prevention after ousting the former director less than a month into her tenure.

President Donald Trump fired on Wednesday CDC Director Susan Monarez after she resisted changes to vaccine policy advanced by Kennedy that she believed contradicted scientific evidence, further destabilizing the already embattled agency.

"Together, we will rebuild this institution into what it was always meant to be: a guardian of America's health and security," Kennedy wrote on Thursday in a memo to CDC staff seen by Reuters. "To help advance this mission, I am pleased to announce that Deputy Secretary of Health and Human Services Jim O'Neill is now also serving as Acting Director of the CDC."

Monarez's firing triggered the resignations of four senior CDC officials who cited anti-vaccine policies and misinformation pushed by Kennedy and his team.

The CDC has faced mounting challenges under Kennedy's leadership, including a shooting at its Atlanta headquarters earlier this month. The union representing CDC workers said the incident "compounds months of mistreatment, neglect, and vilification that CDC staff have endured."

The White House sought to cut the agency's budget by almost $3.6 billion, leaving it with a $4 billion 2026 budget, and Kennedy announced a layoff plan earlier this year that cut 2,400 CDC employees, though some 700 were rehired.

O'Neill, prior to his confirmation as Kennedy's second in command, served as a health policy adviser with ties to several healthcare companies.

He also managed one of tech billionaire and Trump-backer Peter Thiel's venture capital firms, Mithril Capital Management, from 2012 to 2019. O'Neill also served as chief executive of his philanthropic Thiel Foundation and as a managing director at Thiel Capital.

(Reporting by Ahmed Aboulenein and Trevor Hunnicutt; editing by Diane Craft)