Several senior leaders at the US Centers for Disease Control and Prevention (CDC) have resigned just hours after the agency's head was ousted from her role.

Susan Monarez's departure as director comes less than a month after the Senate confirmed her. It is unclear why she was removed from the job.

Three other officials, including the CDC's chief medical officer, have now resigned, according to letters obtained by the BBC's US partner CBS News.

The exodus comes as health experts voice concern over the agency's approach to immunisations under the leadership of US Health Secretary Robert F Kennedy Jr, a vaccine sceptic.

Dr Monarez cancelled an agency-wide meeting on Monday because she had been summoned to Washington DC, CDC officials said.

The US Department of Health and Human Services (HHS) said in a post on X that Kennedy "has full confidence" in the CDC team, "who will continue to be vigilant in protecting Americans against infectious diseases at home and abroad."

The department confirmed Dr Monarez's departure, thanking her "for her dedicated service to the American people".

In a statement shortly after, her lawyer Mark Zaid said his client had "refused to rubber-stamp unscientific, reckless directives and fire dedicated health experts".

"She chose protecting the public over serving an agenda," he said.

Mr Zaid also said Dr Monarez had not resigned or been informed of her dismissal.

A long-time federal government scientist, Dr Monarez was nominated by President Donald Trump to lead the CDC and was confirmed in a Senate vote along party lines in July.

Her nomination followed Trump withdrawing his first pick, former Republican Congressman Dave Weldon, who had come under fire for his views on vaccines and autism.

Shortly after Dr Monarez's departure was announced, at least three senior CDC leaders resigned from the agency.

Daniel Jernigan, who led the National Center for Emerging and Zoonotic Infectious Diseases, quit citing "the current context in the Department".

CDC's Chief Medical Officer Debra Houry and the head of the National Center for Immunization and Respiratory Diseases, Demetre Deskalakis, also resigned.

There are also reports, including by NBC News, that Dr Jennifer Layden, director of the Office of Public Health Data, Surveillance and Technology, has also resigned.

In a letter, Dr Houry warned about the "rise of misinformation" about vaccines. She also argued planned cuts to the agency's budget will hurt the CDC.

Dr Deskalakis said he was no longer able to serve "because of the ongoing weaponising of public health".

Earlier on Wednesday, the Food and Drug Administration (FDA) approved new Covid vaccines while limiting who could receive them.

The vaccines will be available for all seniors, but younger adults and children without underlying health conditions will be excluded.

"The emergency use authorizations for Covid vaccines, once used to justify broad mandates on the general public during the Biden administration, are now rescinded," Robert F Kennedy Jr wrote on X.

Susan Monarez was the first CDC director in 50 years to not hold a medical degree. Her background is in infectious disease research.

In her month as the CDC leader, she helped comfort agency employees after the CDC's headquarters in Atlanta was attacked by a gunman who believed he had been harmed by Covid vaccines.

The attack, in which hundreds of bullets struck the building, killed one police officer.

Earlier this month, current and former employees of the agency wrote an open letter accusing Kennedy of fuelling violence towards healthcare workers with his anti-vaccine rhetoric.

Dr Monarez departure comes about a week after a union representing CDC employees announced that it had fired about 600 employees.

The wide-ranging layoffs included employees working on the government's response to infectious diseases, including bird flu, as well as those researching environmental hazards and handling public record requests.