Nine former directors of the Centers for Disease Control and Prevention (CDC) are speaking out, saying Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. is endangering the health of Americans.

Seven former directors and two former acting directors — whose tenures stretch back to the administration of former President Jimmy Carter — published an op-ed in The New York Times on Monday, just days after the ousting of the CDC’s new director Dr. Susan Monarez.

Sources told ABC News that Kennedy and Stefanie Spear, his principal deputy chief of staff, called on Monarez to support changes to COVID vaccine policy and the firings of high-level staff, which Monarez would not commit to.

The directors said Monarez’s removal is the latest in a series of actions that could have a “wide-ranging impact” on “America’s health security.”

One of the co-authors, Dr. Richard Besser, president and CEO of the Robert Wood Johnson Foundation and acting director of the CDC during the administration of former President Barack Obama, said he and his colleagues are stunned at what they’ve seen.

“What we are seeing taking place in the Department of Health and Human Services, and at CDC in particular, is not businesses as usual,” he told ABC News. “There are always changes, different policy priorities when administration changes. But what we’re seeing under the leadership of Secretary Robert F. Kennedy [Jr.] is something different altogether.”

“He has come into his role as Secretary of Health and Human Services with a strong agenda that is centered on dismantling our vaccine system in America and limiting people’s access to these life-saving, health-preserving interventions,” Besser added.

HHS did not immediately reply to ABC News’ request for comment.

In the op-ed, the former directors point to several decisions made by Kennedy including the firing of thousands of federal health workers, touting unproven treatments as measles was spreading in the U.S., and canceling $500 million in federally funded mRNA vaccine research.

The directors also referenced Kennedy’s removal of all 17 members of the CDC’s vaccine advisory committee and replacing them with his own hand-selected members, many of whom have shared vaccine-skeptic views.

Besser said the ousting of Monarez, along with the resignation of at least four top leaders, compelled him and his colleagues to speak out.

He told ABC News that their departures leave the U.S. vulnerable to every day health challenges as well as public health threats.

“We can’t predict when the next pandemic will be here, but we know there will be future pandemics,” Besser said. “There will be other infectious threats. There will be other public health challenges, and with this Secretary performing in the way that he is, it puts us all at risk.”

He said he and the co-authors “don’t agree on everything, but we agree that our federal public health system is in major jeopardy. The CDC, which had been looked to as the world’s leading public health institution, is on life support and needs our attention immediately.”

The op-ed called on Congress to oversee HHS, which it has authority to do. It echoes a social media post from Sen. Bill Cassidy (R-Louisiana), who said the departure of CDC leaders require oversight from the Senate committee he chairs.

The former directors also called on state and local governments to fill funding gaps left behind by some of Kennedy’s actions.

“We represent individuals who served in every administration from Jimmy Carter through Donald Trump, Republicans and Democrats, and we were unified in our feeling that what we’re seeing is extremely alarming and that Congress needs to step up and perform its oversight function,” Besser told ABC News. “And so, we’re hoping that our voices will add to some of the other voices that have been calling this out and that Congress will do its part.”