Staff at the US Centers for Disease Control and Prevention have been ordered to stop communicating with the World Health Organization, according to a new memo, dealing a significant blow to global health efforts.

The directive was made in accordance with President Donald Trump’s January 20 executive order withdrawing the US from WHO, according to a memo sent late Sunday from CDC Deputy Director of Global Health Dr. John Nkengasong to senior agency leadership, a copy of which was obtained by CNN. It was first reported by the Associated Press.

“Effective immediately all CDC staff engaging with WHO through technical working groups, coordinating centers, advisory boards, cooperative agreements or other means – in person or virtual – must cease their activity and await further guidance,” the memo says.

The US is one of the largest funders of WHO, and federal law requires one year of notice before US support for the organization can be withdrawn.

Trump’s order claims that legal notice of withdrawal was given during his first term, in 2020, so the withdrawal can happen immediately.

The White House, the CDC, WHO and the US Department of Health and Human Services did not respond to requests for comment.

“This move is reckless, basically ordering CDC not to work with WHO to put out fires is going to make Americans far more vulnerable,” Dr. Lawrence Gostin, a global health expert at Georgetown University who runs a WHO coordinating center on national and global health law, told CNN.

With outbreaks of lethal Marburg virus and H5N1 bird flu spreading in animals around the world, he said, a lack of international coordination on public health is dangerous.

Gostin said he was also told that Trump has recalled all CDC staff who are seconded to WHO offices in other countries, a move spelled out in Trump’s January 20 order.

The CDC has staff in a network of international offices in more than 60 countries. These offices can help provide early warning when there are infectious disease outbreaks of concern.

Gostin said that if Trump is trying to negotiate a better deal with WHO, there are better ways to go about it.

“President Trump is asking CDC to hit the pause button. But the truth is that viruses don’t take a break from circulating while the White House figures out its next move,” he said. “If you want to figure out what your future relationship is with WHO, you stay in the fight until you figured it out, and then you make your move. You don’t just quit the battle while you’re trying to figure something out, because the enemy – which is the virus – is still circulating and causing mayhem.”