Experts have warned the US withdrawal could seriously weaken the world’s defences against diseases, as it would put a massive dent in the WHO’s resources.

A year ago, Donald Trump signed an order to remove the US from the World Health Organization (WHO).

The US president singled the WHO executive order out as a “big one” as he signed it alongside 100+ others upon his return to the White House.

The US has now seen out a mandatory one-year notice period, meaning it has officially withdrawn from the WHO.

What was Mr Trump’s rationale for leaving and what impact could it have?

What reasons has Trump given for wanting to leave the WHO?

Mr Trump, who also tried to leave the WHO in his first term, has suggested cost-cutting is the main reason for ending its membership.

“World Health ripped us off, everybody rips off the United States. It’s not going to happen anymore,” Trump said at the signing last year.

WHO is funded through both obligatory and voluntary contributions from governments and other donors in its 193 member states. The obligatory contributions are calculated according to a country’s wealth and population, and fluctuate annually.

The US government’s grievances over payments took up a significant portion of the executive order, with the opening section claiming “WHO continues to demand unfairly onerous payments from the United States, far out of proportion with other countries’ assessed payments”.

It continued: “China, with a population of 1.4 billion, has 300 percent of the population of the United States, yet contributes nearly 90 percent less to the WHO.”

But before the mention of money, there came a hefty criticism of the WHO’s handling of the COVID pandemic and other global health crises, with the executive order claiming it failed to “adopt urgently needed reforms” and showed an “inability to demonstrate independence from the inappropriate political influence of WHO member states”.

As well as the WHO, the Trump administration announced on 7 January plans to withdraw from 35 international and 31 United Nations (UN) entities.

Explaining the decision, a White House memo stated Mr Trump had “prioritised American interests by redirecting focus and resources toward domestic priorities such as infrastructure, military readiness, and border security, and acting swiftly to protect American companies from foreign interference”.

It was previously reported that the US had to meet its financial obligations to the WHO for the 2024-25 fiscal year before leaving.

NPR has reported that the US owed $228m (£169m), but that it hadn’t paid as of 20 January. Sky News has contacted the Trump administration for comment.

What impact could the withdrawal have?

Experts have warned it could weaken the world’s defences against dangerous new outbreaks capable of triggering pandemics, because the US has historically been among its biggest donors and has provided it with hundreds of staffers with specialised public health expertise.

The US has given the WHO about $160m to $815m every year for the past decade, contributing to its yearly budget of about $2bn to $3bn.

Because of this, experts say losing the country’s support could cripple numerous global health initiatives, including the effort to eradicate polio, maternal and child health programmes, and research to identify new viral threats.

Dr Tom Frieden, president and CEO of the US advocacy group Resolve to Save Lives, explained in a statement: "Withdrawing from WHO not only cuts crucial funding from the agency, but it also surrenders our role as a global health leader and silences America’s voice in critical decisions affecting global health security.

“Real reform requires engagement, not abandonment. We cannot make WHO more effective by walking away from it. This decision weakens America’s influence and increases the risk of a deadly pandemic.”

Lawrence Gostin, director of the WHO Collaborating Centre on Global Health Law at Georgetown University, warned the WHO losing American resources would devastate its global surveillance and epidemic response efforts.

“It would make it more likely that we could see novel diseases spinning out of control, crossing borders, and potentially sparking a pandemic,” he said.

How has the WHO responded?

In a statement released after Mr Trump signed the order, the WHO said: "WHO plays a crucial role in protecting the health and security of the world’s people, including Americans, by addressing the root causes of disease, building stronger health systems, and detecting, preventing and responding to health emergencies, including disease outbreaks, often in dangerous places where others cannot go.

"The United States was a founding member of WHO in 1948 and has participated in shaping and governing WHO’s work ever since, alongside 193 other Member States, including through its active participation in the World Health Assembly and Executive Board.

"For over seven decades, WHO and the USA have saved countless lives and protected Americans and all people from health threats. Together, we ended smallpox, and together we have brought polio to the brink of eradication. American institutions have contributed to and benefited from membership of WHO.

"With the participation of the United States and other Member States, WHO has over the past seven years implemented the largest set of reforms in its history, to transform our accountability, cost-effectiveness, and impact in countries. This work continues.

“We hope the United States will reconsider and we look forward to engaging in constructive dialogue to maintain the partnership between the USA and WHO, for the benefit of the health and wellbeing of millions of people around the globe.”

Sky News contacted the WHO for comment ahead of the US’ official departure.