On January 20, his first day in office, US president Donald Trump signed an executive order to withdraw the US from the World Health Organization. Ooh, thats a big one, he said as he was handed the document. The US is the biggest donor to the WHO, and the loss of this income is likely to have a significant impact on the organization, which develops international health guidelines, investigates disease outbreaks, and acts as an information-sharing hub for member states. But the US will also lose out. Its a very tragic and sad event that could only hurt the United States in the long run, says William Moss, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore. A little unfair? Trump appears to take issue with the amount the US donates to the WHO. He points out that it makes a much bigger contribution than China, a country with a population four times that of the US. It seems a little unfair to me, he said as he prepared to sign the executive order. It is true that the US is far and away the biggest financial supporter of the WHO. The US contributed $1.28 billion over the two-year period covering 2022 and 2023. By comparison, the second-largest donor, Germany, contributed $856 million in the same period. The US currently contributes 14.5% of the WHOs total budget. But its not as though the WHO sends a billion-dollar bill to the US. All member states are required to pay membership dues, which are calculated as a percentage of a countrys gross domestic product. For the US, this figure comes to $130 million. China pays $87.6 million. But the vast majority of the USs contributions to the WHO are made on a voluntary basisin recent years, the donations have been part of multibillion-dollar spending on global health by the US government. (Separately, the Bill and Melinda Gates Foundation contributed $830 million over 2022 and 2023.) Its possible that other member nations will increase their donations to help cover the shortfall left by the USs withdrawal. But it is not clear who will step upor what implications it might have to the structure of donations. Martin McKee, professor of European public health at the London School of Hygiene at Tropical Medicine, thinks it is unlikely that European members will increase their contributions by much. The Gulf states, China, India, Brazil, and South Africa, on the other hand, may be more likely to pay more. But again, it isnt clear how this will pan out, or whether any of these countries will expect greater influence over global health policy decisions as a result of increasing their donations. Deep impacts WHO funds are spent on a range of global health projectsprograms to eradicate polio, rapidly respond to health emergencies, improve access to vaccines and medicines, develop pandemic prevention strategies, and more. The loss of US funding is likely to have a significant impact on at least some of these programs. It is not clear which programs will lose funding, or when they will be affected. The US is required to give 12 months notice to withdraw its membership, but voluntary contributions might stop before that time is up. For the last few years, WHO member states have been negotiating a pandemic agreement designed to improve collaboration on preparing for future pandemics. The agreement is set to be finalized in 2025. But these discussions will be disrupted by the US withdrawal, says McKee. It will create confusion about how effective any agreement will be and what it will look like, he says. The agreement itself also wont make as big an impact without the US as a signatory, says Moss, who is also a member of a WHO vaccine advisory committee. The US would not be held to information-sharing standards that other countries could benefit from, and it might not be privy to important health information from other member nations. The global community might also lose out on the USs resources and expertise. Having a major country like the United States not be a part of that really undermines the value of any pandemic agreement, he says. McKee thinks that the loss of funding will also affect efforts to eradicate polio and to control outbreaks of mpox in the Democratic Republic of Congo, Uganda, and Burundi, which continue to report hundreds of cases per week. The virus has the potential to spread, including to the US, he points out. Moss is concerned about the potential for the spread of vaccine-preventable diseases. Robert F. Kennedy Jr., Trumps pick to lead the Department of Health and Human Services, is a prominent antivaccine advocate, and Moss worries about potential changes to vaccination-based health policies in the US. That, combined with a weakening of the WHOs ability to control disease outbreaks, could be a double whammy, he says: Were setting ourselves up for large measles disease outbreaks in the United States. At the same time, the US is up against another growing threat to public health: the circulation of bird flu on poultry and dairy farms. The US has seen outbreaks of the H5N1 virus on poultry farms in all states, and the virus has been detected in 928 dairy herds across 16 states, according to the US Centers for Disease Control and Prevention. There have been 67 reported human cases in the US, and one person has died. While we dont yet have evidence that the virus can spread between people, the US and other countries are already preparing for potential outbreaks. But this preparation relies on a thorough and clear understanding of what is happening on the ground. The WHO provides an important role in information sharingcountries report early signs of outbreaks to the agency, which then shares the information with its members. This kind of information not only allows countries to develop strategies to limit the spread of disease but can also allow them to share genetic sequences of viruses and develop vaccines. Member nations need to know whats happening in the US, and the US needs to know whats happening globally. Both of those channels of communication would be hindered by this, says Moss. As if all of that werent enough, the US also stands to suffer in terms of its reputation as a leader in global public health. By saying to the world We dont care about your health, it sends a message that is likely to reflect badly on it, says McKee. Its a classic lose-lose situation, he adds. Its going to hurt global health, says Moss. Its going to come back to bite us.