Aid cuts could lead to more than 22 million avoidable deaths by 2030, including 5.4 million children under five, according to the most comprehensive modelling to date.

In the past two decades there have been dramatic falls in the number of young children dying from infectious diseases, driven by aid directed to the developing world, researchers wrote in the Lancet Global Health. But that progress was at risk of reversal because of abrupt budget cuts by donor countries, including the US and the UK.

The researchers looked at the link between how much aid countries received and their death rates between 2002 and 2021, and then used the data to forecast three future scenarios.

One was “business-as-usual”, the second assumed a “mild defunding” where aid fell by a similar amount as it had over the past few years, and the third “severe defunding”, where aid fell to about half its 2025 levels until the end of the decade.

Under severe defunding, about 22.6 million more deaths were forecast by 2030, including 5.4 million among under-fives. Mild defunding would mean 9.4 million excess deaths, of which 2.5 million would be young children.

That mild scenario was “not unlikely”, based on current trends, said lead author Prof Davide Rasella of the Barcelona Institute for Global Health (ISGlobal), while the “more drastic” scenario fitted with policies outlined by rightwing political parties in the ascendance in many countries. Reform UK has suggested cutting Britain’s aid budget by a further 90%.

Most traditional donor countries, including Germany, the US and Sweden, have announced large cuts. The US slashed aid spending by more than half in 2025, from $68bn to $32bn. In the UK, spending will fall from 0.5% to 0.3% of GDP by 2028 – about £6bn lower – to fund increased military spending.

Overseas aid in the past has been directly linked to a 39% reduction in deaths of under-fives, and “particularly strong effects” on mortality from infectious diseases including HIV/Aids and malaria, as well as nutritional deficiencies, the researchers found.

“Unfortunately nobody knows at this stage what is going to happen in the future, especially in foreign aid and assistance,” said Rasella.

Previous studies on cuts have focused on “US-funded programmes, a smaller number of recipient countries, or analysed a shorter timeframe, leaving the effects of overall ODA [official development assistance] on global mortality and its projections less understood”, the researchers said.

“As a scientist, we try to provide evidence,” said Rasella. “The problem was that there was very small evidence.”

Reallocation of recipient countries’ domestic resources “will never match the level of assistance we have been seeing”, he said, while “a collapse of some health systems” was a likely scenario.

Rasella said he had visited doctors in rural Mozambique. “They were telling me they had no antibiotics any more for children, because all the stock was distributed by USAID,” he said. “They have dismantled 300 primary care units in Afghanistan because they were also maintained by USAID. The situation is evolving, and now in many countries things are chaotic.”

Eric Pelofsky, vice-president of the Rockefeller Foundation, which helped fund the research, said the chasm of funding gaps were “too huge for any non-government to take on” and so philanthropic organisations were focused on both finding new innovations and “focusing the mind of decision-makers on the actual problem”.

He said aid spending could be hard for political leaders to explain to taxpayers.

“But I think what this report says is there is a genuinely concrete reality to these decisions, that’s consequential to global stability, consequential to our moral and political leadership in the world.”

Gideon Rabinowitz, director of policy and advocacy at Bond, the UK network of NGOs, said the impact of aid budget cuts by governments such as the UK’s was already being felt, with the closure of programmes focused on issues such as HIV, reproductive healthcare and female genital mutilation.

“The evidence is clear,” he said. “ODA funding is one of the most long-term, cost-effective public investments governments can make. It also contributes to making both the UK, and the world, a safer and healthier place for us all – by strengthening global health systems, preventing future pandemics and stopping diseases before they spread. We urge the UK and other governments to heed this evidence, reconsider these cuts and recognise that their choices are costing lives.”