Economists and global health experts have called on G20 leaders to provide trillions of dollars to poorer countries to shore up ailing healthcare systems and economies, or face a disaster that will rebound on wealthier states through migration and health crises.

Twenty experts, among them four Nobel prizewinners, including Joseph Stiglitz, Lord Nicholas Stern and seven chief economists of the World Bank and other development banks, have written to G20 leaders to warn of “unimaginable health and social impacts” as coronavirus rips through the developing world, taking overburdened healthcare systems beyond breaking point, and causing economic and social devastation.

Countries are “falling apart because of the collapse in commodity prices, tourism and remittances”, said the signatories, even before the ravages of the virus itself takes hold across south-east Asia, Latin America and Africa, where at least 43 out of 54 countries have confirmed cases.

The G20 held a virtual meeting by video conference on Thursday to discuss the Covid-19 crisis.

Poorer countries are likely to be hit harder than rich states, since they have less capacity to absorb the shock and overcrowding, poor infrastructure and lack of resources hamper public health efforts. Basic hygiene and a lack of handwashing facilities and soap will be a major issue for poorer people.

“Developing countries are facing an unprecedented collective threat to human life, social cohesion and economic devastation,” the letter to the G20 leaders said. “Massive economic losses will be incurred as countries desperately try to cope, people will migrate out of fear as the epidemic takes hold, leading to social disruption, violence and security issues.”

The group wants swift action to devote emergency spending to stricken developing countries, and longer-term reforms that make nations more resilient to outbreaks and natural disasters.

At least $8bn (£6.5bn) in emergency funding has been requested by the Global Preparedness Monitoring Board, but that should be just the beginning according to the letter’s signatories, who make it clear that the money required will be “of a different order of magnitude” to the amounts pledged so far.

The US is implementing a rescue package worth about $2tn, the UK government is spending more than £330bn on its recovery, and other countries are coming up with packages of their own. Poorer nations will need far greater levels of assistance than have yet been planned, according to the letter.

Erik Berglof, director of the Institute of Global Affairs at the London School of Economics, and an organiser of the letter, told the Guardian: “If it will take more than $2tn, perhaps as much as $6tn, to fix the US, we are not going to fix the rest of the world for less. To attach a specific number now is almost meaningless. What we need are new and creative ways of using the global financial muscle to back up existing international financial institutions.”

He said that emergency resources and medical help could be provided quickly, but it was important to manage the process with a long-term view, expanding existing international institutions and programmes. “You cannot sprinkle money from helicopters in these economies because it would never reach the intended receivers,” he said. “You need carefully managed programmes and projects that can help these countries through this extremely difficult period.”

Infrastructure investment would be vital to ensure recovery from a shock that, the economists said, would be greater than that of the 2008 financial crisis.

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If efforts to control Covid-19 fail, “the virus could become endemic, producing new waves of destructive outbreaks around the world”, the letter warned, adding: “We have a rapidly closing window to ensure that we give these countries at least a fighting chance to manage the crisis and provide some light at the end of what could be a long tunnel.”

The letter concludes: “We are now urging you, the leaders of the G20, to urgently provide the necessary resources to reduce the losses in human life and back up those most vulnerable. The required investment is minute compared to the social and economic costs of inaction. History will judge us harshly if we do not get this right.”

The signatories are: Kaushik Basu, former World Bank chief economist; Erik Berglof, former chief economist of the European Bank of Reconstruction and Development; Tim Besley, former president of the International Economic Association; Victor Dzau, president of the US National Academy of Medicine; Jeremy Farrar, director of the Wellcome Trust; Ahmed Galal, former finance minister of Egypt; Sergei Guriev, former EBRD chief economist; Bengt Holmström, Nobel laureate; Anne Krueger, former World Bank chief economist; Justin Lin, former World Bank chief economist; Nora Lustig, co-founder of the Latin and Caribbean Economic Association; Ngozi Okonjo-Awala, former managing director of the World Bank; Christopher Pissarides, Nobel laureate; Dani Rodrik, incoming president of the International Economic Association; Michael Spence, Nobel laureate; Nicholas Stern, former World Bank chief economist; Joseph Stiglitz, Nobel laureate; Andrés Velasco, former finance minister of Chile; Leonard Wantchekon, founder of the African School of Economics; Shang-Jin Wei, former Asian Development Bank chief economist.