Global leaders came together Thursday to raise at least $2 billion toward providing a future vaccine for the novel coronavirus to people throughout the world — a precarious diplomatic endeavor and one of the biggest unresolved problems in using a vaccine to combat the pandemic.

The virtual summit was convened by a public-private partnership called Gavi, which aims to increase vaccination rates in lower-income countries. At the summit, Gavi unveiled a proposal to ensure poor- and middle-income countries have access to the vaccine. The proposal the group sketched out also includes a way wealthy countries can get access to the same vaccines while supporting equitable global distribution.

The twists and turns of the scientific race to develop and mass produce a vaccine are being closely followed, with mere hints of progress sending the stock market surging. But an even more important challenge looms, because any vaccine will have to be distributed globally to stamp out the pandemic as quickly as possible and avoid a humanitarian disaster in which rich countries restart their economies while people in poorer countries continue to die.

“Lots of countries are working with various manufacturers on [vaccine] supply for their countries. We need to overlay that approach with a global approach, so that most of the output is going to a rational system of allocation of doses to those most at risk,” said Bill Gates, co-chair of the Bill and Melinda Gates Foundation, who pledged $1.6 billion to support Gavi’s mission to expand access to childhood vaccinations and $100 million more toward efforts to buy vaccines against the coronavirus for the poorest countries.

“Sadly, the disease will probably be worse in the developing countries, even though it was slow to get going there, because the health system is weaker and the idea of doing a lockdown is not as practical,” Bill Gates said.

Gavi — which is backed by the Gates Foundation, the World Health Organization, World Bank, UNICEF and governments of countries around the world — is seeking donations from many of those same groups and countries for the coronavirus vaccine campaign.

At Thursday’s virtual summit, hosted by British Prime Minister Boris Johnson, international health officials will unveil a global funding mechanism they hope will help secure access to a coronavirus vaccine for poorer countries. The mechanism — called an “advance market commitment” — functions as a preorder for the developing world. It’s a guarantee that Gavi will purchase vaccines at a particular price for developing countries to make the process less financially risky for companies scaling up now.

On Thursday, British company AstraZeneca announced an agreement to set aside 300 million doses of a vaccine it is developing for Gavi’s new effort. And the Coalition for Epidemic Preparedness Innovations (CEPI) said it would invest $383 million into that partnership.

The idea for such a mechanism began gaining traction about 15 years ago and has been used to create a market for a pneumococcal vaccine for lower-income countries. In 2009, governments came together to pledge $1.5 billion to buy that vaccine, with companies bidding for contracts to supply the projected global demand.

Seth Berkley, CEO of Gavi, said manufacturers are drawn to that arrangement because it guarantees them payment for vaccines sent to developing countries, reducing the financial risk for drug companies. At the same time, the large orders allow those companies to scale up production, dramatically lower their cost and achieve more profit when they sell those vaccines at higher cost to higher-income countries.

The Gavi mechanism also would allow high- and upper-income countries to pledge money, join the effort and in return gain access to vaccines covered under the agreement. That could be a way for countries that are advancing individual vaccine efforts to have access to a broader range of inoculation candidates in case their experimental vaccines fail. Despite the tremendous energy and funding flowing into research for a coronavirus vaccine, the success of any individual effort is far from guaranteed.

“The reality is most vaccines will fail. If you as a country just buy into one vaccine, and it ends up working, that’s great. If it doesn’t, you end up with nothing. In a sense, this is like an insurance policy,” Berkley said.

Gavi, the WHO and other international groups note that without global cooperation, the coronavirus will continue spreading. Even if countries think selfishly and hoard supplies, they will struggle with new outbreaks as long as the virus continues to rage elsewhere in the world because of how easily it spreads.

It is a particularly tough time for such international cooperation.

Borders around the world remain heavily restricted or closed. America’s future participation in the WHO, a key international body for facilitating global cooperation, remains unclear as President Trump has threatened to terminate the relationship. The blame game for the pandemic rages between the United States and China.

Nations have found themselves competing for limited protective equipment, tests and ventilators.

The vaccine will be a precious and initially limited resource that appears destined to further strain the fragile framework for international cooperation. In past pandemics, even when the threat has been less acute and the impulse for global cooperation stronger, efforts to equitably distribute vaccines have been among the most controversial and fraught areas of global health policy.

“We haven’t had to deal with a serious infectious-disease outbreak in the context in which the balance of power mattered since the end of the Cold War,” said David P. Fidler, an adjunct senior fellow at the Council on Foreign Relations. “There are typically some centers of gravity around these successful endeavors. And in the past, one of the critical centers of gravity has been the World Health Organization.”

Some chief executives of pharmaceutical companies have vowed they will not profit off the vaccine during the pandemic and will participate in efforts to make sure the world has access. But it is unclear how that will unfold.

Reacting to news of the new Gavi mechanism, international medical humanitarian organization Doctors Without Borders expressed concern it would not prevent pharmaceutical companies from making decisions purely driven by profit. The group urged global leaders to demand pharmaceutical firms commit to selling any potential coronavirus vaccines at the cost of production.

“Everyone seems to agree that we can’t apply business-as-usual principles here, where the highest bidders get to protect their people from this disease first, while the rest of the world is left behind,” Kate Elder, the group’s senior vaccines policy adviser, said in a statement. “Governments must ensure any future COVID-19 vaccines are sold at cost and universally accessible to all across the world.”

In recent weeks, the three international bodies most involved in the pursuit of a vaccine have developed a working framework for research, procurement and distribution. Under that framework, CEPI, a global alliance set up to develop vaccines, is helping to lead and coordinate the research. Gavi — which already helps vaccinate nearly half the world’s children against diseases and was created to deliver vaccines to poorer countries — has the task of figuring out the economics and incentives needed to scale up coronavirus vaccine manufacturing and ensuring poorer countries have access.

Meanwhile, the WHO will lead the effort to ensure fair allocation among nations and to develop recommendations about who should be at the front of the line to get a vaccine — health-care workers, for instance.

Each of those tasks is riddled with challenges. Perhaps the greatest challenge running through all of them is the specter of nationalism. Without international agreements worked out beforehand, the initial supply of a vaccine could devolve into bidding wars, hoarding and ineffective vaccination campaigns.

“It’s a fragmented landscape of what is being proposed of different national initiatives, different international initiatives,” said Rachel Silverman, at the Center for Global Development, a nonprofit think tank that has proposed paying different amounts for vaccines depending on their effectiveness. “How it would work is a bit of an open question.”