In the global race to develop a COVID-19 vaccine, clinical trials have ramped up in the U.S., U.K., China, and beyond. Researchers and pharmaceutical companies face growing political pressure to develop and distribute one soon.

But when and if there is an effective vaccine, the supply could be limited. So, who gets it first? That is a critical question playing out on the world stage in a pandemic that knows no borders.

Several countries have already secured big manufacturing contracts for the production of select vaccine candidates. Some global health experts worry it's a sign of "vaccine nationalism," in which nations with the most resources or best access to vaccines that end up working buy up the available supply.

In response, a group of international organizations has set up a new initiative called COVID-19 Vaccines Global Access Facility, or COVAX, to ensure fair and equitable access to COVID-19 vaccines.

The effort is meant to prevent a repeat of what happened during the 2009 H1N1 pandemic, in which richer countries bought up virtually all available supplies of the vaccine as poorer nations were shoved to the back of the line. COVAX is co-led by the Coalition for Epidemic Preparedness (CEPI), the World Health Organization (WHO) and Gavi, the Vaccine Alliance.

COVAX faces major opportunities — and hurdles — in the coming months. To compete with national interests, the initiative will require billions of dollars and the cooperation of as many countries as possible. So far at least 76 middle and high-income countries have committed to join — but not the U.S.

The European Union pledged €400 million euro (about $472 million) and threw its full support behind the effort.

The U.S., meanwhile, said it would not participate, creating a gaping hole in the global effort. In an emailed statement, deputy White House press secretary Judd Deere said that the U.S. "will continue to engage our international partners to ensure we defeat this virus," but doesn't want to be "constrained" by multilateral organizations.

Though the U.S. has historically been a leader in global health efforts, President Donald Trump's White House has been critical of the WHO and its response to China's handling of the pandemic.

The United States' decision worries Lawrence Gostin, a professor of global health law at Georgetown University in Washington, DC, and an adviser to the WHO.

"COVAX is the world's one chance to actually rally together for the most important medical commodity in our lifetimes, which is a highly safe and effective COVID[-19] vaccine, and probably many vaccines because we don't know which will work best," Gostin said.

Lessons from H1N1

COVAX works as a financing mechanism or an insurance policy, in which countries pool resources to find a vaccine that works without taking on the risk of going solo. Countries may choose to join, and wealthier countries also contribute money. Those funds support the development and production of different vaccines, nine of which have been selected for development and evaluation by COVAX. The idea is that if one or several of those vaccines prove effective, COVAX would have the leverage to buy and distribute vaccines — ideally 2 billion doses — in an equitable way across countries to populations that need it most, such as health care workers, older people, and those with high-risk health conditions.

The case for an international effort like COVAX was urgent well before any COVID-19 vaccine trials began and before the WHO declared it a pandemic. The 2009 H1N1 pandemic provided a cautionary tale: There was a clear division between the haves and have-nots, says CEPI board chair Jane Halton, who has spent decades working in pandemic preparedness.

"All of the wealthy countries basically bought up the supply of the vaccine, and some of the less wealthy countries and smaller countries found it hard to get access," said Halton, who once headed Australia's health and finance departments. CEPI emerged out of the H1N1 pandemic to support creating and fairly distributing future vaccines, particularly in low-income countries.

"How do we purchase for the people who need the vaccine across poor countries, and for people who can't self-finance?" Halton said. "And then, how do we evenly distribute to the people most in need first around the world so we can bring this to a speedy conclusion?"

'A test of human ingenuity'

So far, about 172 countries are in talks to participate in COVAX.

In a WHO conference call, participating organizations and countries gave an update on the initiative's progress.

"...COVID-19 is also a test of our human ingenuity to transcend our individualities and destroy all barriers, looking to act with a common purpose," said Mitoha Ondo'O Ayekaba, vice-minister for health and social welfare for Equatorial Guinea.

Being part of a bigger pool ensures his country can actually compete with bigger ones to get an effective vaccine, he added.

The European Commission's financial backing was a major boost for the initiative.

"The European Commission is ready to join the COVAX facility," European Commission president Ursula von der Leyen announced. "We will join COVAX together with our E.U. member states as Team Europe. And the Commission will also make a contribution of €400 million euros in guarantees to COVAX facility and its goals."

Among the middle and high-income countries that have committed to join are Japan and Germany, two U.S. allies. The initiative set a deadline of Sept. 18 for nations to join and agree to a financial pledge.

The United States' refusal to participate "sets a negative precedent," said Suerie Moon, co-director of the Global Health Centre at the Graduate Institute in Geneva. But it won't "cripple" the effort.

Other big challenges lie ahead, too, Moon pointed out: Countries may try to buy up vaccines for themselves, whether or not they're participating in COVAX. That could make it harder for COVAX to get the same access.

"The COVAX facility still can deliver a lot. I think there's still a lot of potential," Moon said. "There's very strong political backing and certainly a lot of interest from countries around the world in making it work."

This article originally appeared at PRI's The World.