A global initiative to ensure equitable distribution of coronavirus vaccines now includes most of the world — but not the U.S., China or Russia.

Why it matters: Assuming one or more vaccines ultimately gain approval, there will be a period of months or even years in which supply lags far behind global demand. The COVAX initiative is an attempt to ensure doses go where they're most needed, rather than simply to countries that can produce or buy them at scale.

How it works: COVAX — led by the WHO, the Coalition for Epidemic Preparedness Innovations and the Gavi vaccine alliance — is investing in the production of nine vaccines, including candidates from the U.S., Europe and China. It plans to distribute any that are approved to all participating countries.

• The funding will come from wealthier countries and other donors, with poorer countries receiving subsidized access.

A draft plan envisions a first phase of proportional distribution across all countries until about 20% of their populations are vaccinated.

• The guidelines suggest health workers be vaccinated first, followed by the elderly and those with serious health conditions (countries will make their own allocation decisions).
• In a second wave of more widespread distribution, each country’s "COVID threat and vulnerability" will be taken into account.

Where things stand: 65 higher-income economies have now joined the 92 that are eligible for subsidized access. Together, they comprise 64% of the global population.

• A further 35 higher-income countries or territories have said they intend to join.

The other side: The U.S. is independently buying up doses of six vaccine candidates and has said it won't participate in COVAX, citing the influence of "the corrupt World Health Organization and China."

• China isn't currently part of COVAX. It may still join, but it plans to give its citizens and some friendly countries priority access to any vaccine it produces.
• Russia hasn't joined either. Like China, it says it will make its vaccine available to other countries, but it wants to control that process.
• President Trump, meanwhile, has said the U.S. will vaccinate Americans first and then share any remaining doses.

Between the lines: COVAX will be competing for a scarce supply of vaccines with both the U.S. and some of its own participants.

• The U.K., Japan and EU plan to participate in COVAX, but they have also been purchasing tens of millions of doses for their own populations.
• Meanwhile, India is eligible for access under COVAX, but it has also been promised some 500 million doses by the Serum Institute of India, the world's highest-capacity vaccine manufacturer.

What to watch: The groups behind COVAX argue that its diversified "portfolio" of vaccines gives countries their best shot at obtaining access to one that works.

• Russia, by contrast, appears to be putting all of its hopes into a single vaccine, called Sputnik.
• The White House says it's confident that at least one of the six vaccines the U.S. has invested in will work.
• The latest: Johnson & Johnson's vaccine became the fourth to begin phase 3 trials in the U.S. this week.

The bottom line: We're sprinting toward a potential vaccine, and there's now a plan in place to distribute it around the world. But this is one of the most complex global undertakings in decades, and success is far from guaranteed.

Go deeper:Where the vaccine race stands in the U.S.