China's entry into the COVAX initiative means the list of non-participants in the global effort to develop and distribute coronavirus vaccines has dwindled down to Belarus, Kazakhstan, Malaysia, Russia, the U.S. and five small island countries or micro-states.

Breaking it down: 183 countries with a combined 93% of the world's population are either eligible for subsidized access or have said they intend to participate, though some have yet to sign formal agreements.

Why it matters: The distribution of coronavirus vaccines may be the defining global challenge of 2021.

• Led by the World Health Organization, the GAVI vaccine alliance and the Coalition for Epidemic Preparedness Innovations, COVAX is the one genuinely global effort to address it.

How it works: By pooling resources, COVAX intends to invest in the development of at least nine vaccine candidates, secure lower-cost bulk access and distribute 2 billion doses to all participant countries by the end of next year.

• Distribution will initially be proportional to population, with guidelines calling for health workers and vulnerable groups to be vaccinated first.
• What to watch: It will not be easy to balance the interests of 180+ countries, some of which are building up their own vaccine stockpiles or attempting to negotiate the terms of their participation.

Driving the news: China was late to join COVAX, and the terms of its commitment aren't clear. But Beijing is attempting to contrast itself with the U.S. and counter the reputational damage it has suffered during the pandemic.

• "We are taking this concrete step to ensure equitable distribution of ­vaccines, especially to developing countries, and hope more-capable countries will also join and support COVAX," the Chinese Foreign Ministry announced on Friday.
• Polls suggest distrust of Beijing is growing around the world, particularly after a suspected cover-up of the initial outbreak in Wuhan.
• But China now has the virus largely under control. It also has four vaccine candidates in phase III trials, and it's promised that some neighbors and strategic partners will have priority access.

The flipside: The U.S. appears to be the only country to have publicly rejected the COVAX initiative outright. It cited the influence of "the corrupt World Health Organization and China."

• President Trump has instead invested in six vaccine candidates through the $10 billion Operation Warp Speed.

The bottom line: The U.S. is not alone in prioritizing access for its own population — though it is very nearly alone in refusing to join COVAX and withdrawing from the WHO.

• Joe Biden has said he'd reverse Trump's WHO decision. Asked by Axios, his campaign didn't say whether he'd consider bringing the U.S. into COVAX.