In the United States, the good vaccine news keeps coming. For much of the world, things look bleak.

As of Thursday, just short of 20 percent of the U.S. population was fully vaccinated, giving some 66 million people a strong measure of protection against a disease that has already killed more than 500,000 Americans.

By contrast, Covax — a World Health Organization-backed push for equitable distribution — aims to secure enough doses to cover up to 20 percent of the people in participating countries by the end of 2021, but it may not meet that relatively modest goal, experts warn.

The gap between the vaccine “haves” and “have-nots” is widening, fueling frustration and potentially extending the pandemic.

“It’s unconscionable,” said Zain Rizvi, an expert on access to medicine at Public Citizen, a watchdog group. “Many countries will be lucky if by the end of the year they are close to where the U.S. is now.”

So far, the vaccine race has been dominated by a handful of relatively wealthy nations: most notably Israel, where nearly 57 percent of the population was fully vaccinated as of April 7; Chile, at about 22 percent; and the United States. Britain has been vaccinating rapidly, as well, but it has delayed second doses as it tries to get a first to as many people as possible.

Meanwhile, Our World in Data estimates based on publicly reported data that at least 5 percent of the global population has had a dose, with the real number (incorporating China’s nonpublic tally) perhaps between 6 and 7 percent.

Priority-supply deals, export restrictions and other means of hoarding by rich nations have contributed to a severe global supply crunch and left many countries scrambling.

Covax has delivered 38 million doses, providing potentially lifesaving shots to places and people that might otherwise go without. Yet divided between 100 economies, those doses amount to only a thin layer of protection.

“It’s been heartening to see a small number of doses reaching countries around the world,” said Suerie Moon, co-director of the Global Health Center at the Graduate Institute of International and Development Studies in Geneva. “But the big picture is more troubling than reassuring because we have a lot of things that are not going well.”

While the United States administers millions of inoculations a day, some countries are still waiting for their first shots to arrive, or have just started vaccinating. A recent WHO estimate suggested that just 2 percent of the 690 million doses administered to date globally went to Africa.

A chorus of experts and officials have argued — for months — that rich countries have not only a moral obligation to close the gap but an interest in doing so. With a fraction of the world’s population vaccinated, they argue, the global economy won’t recover and the virus will mutate and spread.

Treasury Secretary Janet Yellen on Monday called for speeding up distribution to poorer nations, warning that the pandemic may force 150 million people into poverty, hurting growth.

“Our first task must clearly be stopping the virus by ensuring that vaccinations, testing and therapeutics are available as widely as possible,” she said in remarks delivered to the Chicago Council on Global Affairs.

The same day, while introducing a new coordinator for global coronavirus response and health security, Secretary of State Antony Blinken addressed the danger of variants.

“Even if we vaccinate all 332 million people in the United States tomorrow, we would still not be fully safe from the virus, not while it’s still replicating around the world and turning into new variants that could easily come here and spread across our communities again,” he said.

Still, Blinken defended the effort to vaccinate Americans first and suggested that further action would have to wait until the United States was more confident of its vaccine supply.

“I know that many countries are asking for the United States to do more, some with growing desperation because of the scope and scale of their covid emergencies,” he said. “We hear you. And I promise, we’re moving as fast as possible.”

The woman he introduced as the new global coronavirus response coordinator, Gayle Smith, served as chief executive of the ONE Campaign, a nonprofit organization that has called on wealthy countries to donate 5 percent of their surplus doses once they’ve vaccinated 20 percent of their population.

For its part, the Biden administration announced the “loan” of a combined 4 million doses of AstraZeneca’s coronavirus vaccine — not yet authorized by U.S. regulators — to Mexico and Canada.

It is not clear, however, if or when the administration will offer up a more substantial portion of the hundreds of millions of surplus doses the country has secured.

A recent survey of 788 Americans by researchers at Virginia Commonwealth University found strong support for the idea of donating 10 percent or more of the U.S. supply to less prosperous countries, but views were divided on the timing. While 41 percent of respondents said donations should happen immediately, 28 percent wanted to wait until at-risk Americans had been vaccinated and 31 percent said donations should happen only after everyone in the United States who wanted to be vaccinated had been.

In Israel, Prime Minister Benjamin Netanyahu offered in February to donate doses to 20 foreign allies, but the plan was put on hold in the face of domestic pushback and lawsuits.

The Biden administration’s moves so far have focused on longer-term efforts to bolster the global rollout.

In February, the White House threw its support behind Covax, announcing funding of up to $4 billion, including an initial contribution of $2 billion that Congress appropriated in December.

And last month, the United States, India, Japan and Australia pledged to jointly manufacture and distribute up to 1 billion doses of coronavirus vaccines with a focus on Southeast Asia. But the timeline is long, with a goal of having things up and running before the end of next year.

The Biden administration has so far resisted pressure to waive patent protections in a way that would allow more countries to make coronavirus vaccines.

However, recent statements from Blinken suggest some new initiative could be on the way.

“The clock is ticking,” Moon said, “and the situation is not getting better.”