The swift development of effective coronavirus vaccines has been one of the few bright points of the pandemic. Since Britain administered the first fully tested vaccine in December, well over 130 million doses have been delivered around the world — 44 million in the United States alone.

But most countries have yet to see the benefits of this accomplishment. Months into the global vaccine rollout, the pace remains staggeringly unequal, with wealthy countries leaving poorer ones in the dust. And they aren’t competing in a vacuum: The success of the former has come very much at the expense of the latter.

It remains to a large degree a zero-sum game, which means that every dose that goes to the U.S. or the U.K. or an E.U. country is a dose that’s off the shelves,” said Andrea Taylor, a researcher at Duke University’s Global Health Innovation Center who tracks vaccines. “And the shelves aren’t going to be restocked for a while.”

The zero-sum nature of vaccine supply is rooted in a wide variety of factors, but boils down to the simple fact that manufacturers cannot yet meet demand. The potential consequences are wide-ranging. Unmitigated spread in any country, rich or poor, can lead to variants that may be more virulent or resistant to vaccines.

“As long as the epidemic is ongoing and more virus is circulating, more variants are likely to emerge,” said Willem Hanekom, director of the Africa Health Research Institute in South Africa, which earlier this week halted the rollout of a vaccine developed at Oxford University and made by AstraZeneca after a study suggested it may not be adequately effective against a variant widespread in the country.

“The longer this takes, the greater the risk of mutations which could render the vaccines impotent, as is already apparently happening in South Africa, which poses a risk to people everywhere and to the global economic recovery,” said Ian Goldin, a professor of globalization and development at Oxford University.

The delayed introduction of vaccines in poorer countries is also likely to lead to a longer pandemic, with a deadlier human toll and more long-lasting economic damage.

Some experts have urged wealthy nations to address the situation head-on, through a policy likely to find little domestic support: the donation of doses to other countries that need them, before fully vaccinating at home.

The idea has sway among those who study global public health and inequality. The first focus should not be on getting an entire country vaccinated, but on working to vaccinate the people who are most at risk around the world, said Sema Sgaier, an assistant professor at Harvard and director of the Surgo Foundation.

“It is not unfair that countries vaccinate their population — they definitely have to meet local demand and open local economies,” Sgaier wrote in an email. “But once each country reaches a threshold (e.g. age and risk groups), there should be some sharing.”

Beyond sharing their guaranteed orders abroad, countries could relax intellectual property and move to ensure manufacturing resources go to vaccines that are not only successful but also easy to store and transport, Goldin said.

So far, buy-in to this way of thinking remains limited. Norway has offered a plan to donate excess vaccine doses. Mexico, despite recording the third-worst death toll globally, has limited its purchases of the Pfizer vaccine after a U.N. request. Most other nations have not detailed such plans.

The Biden administration has pledged to pursue the idea of donating vials, but has not offered a timeline or other specifics.

The United States would “develop a framework for providing surplus U.S. government vaccine doses to countries in need, once there is sufficient supply in the United States,” a State Department spokesperson said. The government has not said how it would define surplus in light of vaccine hesitancy.

The spokesperson said the United States might consider donating excess doses through Covax Facility, a program backed by the World Health Organization that is designed to ensure global access to vaccines. The Biden administration has pledged to support the program, in which the Trump administration did not participate.

Such a move could strengthen Covax, which was designed to attract wealthy donors by placing guaranteed orders among a range of vaccine manufacturers — avoiding the risk of nation-level deals to acquire vaccines that could later prove inefficient. So far, the initiative has struggled to find funding and accrue supply.

“Not only did most wealthy nations not rely on Covax for their vaccines, [but] the portfolio approach that these nations took to their advance vaccine purchases consumed most of the resources and vaccine manufacturing capacity that otherwise might have gone to Covax,” said Thomas J. Bollyky, a senior fellow at the Council on Foreign Relations.

Speaking from South Africa, where authorities are scrambling to make their own deals after Covax fell short, Hanekom said many nations failed to see that vaccination is a fundamentally global issue. “Thinking that we can protect our own population, when others are not protected? There’s incredible risk with that approach,” he said.

Some countries have ordered more doses than they need to reach full vaccination. By the end of January, Canada had arranged deals to acquire 338 million doses for its population of 38 million — giving it the potential for 500 percent coverage, according to Taylor’s team at Duke.

Covax announced last week that it plans to distribute 330 million doses to poorer countries in the first half of the year, but that remains a fraction of the total number required. At the same time, wealthy nations with their own supply of vaccines, including Canada, are set to receive doses through Covax.

The effects of and solutions for vaccine inequality are not clear-cut. Canadian officials last week defended their decision to accept 1.9 million doses of the AstraZeneca vaccine by summer through Covax, arguing that their responsibility was to their citizens first and foremost. Officials told The Washington Post in December that Canada would donate excess doses, but have not given a firm time frame.

The portfolio approach to vaccine acquisition, by which wealthy nations pieced together sets of orders across promising vaccine prospects, may have had some positive knock-on effects for global availability. “The scale of the preorders was staggering,” Taylor said. “And what the preorders at that scale allowed was a massive investment in manufacturing early in the process.”

Wealthy countries may have helped spur more initial supply, but “we were always going to have demand outstripping supply,” Taylor said. But the ceiling could eventually go up. Some drug manufacturers are moving their resources to support rivals after their own vaccine candidates fizzled.

In the United States, where the Biden administration recently announced additional vaccination plans, some believe that supply could soon outpace demand.

“We’re going to run out of demand sooner than we think,” Scott Gottlieb, a former commissioner of the Food and Drug Administration, said during an appearance on CNBC this week.

On the global stage, vaccines made by Russia and China, though initially viewed with skepticism in the West, are already making a difference. Data compiled by the Think Global Health project at the Council on Foreign Relations shows these vaccines already account for much of limited supply in middle- and low-income countries.

The redistribution of doses to the world’s most vulnerable could help avert a steep human toll, buying manufactures more time to meet global demand — which they might, in theory, by next year, Taylor said. But the very nature of the virus means any timeline could change.