Moderna, whose coronavirus vaccine appears to be the world's best defense against COVID-19, has been supplying its shots almost exclusively to wealthy nations, keeping poorer countries waiting and earning billions in profit.

After developing a breakthrough vaccine with the financial and scientific support of the U.S. government, Moderna has shipped a greater share of its doses to wealthy countries than any other vaccine manufacturer, according to Airfinity, a data firm that tracks vaccine shipments.

About 1 million doses of Moderna's vaccine have gone to countries that the World Bank classifies as low income. By contrast, 8.4 million Pfizer doses and about 25 million single-shot Johnson & Johnson doses have gone to those countries.

Of the handful of middle-income countries that have reached deals to buy Moderna's shots, most have not yet received any doses, and at least three have had to pay more than the United States or the European Union did, according to government officials in those countries.

Thailand and Colombia are paying a premium. Botswana's doses are late. Tunisia could not get in touch with Moderna.

Unlike Pfizer, Johnson & Johnson and AstraZeneca, which have diverse rosters of drugs and other products, Moderna sells only the COVID vaccine. The Massachusetts company's future hinges on the commercial success of its vaccine.

"They are behaving as if they have absolutely no responsibility beyond maximizing the return on investment," said Dr. Tom Frieden, a former head of the Centers for Disease Control and Prevention.

Moderna executives have said that they are doing all they can to make as many doses as possible as quickly as possible but that their production capacity remains limited. All of the doses they produce this year are filling existing orders from governments such as the E.U.

Even so, the Biden administration has grown increasingly frustrated with Moderna for not making its vaccine more available to poorer countries, two senior administration officials said. The administration has been pressing Moderna executives to increase production at U.S. plants and to license the company's technology to overseas manufacturers that could make doses for foreign markets.

Moderna is now scrambling to defend itself against accusations that it is putting a priority on the rich.

On Friday, after the New York Times sent detailed questions about how few poor countries had been given access to Moderna's vaccine, the company announced it was "currently investing" to increase its output so it could deliver 1 billion doses to low-income countries in 2022. The company also said this past week that it would open a factory in Africa, without specifying when.

Moderna executives have talked with the Biden administration about selling low-cost doses to the federal government, which would donate them to poorer countries, as Pfizer has agreed to do, the two senior officials said. The negotiations are continuing.

In an interview Friday, Moderna CEO Stéphane Bancel said that "it is sad" that his company's vaccine had not reached more people in poorer countries but that the situation was out of his control.

He said Moderna tried and failed last year to get governments to kick in money to expand the company's scant production capacity and that the company decides how much to charge based on factors including how many doses are ordered and how wealthy a country is. (A Moderna spokesperson disputed Airfinity's calculation that the company had provided 900,000 doses to low-income countries, but she did not provide an alternate figure.)

Nearly a year after Western countries began sprinting to vaccinate their populations, the focus in recent months has shifted to the severe vaccine shortages in many parts of the world. Dozens of poorer countries, mostly in Africa and the Middle East, had vaccinated less than 10% of their populations as of Sept. 30.

In August, for example, Johnson & Johnson faced rebukes from the director-general of the World Health Organization and public health activists after the Times reported that doses of that shot produced in South Africa were being exported to wealthier countries.

Biden administration officials are especially frustrated with what they see as Moderna's lack of cooperation, because the U.S. government has provided the company with critical assistance.

Scientists at the National Institutes of Health worked with the company to develop the vaccine. The United States kicked in $1.3 billion for clinical trials and other research. And in August 2020, the government agreed to preorder $1.5 billion of the vaccine, guaranteeing that Moderna would have a market for what was an unproven product.

Although clinical trials last year found that the Moderna and Pfizer vaccines were similarly effective, more-recent studies suggest that Moderna's shot is superior. It offers longer-lasting protection and is easier to transport and store.

Moderna's shot is "essentially the premium vaccine," said Karen Andersen, an industry analyst at Morningstar. "They're in a position where they probably don't need to sacrifice too much on pricing in a lot of these deals."

Only a handful of governments have disclosed how much they are paying for Moderna doses. The United States paid $15-$16.50 for each shot, on top of the $1.3 billion the government gave Moderna to develop its vaccine. The EU has paid $22.60-$25.50 for its Moderna doses.

Botswana, Thailand and Colombia, which the World Bank classifies as upper-middle-income countries, have said they are paying $27-$30 per Moderna dose.

Moderna's COVID vaccine has been transformative for the company and its leaders. The company has said it expects its vaccine to generate at least $20 billion in revenue this year, which would make it one of the most lucrative medical products in history. Andersen, the Morningstar analyst, projected that the company's profits on the vaccine could be as high as $14 billion. In 2019, Moderna reported total revenue of $60 million.