NEW YORK (NYTIMES) - Moderna, whose coronavirus vaccine appears to be the world's best defence 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 United States 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 one 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 US or 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-19 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 maximising the return on investment," said Dr Tom Frieden, a former head of the Centres for Disease Control and Prevention.

Moderna executives have said that they are doing all they can to make as many doses as possible and 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 EU.

Even so, the Joe 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 US 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.

Last Friday (Oct 8), 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 one billion doses to low-income countries next year. The company also said this past week that it would open a factory in Africa, without specifying when.

Moderna executives have been talking 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 on Friday, Moderna chief executive Stephane 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 that Moderna tried and failed last year to get governments to kick in money to expand the company's scant production capacity and that it decides how much to charge based on factors, including how many doses are ordered and how wealthy a country is.

A Moderna spokesman 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 per cent of their populations as at Sept 30.

In August, for example, Johnson & Johnson faced rebukes from the director-general of the World Health Organisation 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 US government has provided the company with critical assistance.

Scientists at the National Institutes of Health worked with the company to develop the vaccine.

The US kicked in US$1.3 billion (S$1.8 billion) for clinical trials and other research. And in August last year, the government agreed to pre-order US$1.5 billion worth 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 Ms 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."

The lack of transparency about how much other governments are paying has put relatively poor countries in a weak bargaining position. They are "negotiating totally in the dark", said Ms Kate Elder, who advises Doctors Without Borders on vaccine policy.

In some cases, Moderna has offered to provide poorer countries the vaccine at relatively low prices, but only after it has fulfilled other countries' orders.

In May, Moderna offered the African Union doses for about US$10 each, according to a bloc official involved in the discussions. But the doses would not be available until next year, causing the talks to fall apart, according to two African Union officials.

Dr Ayoade Alakija, who helps run the African Union's vaccine delivery programme but was not involved in the procurement discussions, said Moderna's attitude amounted to: "We're here to make money. We've stumbled upon a good thing, and we're not even trying to pretend that we're trying to save the world."

Moderna's Covid-19 vaccine has been transformative for the company and its leaders. The company has said it expects its vaccine to generate at least US$20 billion in revenue this year, which would make it one of the most lucrative medical products in history.

Ms Andersen, the Morningstar analyst, projected that the company's profits on the vaccine could be as high as US$14 billion. In 2019, Moderna reported total revenue of US$60 million.

Moderna's market value has nearly tripled this year to more than US$120 billion. Two of its founders, as well as an early investor, this month made Forbes magazine's list of the 400 richest people in the US.

As the coronavirus spread early last year, Moderna raced to design its vaccine - which uses a new technology known as messenger RNA - and to plan a safety study. To manufacture the doses for that trial, the company received US$900,000 from non-profit organisation Coalition for Epidemic Preparedness Innovations.

The non-profit group said Moderna had agreed to its "equitable access principles". That meant, according to the coalition, that the vaccine would be "first available to populations when and where they are needed and at prices that are affordable to the populations at risk, especially low- and middle-income countries or to public sector entities that procure on their behalf".

Moderna agreed in May to provide up to 34 million vaccine doses this year as well as up to 466 million doses in 2022 to Covax, the struggling United Nations-backed programme to vaccinate the world's poor.

The company has not yet shipped any of those doses, according to a Covax spokesman, although Covax has distributed tens of millions of Moderna doses donated by the US.

Mr Bancel said many more doses would have gone to Covax this year had the two parties reached a supply deal last year.

Covax official Aurelia Nguyen denied that, saying: "It became clear early on that the best we could expect was minimal doses in 2021."