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 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 United States or European Union did, according to government officials in those countries.