Pfizer and BioNTech will provide an additional 500 million doses of their Covid-19 vaccine to the U.S. government to be donated to low and lower-middle-income countries.

The move announced Wednesday represents an expansion of the companies’ existing agreement with the U.S. government to provide extra vaccine doses at a not-for-profit price for less-advantaged nations, and brings the total number of doses to be supplied for donation to these countries to a billion.

In line with the initial agreement, the U.S. government will allocate doses of the Pfizer-BioNTech Covid vaccine to 92 low and lower-middle-income countries and the 55 member states of the African Union, Pfizer said in a press statement Wednesday.

Deliveries of the initial 500 million doses began in August, and the total 1 billion doses under the expanded agreement are expected to be delivered by the end of Sept. 2022, the company added.

The first doses allocated through this program arrived in Rwanda in mid-August and since then, more than 30 million doses have been shipped to 22 countries.

Pfizer and BioNTech have an existing agreement in place to supply vaccine doses to the COVAX Facility, a mechanism established by Gavi, the Vaccine Alliance, the Coalition for Epidemic Preparedness Innovations and World Health Organization that aims to provide poorer countries with early access to Covid-19 vaccines.

Meanwhile, developed nations like the U.S. and those in Europe have had plentiful supplies of Covid shots since a number of vaccine candidates were developed at breakneck speed and authorized for emergency use last year before being rolled out to their general populations in mass vaccination campaigns.

While a majority of adults in the U.S. and Europe are now fully vaccinated, millions of people around the world do not have such ready access to Covid vaccines which greatly reduce the risk of severe Covid infection, hospitalization and death.

In the U.S., 64.1% of the population above 12 years of age is fully vaccinated, according to CDC data, while in the U.K., 81.9% of people over 16 are fully vaccinated, British government data shows. In the EU, 71.7% of adults are fully vaccinated, according to the European Centre for Disease Prevention and Control.

Our World in Data figures note that while 43.5% of the world population has received at least one dose of a Covid vaccine, only 2% of people in low-income countries have received at least one dose.