Up to 100 million additional doses of any eventual Covid-19 vaccines will be secured for delivery to poorer countries in 2021, Gavi, the Vaccine Alliance, announced Tuesday.

The announcement doubles the number of doses already secured from the Serum Institute of India (SII), the world's largest vaccine manufacturer by volume, by Gavi and the Bill & Melinda Gates Foundation, following an initial agreement last month.

"This brings the total number of vaccine doses to be covered by the partnership between SII, Gavi, and the Gates Foundation to an aggregate of up to 200 million doses," Gavi said in a statement. It stressed that the agreement "provides an option to secure additional doses, potentially several times the 200 million dose total," if needed, adding that the vaccines will have a ceiling price of $3 per dose.

Under the agreement, SII will receive upfront capital to scale up its manufacturing capacity to produce candidate vaccines being developed by AstraZeneca and by Novovax. The idea is for the manufacturer to be able to begin rolling out millions of doses as soon as a vaccine, or vaccines, receive approval from regulators and the World Health Organization.

The doses will be distributed through Covax, the international coronavirus vaccine procurement pool created by the WHO to ensure equitable access to the jabs, "as early as the first half of 2021", the statement said.

"No country, rich or poor, should be left at the back of the queue when it comes to COVID-19 vaccines; this collaboration brings us another step closer to achieving this goal," Gavi chief Seth Berkley said in the statement.

Covax, which aims to lay its hands on two billion doses of safe and effective vaccines by the end of next year, has struggled to raise the funds needed to provide for the 92 low-income countries and other economies that quickly signed up.

WHO announced last week that more than 60 wealthy nations had joined, but a number were not yet on the list, including the United States, China and Russia. Gavi also signed a memorandum of understanding with AstraZeneca back in June for access to another 300 million doses of its candidate vaccine, but those would be made available to all participants in Covax, not just the poorer countries.