By Eric Bellman

The Bill & Melinda Gates Foundation said it is backing the world's largest vaccine maker, Serum Institute of India, to churn out 100 million doses of coronavirus vaccine for poorer countries and price them at less than $3.

The move comes as governments around the world, including the U.S. and U.K., strike vaccine production deals with the manufacturers of a handful of promising, late-stage vaccine development projects.

The Gates Foundation as well as Gavi, the Vaccine Alliance -- an organization which helps negotiate and finance vaccines for poor countries -- said they would back privately held Serum Institute, or SII, to speed up the manufacturing of Covid-19 vaccine doses for the developing countries once any are proven effective. SII is one of several contracted manufacturers already tapped by AstraZeneca PLC to make a vaccine in development at the University of Oxford.

The Pune, India-based SII is the go-to vaccine supplier for the World Health Organization and others and produces 1.5 billion doses of other vaccines every year, making it the largest in the world by volume. The three organizations said the collaboration will help ensure that lower and middle-income countries won't be forgotten if a coronavirus vaccine is found.

"Researchers are making good progress on developing safe and effective vaccines for Covid-19," said Bill Gates in a statement. "But making sure everyone has access to them, as soon as possible, will require tremendous manufacturing capacity and a global distribution network."

Indian drug giant SII is little-known outside the vaccine world. It had already announced plans to make and distribute a billion doses of Oxford's yet-to-be approved coronavirus vaccine. Oxford has previously agreed with AstraZeneca, the U.K. based pharmaceutical giant, to coordinate the global production of the vaccine. AstraZeneca, in turn, agreed that SII would be the main contract manufacturer for low- and middle-income countries if the vaccine proves safe and effective.

The Oxford vaccine has shown promise in early trials. SII has a similar agreement to manufacture and distribute another vaccine being developed by Novovax Inc.

The deal with the Gates Foundation and Gavi would help finance, accelerate and organize those efforts.

With some vaccines in late-stage testing, drugmakers have been signaling how much they might charge initially, with prices spanning from several dollars a dose to more than $70 for a multiple-dose course. Oxford has specifically stipulated any successful vaccine it creates should be sold at cost during the pandemic.

Moderna Inc. said it signed small-volume supply contracts with governments, pricing a two-dose regimen at as much as $74. Johnson & Johnson said it agreed to provide the U.S. with 100 million doses of its vaccine for more than $1 billion from the federal government, implying a per-dose price of about $10.

Under a deal announced last month, the U.S. agreed to pay Pfizer Inc. and Germany's BioNTech $1.95 billion for 100 million doses, which suggests a price of $19.50 a dose and $39 for two doses. AstraZeneca, meanwhile, agreed to provide 300 million doses to the U.S. for $1.2 billion, implying a cost of $4 a dose.

Write to Eric Bellman at eric.bellman@wsj.com