The leaders of the US, Australia, India and Japan have agreed to deliver one billion doses of coronavirus vaccine to much of Asia by the end of 2022.
The joint commitment was made following the first leaders' meeting of the so-called Quad - a group formed in 2007.
The vaccines - expected to be the single-dose Johnson & Johnson product - are set to be manufactured in India.
The US said the "massive joint commitment" would initially focus on delivering doses to South East Asia.
"With Indian manufacturing, US technology, Japanese and American financing and Australian logistics... [we] committed to delivering up to one billion doses," US National Security Adviser Jake Sullivan said shortly after the virtual summit on Friday.
He said the vaccines would go to the Association of South East Asian Nations (Asean) as well as "the Pacific and beyond".
Asean is a 10-member international body that represents more than 500 million people. Thailand, Indonesia and the Philippines are all members.
The Indian company Biological Ltd will manufacture the extra doses of the Johnson & Johnson jab, which received initial World Health Organization (WHO) approval on Friday.
"India's formidable vaccine production capacity will be expanded with support from Japan, US and Australia," India's Prime Minister Narendra Modi wrote on Twitter after the meeting.
What else was discussed?
The talks on Friday were the group's first at a leaders' level, and Mr Modi said "vaccines, climate change, and emerging technologies" were all on the agenda.
"The four countries have agreed to a plan to pool their financial resources, manufacturing capabilities and... and logistical strengths so as to ramp up the manufacturing and distribution of Covid-19 vaccines," Indian Foreign Secretary Harsh Vardhan Shringla said.
The Quad group is often regarded as a counterweight to China's growing assertiveness in Asia, and comments from the four leaders after the meeting appeared to take indirect aim at Beijing. All nations pledged to defend a "free and open" continent.
"We're renewing our commitment to ensure that our region is governed by international law, committed to upholding universal values and free from coercion," US President Joe Biden, who chaired the meeting, said in a statement.
Australian Prime Minister Scott Morrison said the summit represented "a new dawn".
Japanese Prime Minister Yoshihide Suga, however, took a more direct line against Beijing. He told reporters he had raised "strong opposition to China's unilateral attempts to change the status quo", adding that the other leaders had expressed support for his comments.
China looms in the background
The co-operation between these four democracies began with their joint response to the tsunami of 2004.
But President Biden has taken it to the leadership level for the first time as part of his policy of strengthening a strategic counterweight to Beijing.
His administration has been careful not to link the Quad explicitly to China, but the US is in the midst of a diplomatic drive to solidify alliances in response to Beijing's increasingly assertive global and regional role.
And there were plenty of implicit references to China in the comments after the meeting. The Quad leaders emphasised a "free and open Indo-Pacific" at a time when they're facing security challenges from Beijing.
Their plan to massively increase the production and distribution of coronavirus vaccines in Asia would put them in a position to compete with China's own vaccine diplomacy. And their intention to cooperate on critical and emerging technologies arises from concerns surrounding Chinese influence and actions in the cyber-sphere.
What is the Quad?
The Quad, which is shorthand for the Quadrilateral Security Dialogue, is an informal strategic forum for the four nations.
Although it was formed in 2007, it was on hiatus for nearly a decade after Australia’s then-Prime Minister Kevin Rudd withdrew Australia in 2008. The group was resuscitated in late 2017 as the Trump administration ramped up confrontation with Beijing.
Although the group is often regarded as an effort to contain China's growing ambitions, official statements leading up to Friday’s meeting said little about the country.
Nevertheless, Chinese state media has criticised the group.
The Global Times quoted Chinese experts who suggested that the Quad members are likely to follow their own interests above the interest of the group, rendering the alliance an “empty talk club”.
China is also engaged in what some have described as vaccine diplomacy, particularly in the Asia Pacific region.
China's ministry of foreign affairs has said the country will donate vaccines to 69 developing countries in urgent need, and is exporting vaccines to 43 further nations.