The Covax vaccine-sharing initiative said on Thursday that it needed $2 billion in extra funding by next week to fill a gap caused by India halting exports as it tackles a strong Covid outbreak.
Covax, a scheme to distribute vaccines to vulnerable countries, said it also urgently needs 190 million vaccine doses.
It said it could fall behind on its goal of delivering two billion doses by the end of the year.
"We need an additional $2 billion to lift coverage in [recipient countries] up to nearly 30 per cent, and we need it by June 2 to lock in supplies now so that doses can be delivered through 2021 and into early 2022," the Covax group said.
The shortfall comes after India stopped exports from its Serum Institute, the world's largest vaccine maker by volume, which was producing 1.5 billion doses of vaccines a year even before the pandemic hit.
Covax said it had delivered 70 million vaccine doses to 126 countries, but faces a shortfall of 190 million at the end of June because of the "severe impact on Covax's supply in the second quarter of this year".
Covax will have more vaccinations available later in the year through deals it secured with several makers, but it said there could be “catastrophic” consequences if the current shortfall was not addressed.
"If the world's leaders rally together, the original Covax objectives – delivery of two billion doses of vaccines worldwide in 2021, and 1.8 billion doses to 92 lower-income countries by early 2022 – are still well within reach," Covax said.
"But it will require governments and the private sector to urgently unlock new sources of doses, with deliveries starting in June, and funding so we can deliver."
Covax welcomed vaccine pledges made so far by countries such as the UAE, France, Germany, Sweden, Italy, Spain and New Zealand.
The US has promised to supply about 80 million doses, the largest donation from a country, but has so far not said how the shots would be distributed or which countries would receive them.