The international scheme to ensure equal access to Covid-19 vaccines is 140 million doses short because of India's continuing Covid crisis.
The Serum Institute of India (SII), the largest single supplier to the Covax scheme, has made none of its planned shipments since exports were suspended in March.
The UN children's agency Unicef buys and distributes vaccines for Covax.
It is urging leaders of G7 nations and EU states to share their doses.
They are due to meet in the UK next month.
Unicef says data it has commissioned suggests that together this group of countries could donate around 153 million doses, while still meeting their commitments to vaccinate their own populations.
'A huge concern'
The SII was due to supply around half of the two billion vaccines for Covax this year but there were no shipments for March, April or May. The shortfall is expected to rise to 190 million doses by the end of June.
"Unfortunately, we're in a situation where we just don't know when the next set of doses will materialise," said Gian Gandhi, Unicef's Covax co-ordinator for supply.
"Our hope is, things will get back on track, but the situation in India is uncertain… and a huge concern."
Unicef is calling on the G7 countries - Canada, France, Germany, Italy, Japan, the UK and the US, as well as the EU, to donate their surplus supplies urgently.
Some countries have ordered enough to vaccinate their population many times over, including the UK, US and Canada.
In February British Prime Minister Boris Johnson promised to donate most of the UK's surplus supply to poorer countries but he has so far given no specific timescale. It is a similar story for the US. So far France is the only G7 country to donate doses in view of the crisis in India.
Unicef said the rich and powerful G7 countries could make a huge dent in the vaccine deficit for poorer countries by donating 20% of their supplies in June, July and August, which would release around 153 million doses for the Covax scheme.
France has pledged half a million doses by mid-June while Belgium has promised 100,000 from its domestic supply in the coming weeks.
Spain, Sweden and the United Arab Emirates are some of the very few others pledging to share their supplies now.
There are grave concerns that events in India could play out in other countries too - both near and far from the region.
"Cases are exploding and health systems are struggling in countries like Nepal, Sri Lanka and Maldives… and also in Argentina and Brazil," said Unicef director Henrietta Fore. "The cost for children and families will be incalculable."
Booster jab dilemma
Countries in Africa are some of the most reliant on doses through the Covax scheme.
But, like in many parts of the world, there has also been hesitancy around receiving the vaccine among some communities. Another major challenge is physically getting the doses into people's arms - all that requires health workers to be specially trained and the vials to be transported to far-flung parts of countries where infrastructure can be limited.
Some nations are now facing the prospect of deciding whether to give second doses to the most vulnerable who have already been given one jab or continue vaccinating more people as planned in the hope that the next shipments turn up soon.
"We're in a situation now where healthcare workers and frontline workers in many countries in Africa are yet to be vaccinated," said Gian Gandhi. "And yet higher-income countries are vaccinating lower-risk populations, such as teenagers."
Nations including Rwanda, Senegal and Ghana are already using some of their last remaining doses, according to the World Health Organization (WHO).
Covax doses in Africa
- Seven countries in Africa have used almost 100% of their Covax doses including Botswana, Ghana, Rwanda and Senegal
- Kenya and Malawi have used nearly 90% of their Covax doses
- Cabo Verde and the Gambia have used 60% of their Covax doses
- 1.3 million doses have been redistributed from Democratic Republic of Congo to other parts of Africa because the country will not be able to use them all before their expiry date in June
"We really do sympathise with the situation in India," said Dr Richard Mihigo, who heads up the immunisation and vaccine development programme for the WHO in Africa.
"Most of our [18 million] Covax doses so far have come from India.
"I think it's very important [to keep] the global promise of solidarity for those countries that have enough vaccines - to distribute and share them because unless we stop the transmission everywhere, it will be very difficult to end this pandemic, even in places where people have been fully vaccinated."
What is Covax?
- Aim is to distribute two billion Covid-19 vaccine doses by the end of 2021
- No country to receive vaccines for more than 20% of their population before all countries have vaccinated at least 20% of the population
- Scheme has so far shipped around 60 million doses to 122 participants
- Co-led by the WHO and the vaccine alliance - Gavi and the Coalition for Epidemic Preparedness Innovations (Cepi)
- Unicef is the key delivery partner
The only way to fill the gaping hole for poorer countries right now is for richer countries to donate some of their supplies.
"We have issued repeated warnings of the risks of letting down our guard and leaving low- and middle-income countries without equitable access to vaccines, diagnostics and therapeutics," said Ms Fore.
"We are concerned that the deadly spike in India is a precursor to what will happen if those warnings remain unheeded. The longer the virus continues to spread unchecked, the higher the risk of more deadly or contagious variants emerging."
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