Vaccines are being administered across the world to try and stop the coronavirus pandemic.
An international effort, known as the Covax scheme, was set up last year to try to ensure fair access to vaccines among rich and poor nations but the global situation remains vastly uneven.
More than 49 million vaccine doses have been delivered through Covax so far.
But a further $35bn-$45bn (£25bn-£32bn) is needed over the course of next year to ensure most adults are immunised, according to the Director-General of the World Health Organization (WHO) Tedros Adhanom Ghebreyesus.
Covax is co-led by the WHO, the Global Vaccine Alliance (Gavi) and the Coalition for Epidemic Preparedness Innovations (Cepi), with the UN children's fund, Unicef, as key implementation partner.
Why is a scheme to share vaccines needed?
The coronavirus pandemic has destroyed livelihoods and claimed the lives of more than three million people worldwide.
Health experts, including the head of the WHO, have been clear that the fight against coronavirus is a global one and have urged wealthier nations to do more to help others to bring the situation under control.
Covax was created to pool the global vaccine effort and ensure fairer distribution in a mechanism by which richer countries offset the costs of getting vaccines to poorer ones.
Ghana was the first country to receive Covax vaccines in February. Tens of millions of doses have been shipped across six continents since and experts hope to distribute two billion doses by the end of the year.
Dr Tedros has criticised wealthier nations for undermining Covax, accusing them of "gobbling up" the global vaccine supply by ordering multiple times more than they need for their own populations.
He said in April that only 0.3% of the vaccines administered around the world so far had gone to people in low-income countries.
Covax now also needs about 20 million extra doses by the end of June to make up a shortfall on deliveries caused by the spiralling health crisis in India.
Sweden is the latest country to donate to that pot, pledging one million AstraZeneca doses at the start of May.
What vaccines is Covax using?
Five vaccines have already been given emergency use authorisation by the WHO, which is a pre-requisite for Covax use.
The latest one is by US drug-maker Moderna, which has agreed a deal with Covax for 500 million doses at its "lowest-tiered price". However, the bulk of these will not be available until 2022.
Only Pfizer and Oxford Astra-Zeneca jabs have been distributed by Covax so far.
Low-income countries including Algeria, Malawi and Uganda in Africa, Iran and Iraq in the Middle East, and Barbados, El Salvador and Nicaragua in the Americas are among the recipients.
Although most of the first doses available will be delivered to low- and middle-income countries, some will be sent to high-income countries such as Canada, which has defended its decision to draw on Covax's early supply.
What issues have there been with Covax's rollout?
Some people say Covax has not moved quickly enough.
Initial targets were missed and problems with deliveries are likely to be exacerbated by the deteriorating situation in India.
Officials there have restricted exports to protect their own population amid record-breaking number of cases and deaths.
The Serum Institute factory in India was a major supplier of the Oxford-AstraZeneca vaccine to Covax and the scheme now faces a shortfall of tens of millions of doses, exacerbating concerns about protecting people in poorer nations.
Could Covax help end the pandemic?
Covax hopes to distribute enough vaccines to protect at least 20% of the population in the 92 poorer countries signed up.
But even if that target is met, it would still fall short of the level of immunity that experts say is needed to end the pandemic.
The WHO has suggested that stopping Covid-19 will require at least 70% of the global population to have immunity.
Some nations, including the UK, have said they will donate surplus doses to poorer nations but the WHO is urging nations to do more earlier.
Pictures of the first people being vaccinated against Covid-19 haven't filled everyone around the world with joy. In some places - in countries such as Zimbabwe, Mexico and Pakistan - the battle to get hold of the vaccine is likely to be long and tortuous.
Watching the vaccine roll out in the UK, Lois Chingandu wasn't excited - she was worried. Like most of us, she's looking forward to getting vaccinated and getting life back to normal. But unlike many people right now, she doesn't see light at the end of the tunnel.
It's not clear when her country, Zimbabwe, will get a vaccine. "It's now just an issue of sitting and hoping if we will get it in my lifetime," she says. "I live in fear that I will contract Covid and die because of where I am sitting." It may sound like an exaggeration, but she has seen something very similar happen before.
Ms Chingandu works in HIV prevention and in the late 1990s in Harare, Zimbabwe's capital city, she watched thousands of people die from AIDS each day. Medicine was available to stop it - but only to those who could afford it. "Eventually when the privileged decide that it's time to save the poor people, then we will get the vaccine," she says.
Ms Chingandu is a member of a campaign called the People's Vaccine Alliance, which has admonished rich countries - particularly the US, UK, EU countries and Canada - for hoarding vaccines.
According to researchers at Duke University, which is keeping track of deals between governments and vaccine companies, a handful of countries have secured more supply than their populations actually need. Canada has secured enough vaccines to vaccinate its entire population five times.
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