The US has thrown its support behind a move at the World Trade Organization (WTO) to temporarily lift patent protection for coronavirus vaccines.
India and South Africa proposed the plan, which they said would increase vaccine production around the world.
But drugs manufacturers argue it may not have the desired effect.
US Trade Representative Katherine Tai said that "extraordinary times call for extraordinary measures".
And she warned that it would take time to reach an agreement on the matter.
One hundred members of the World Trade Organisation (WHO) are said to be in favour, and a panel on intellectual property is expected to discuss the issue next month.
Intellectual property means creations, such as inventions, which are protected by patents, copyrights and trademarks - allowing the originator to be financially rewarded.
India and South Africa were the leading voices in a group of about 60 countries which for the last six months has been trying to get the patents on vaccines set aside.
However, they met with strong opposition from the previous US administration of Donald Trump, the UK and the EU.
But Mr Trump's successor as US President, Joe Biden, has taken a different tack. He backed a waiver on intellectual property associated with Covid vaccines during the 2020 presidential campaign and repeated his support on Wednesday.
The head of the WHO called the move a "monumental moment" in the fight against Covid-19.
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What would the waiver mean?
If approved, supporters say, the waiver would allow production of vaccines to be ramped up and provide more affordable doses for less wealthy countries.
Many developing countries have argued that rules requiring countries to protect patents and other forms of intellectual property are an obstacle to increasing the production of vaccines and other products needed to tackle the pandemic.
The US had previously helped block WTO negotiations about the waiver proposal led by India and South Africa aimed at helping developing countries to produce vaccines using the intellectual property of pharmaceutical companies.
Ms Tai said the US would now embark on negotiations at the WTO to try to secure the waiver. This could take time as WTO decisions require a consensus of all 164 members.
It is a truly extraordinary moment.
The campaign for this has been going on amongst NGOs, some US Congressional Democrats and some developing countries such as India and South Africa. And as recently as March the US, the UK and the EU were resisting the moves in negotiations at the WTO in Geneva.
Katherine Tai has been seeing top pharmaceutical companies and also raised the issue with UK International Trade Secretary Liz Truss at a virtual meeting last month.
In an interview with the BBC last month, WTO chief Ngozi Okonjo-Iweala said that if vaccine manufacturers failed to supply the world, they would have to transfer the know-how.
Drugs companies are adamant that the patents are not the bottleneck here, it is manufacturing capacity. But the Indians and South Africans have disagreed, with South African President Cyril Ramaphosa decrying what he called "vaccine apartheid", saying his country and some in South America could ramp up production.
The US move does tilt the balance of power in the WTO towards a waiver. But the position of both the UK and the European Union will now also come under careful scrutiny.
What has the reaction been?
Tedros Adhanom Ghebreyesus, head of the WHO, called the US decision "historic" and marked "a monumental moment in the fight against Covid-19".
But pharmaceutical companies have voiced their opposition, insisting that patents are not the primary obstacle, and cautioned that the move could stifle innovation.
The International Federation of Pharmaceutical Manufacturers and Associations called the move "disappointing".
"A waiver is the simple but the wrong answer to what is a complex problem," the Geneva-based lobby group said.
Dr Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, told the Reuters news agency that the waiver "amounts to the expropriation of the property of the pharmaceutical companies whose innovation and financial investments made the development of Covid-19 vaccines possible in the first place".
Some experts say pharmaceutical companies would also need to share know-how, such as production techniques, with poorer countries to have any real beneficial effect.
Licensing agreements are another way of boosting production.
One of the largest Covid vaccine producers is the Serum Institute of India which manufactures the Oxford-AstraZeneca jab under a licensing agreement with the Anglo-Swedish company.
A potential link between the Oxford-AstraZeneca vaccine and very rare cases of blood clotting is being investigated.
Regulators have been saying there is no evidence that the vaccine is linked to or responsible for these events.
Some recently immunised people suffered an extremely rare form of blood clot - cerebral sinus vein thrombosis (CSVT) - in the brain.
In the UK, 30 cases of CSVT - seven of them fatal - were reported among 18 million people who received the vaccine.
The European Medicines agency was investigating 14 deaths as of 22 March.
It can occur naturally and no link to the vaccine has been confirmed.
But because, in some very rare cases, people have displayed unusual combinations of blood clotting alongside a very low platelet count - cell fragments that normally form clots - suspicions have been raised that there might be some connection.
As a precaution, regulators will continue to monitor the situation and advise symptoms, such as unusual bruising or persistent headache after vaccination, should prompt a medical check.
Covid infection itself can also make clots more likely.
Experts have continued to stress the benefits of vaccination outweigh the risks.
How do we know a vaccine is safe?
Safety trials begin in the lab, with tests and research on cells and animals, before moving on to human studies.
The principle is to start small and only move to the next stage of testing if there are no outstanding safety concerns.
When will I get a coronavirus vaccine?
What role do trials have?
If the safety data from the labs is good, scientists can check the vaccine or treatment is effective.
That means tests on large numbers of volunteers - about 40,000 in the case of Pfizer-BioNTech, the first to be approved in the UK.
Half are given the vaccine and the other half a placebo jab. The researchers and participants are not told which group is which, until after the results, to avoid bias.
All of the work and findings are checked and verified independently.
The Covid vaccine trials happened at breakneck speed, but they didn't skip any steps - they were able to move faster because so many people were involved and other projects were put aside.
What is the fast-track approval for vaccines against new variants?
Scientists hope to tweak coronavirus vaccines to ensure they continue to offer high protection as new variants of the disease emerge.
The UK's drug regulator says new vaccines can be fast-tracked for approval if needed.
No corners will be cut, with safety paramount.
But lengthy clinical trials with thousands of volunteers will not be needed.
Instead, data from existing studies and ongoing real-world use could be used.
Will the vaccine give me side effects?
Vaccines do not give you a disease. Instead, they teach your body's immune system to recognise and fight the infection they have been designed to protect against.
Some people do experience mild to moderate symptoms after being vaccinated. This is not the disease itself, but the body's response to the vaccine.
Common reactions that may affect more than one in 10 people and typically get better within days include chills, tiredness and aching muscles.
Who approves vaccines or treatments?
Approval is only given in the UK if the regulator is happy that a vaccine is safe and effective.
Checks continue after approval to make sure there are no further side effects or long-term risks.
Independent experts on the Joint Committee on Vaccination and Immunisation decide how best to use a vaccine and who should get it.
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