If or when scientists succeed in making a coronavirus vaccine, there won't be enough to go around.
Research labs and pharmaceutical companies are rewriting the rulebook on the time it takes to develop, test and manufacture an effective vaccine.
Unprecedented steps are being taken to ensure roll-out of the vaccine is global. But there are concerns that the race to get one will be won by the richest countries, at the expense of the most vulnerable.
So who will get it first, how much will it cost and, in a global crisis, how do we make sure nobody gets left behind?
Vaccines to fight infectious diseases usually take years to develop, test and deliver. Even then, their success is not guaranteed.
To date, only one human infectious disease has been totally eradicated - smallpox - and that took 200 years.
The rest - from polio to tetanus, measles, mumps and TB - we live with, or without, thanks to vaccinations.
How soon can we expect a coronavirus vaccine?
Trials involving thousands of people are already underway to see which vaccine can protect against Covid-19, the respiratory disease caused by coronavirus.
A process that usually takes five to 10 years, from research to delivery, is being pared down to months. In the meantime, manufacturing is being scaled up - with investors and manufacturers risking billions of dollars to be ready to produce an effective vaccine.
Russia says trials of its Sputnik-V vaccine have shown signs of an immune response in patients and mass vaccination will start in October. China says it has developed a successful vaccine that is being made available to its military personnel. But concerns have been raised about the speed at which both vaccines have been produced.
Neither are on the World Health Organization's list of vaccines that have reached phase three clinical trials - the stage that involves more widespread testing in humans.
Some of these leading candidates hope to get their vaccine approved by the end of the year - although the WHO has said it does not expect to see widespread vaccinations against Covid-19 until the middle of 2021.
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