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Pfizer and its partner BioNTech are on Friday filing for emergency authorisation in the US of their Covid-19 vaccine.
It will be the job of the US Food and Drug Administration (FDA) to decide if the vaccine is safe to roll out.
It is not clear how long the FDA will take to study the data. However, the US government expects to approve the vaccine in the first half of December.
Data from an advanced trial showed the vaccine protects 94% of adults over 65.
The trial involved 41,000 people worldwide. Half were given the vaccine, and half a placebo.
The UK has pre-ordered 40 million doses and should get 10 million by the end of the year.
So when could we expect vaccinations?
If FDA authorisation does come in the first half of next month, Pfizer and BioNTech will "be ready to distribute the vaccine candidate within hours", the two companies said.
This would be remarkably quick for vaccine development - within 10 months of detailing the genetic code. The average wait for approval in the US is nearer eight years.
Pfizer CEO Albert Bourla said on Thursday that the filing for emergency use was a "milestone in our journey to deliver a Covid-19 vaccine to the world".
Initial doses would be scarce, though, and the Centers for Disease Control and Prevention (CDCP) will decide who is first in line.
European Commission president Ursula von der Leyen said the EU could move quickly too - by the end of the year.
But there are caveats. Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said both the FDA and the European Medicines Agency (EMA) would "conduct a very careful evaluation".
And BBC health correspondent Naomi Grimley says this vaccine is still a long way off widespread use, not least because it adopts an experimental technology that has never been approved before.
How effective is it?
Last week, Pfizer and BioNTech published preliminary data suggesting the vaccine offered 90% protection against Covid-19 and said there were no safety concerns.
Subsequent data released on Wednesday suggested 95% effectiveness.
This effectiveness was also consistent across age groups - essential given the vulnerability of the elderly - as well as ethnicities and gender.
The vaccine also had only mild-to-moderate and short-lived side-effects.
It uses an experimental approach, called mRNA, which involves injecting part of the virus's genetic code into the body to train the immune system.
Antibodies and T-cells are then made by the body to fight the coronavirus.
What's the Covid situation in the US?
The US this week passed 250,000 deaths in the coronavirus outbreak, by far the largest number in the world.
Its confirmed cases since the pandemic began stand at 11.7 million, according to Johns Hopkins University research, again a global first.
Cases have also been soaring over the past week, reaching record daily highs.
image copyrightReutersimage captionA nurse and patient in Burbank, California. The health system is strained in many areas of the US
The health system in many areas is struggling to cope, with makeshift wards being created.
The CDCP has issued a "strong recommendation" that Americans refrain from travelling during the Thanksgiving holiday.
In California from Saturday there will be a 22:00 to 05:00 stay-at-home curfew for the vast majority of the population.
Ohio, Minnesota and New York are among other states imposing tough restrictions.
The transition of administrations has also not helped, with President-elect Joe Biden complaining of a lack of co-operation from Donald Trump's government.
What other vaccines are being developed?
Data on a vaccine made by US company Moderna also suggests nearly 95% protection. It too uses the mRNA approach.
In terms of development it is believed to be not far behind the Pfizer/BioNTech vaccine.
An Oxford University/AstraZeneca vaccine is still undergoing trials but has shown promising results, with a strong immune response in the elderly. The UK has ordered 100 million doses.
It is made from a a weakened version of a common cold virus from chimpanzees, that has been modified to not grow in humans.
Other vaccines are in late testing stages in China and Russia.