The UK has committed to buying a new potential coronavirus vaccine for 2.5 million people.
Health Secretary Matt Hancock made the announcement hours after Moderna said its drug was 94.5 per cent successful, leading him to say: "We can see the candle of hope."
To vaccinate 2.5 million people, 5 million doses of the Moderna drug are needed.
The UK also has previous purchase agreements with Astrazeneca and Pfizer, for around 100 million doses of other trial vaccines.
Mr Hancock said progress with the Moderna shot was an "encouraging step forward".
"Great advances of medical science are coming to the rescue. While there is much uncertainty, we can see the candle of hope and we must do all we can to nurture its flame,” he said.
"But we're not there yet; until the science can make us safe we must remain vigilant and keep following the rules that we know can keep this virus under control."
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Two women wearing face masks walk down the street outside Hull Royal Infirmary in Hull, England. Hull recorded 726.8 new cases per 100,000 people in the week to November 7, nearly triple the national rate for England that same week. Getty Images -
An ambulance is parked outside the main entrance of Hull Royal Infirmary. Getty Images -
General view of Hull Royal Infirmary. Getty Images -
A man wearing a face shield walks through Hull city centre. Getty Images -
A woman pushes a child past a Covid-19 information sign in Hull city centre. Getty Images -
A couple wearing face masks walk through Hull city centre. Getty Images -
A Covid 19 sign is displayed on an empty street in Hull. Getty Images -
NHS information signs are displayed in Hull city centre. Getty Images -
A man reacts as pigeons take flight around him in Hull city centre. Getty Images
Prof Jonathan Van-Tam, England's deputy chief medical officer, explained the significance with a football analogy.
"It's brilliant news, absolutely brilliant. It's the second penalty now - that's also gone into the back of the net, so we're starting to feel in a better position."
Moderna announced its experimental vaccine was 94.5 per cent effective in preventing the virus, the second US company to report results that far exceeded previous expectations.
A week ago, competitor Pfizer announced that its own Covid-19 vaccine appeared more than 90 per cent effective.
Both the Pfizer and Moderna shots rely on a technology called messenger RNA that has never before been used to build an approved vaccine.
A key advantage of Moderna's vaccine is that, unlike Pfizer’s, it does not need ultra-cold storage, making it far easier to distribute and store.
Moderna, part of the US government's Operation Warp Speed programme, expects to produce about 20 million doses of the vaccine for the US this year.
England, meanwhile, has entered a second national lockdown and UK Prime Minister Boris Johnson is also self-isolating after coming into contact with a symptom of Covid-19.
Mr Johnson needed intensive hospital treatment earlier this year when he was struck down by cornoavirus.
