In a potentially historic announcement on Monday, Pfizer Inc said that its experimental vaccine was more than 90 per cent effective in preventing Covid-19 based on initial data from a large study.
Pfizer and German partner BioNTech SE are the first drugmakers to show successful data from a large-scale clinical trial of a coronavirus vaccine.
Governments, politicians and scientists have been reacting to the news from around the world - here is a snapshot.
WHO - Could 'fundamentally change' crisis
The world could be in a position to "fundamentally change" the direction of the Covid-19 crisis by March, according to the World Health Organisation (WHO).
Senior WHO official Bruce Aylward said this could be done with a vaccine roll-out to the most vulnerable by the third month of 2021.
Mr Aylward told the WHO's annual ministerial assembly that interim results announced earlier in the day from Pfizer Inc's phase-three vaccine trials were "very positive" but added that more work needs to be done.
United States - 'Such great news'
US President Donald Trump welcomed the experimental vaccine results and the stock market boost it brought, tweeting "SUCH GREAT NEWS!"
Meanwhile, US president-elect Joe Biden greeted the news but warned that widespread vaccination remained months away and reminded of the need to wear masks and maintain social distancing.
"Today’s news is great news, but it doesn't change that fact," Mr Biden said in a statement. "Today's announcement promises the chance to change that next year, but the tasks before us now remain the same."
United Kingdom - 'Made me smile from ear to ear'
Britain expects to have 10 million doses of the vaccine available by the end of the year if regulators approve it, Prime Minister Boris Johnson's spokesman said on Monday.
The spokesman said that Britain had now ordered 40 million doses of the candidate vaccine developed by Pfizer and BioNTech, one of six that Britain has agreed supply deals for.
"This news made me smile from ear to ear," Peter Horby, Professor of Emerging Infectious Diseases at the University of Oxford said.
"It is a relief to see such positive results on this vaccine and bodes well for Covid-19 vaccines in general.
"Of course we need to see more detail and await the final results, and there is a long, long way to go before vaccines will start to make a real difference, but this feels to me like a watershed moment."
Germany - 'A good signal'
Data from Pfizer and its partner BioNTech on their experimental vaccine was encouraging, Germany's health minister said on Monday.
"If this should prove true ... then it would be a good signal because it shows that this vaccine makes a difference," Jens Spahn told a news conference. "This is encouraging."
Russia - 'We welcome progress by foreign colleagues'
Alexander Gintsburg, Director of Moscow's Gamaleya Institute which developed Russia's main vaccine candidate, Sputnik V, said: "We welcome the progress shown by our foreign colleagues in the development of a vaccine against the new coronavirus.
"The more vaccines are available in the world, the better people will be protected."
Italy - 'Caution is still needed'
"Today's news about the anti-Covid vaccine is encouraging," Italian Health Minister Roberto Speranza said.
"But caution is still needed. Scientific research is the real key to overcoming the emergency. In the meantime, we must never forget that the behaviour of each of us is essential to bend the curve."
France - 'We feel energised'
"Congratulations to our Pfizer colleagues. We feel energised to move on and bring more vaccines to the world to fight against this pandemic," Quentin Vivant, Spokesman for French drugmaker Sanofi said.
Canada - 'Light at the end of the tunnel'
Canadian Prime Minister Justin Trudeau said the vaccine announcement was "very encouraging".
"We hope a vaccine is coming early next year. We're seeing a light at the end of the tunnel," he told a news conference.
Canada has a deal with Pfizer for a minimum of 20 million doses of the vaccine.
