Donald Trump has defied scientists by claiming a vaccine for coronavirus will be ready by the end of this year.
His prediction flies in the face of the views of many scientists, who say it will take between a year and 18 months to develop a vaccine since the emergence of COVID-19.
The US president’s forecast came as Boris Johnson prepares to urge world leaders to work together in the race to find a vaccine, saying it is “not a competition between countries”.
In the UK, human trials of a vaccine being developed by Oxford University began last month. Pharmaceutical giant AstraZeneca, which has agreed to manufacture and distribute Oxford’s vaccine if it is effective, says it could be available for limited use by the end of the year.
However, most scientists predict a vaccine will be available in mid-2021 by the earliest.
In a virtual town hall with Fox News at the Lincoln Memorial in Washington on Sunday, Trump said: “We are very confident we are going to have a vaccine by the end of the year.”
He warned that up to 100,000 Americans could lose their lives in the pandemic, having predicted last month the figure would be 60,000.
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US health experts have repeatedly said a vaccine is probably a year to 18 months away, although the White House coronavirus task force's Dr Anthony Fauci said last month that one could conceivably be in wide distribution as early as January.
On Sunday, US secretary of state Mike Pompeo said there was “enormous evidence” that the coronavirus pandemic originated in a laboratory in the Chinese city of Wuhan, but did not present anything to support his claim.
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On Monday, Johnson will urge nations to “pull together” and share their expertise in order to find a successful vaccine.
The UK has pledged £388m in aid funding for research into vaccines, tests and treatments.
European leaders have pledged more than £6.6bn in funding to support the global response to the pandemic.
The prime minister is expected to tell Monday’s online pledging conference: "We are in this together and together we will prevail.
"To win this battle, we must work together to build an impregnable shield around all our people and that can only be achieved by developing and mass producing a vaccine.
"The more we pull together and share our expertise, the faster our scientists will succeed.
"The race to discover the vaccine to defeat this virus is not a competition between countries but the most urgent shared endeavour of our lifetimes.
"It's humanity against the virus – we are in this together and together we will prevail."
International development secretary Anne-Marie Trevelyan said: "It is only by working together that we will prevent future waves of infection and end this pandemic as quickly as possible.
"By strengthening developing countries' health systems and working to find a vaccine, the UK is playing its part in stopping the global spread of coronavirus to save lives everywhere and protect our NHS."