In a separate BBC interview, a member of the WHO's delegation to China said that, while Chinese authorities had not given them all raw data, they had seen a lot of information and discussed analysis of the first cases.
Nearly 1.2 million letters were due to have landed on the doorsteps of over-65s and the clinically vulnerable by Saturday asking people to log on to the national booking service, NHS England said.
A further 1.2 million are due to arrive this week.
GP-led vaccination sites will focus initially on the clinically vulnerable because of the relationship between general practice and those with long-term conditions.
People aged 70 or over in England have already been asked to contact the NHS to arrange their jab. "And we have to continue getting vaccinated as soon as that vaccine is available to us".
"The solutions to the challenges we face ... lie in the discussions we have with our friends and partners around the world", Johnson said in a statement released late Saturday.
Prof. Stephen Powis, NHS medical director, said the UK's vaccination programme was "off to a strong start".
"Don't hesitate at all".
He added "quantum leaps in science" had helped produce the Covid-19 vaccines needed to end the pandemic, and that world governments now had a responsibility to work together to distribute them.
This is the equivalent of 22.4 percent of the total population of Scotland, and 27.6 percent of people aged 18 and over.
"Once all nine priority groups have been protected by the end of April, there is no justification for any legislative restrictions to remain", the letter organised by the Covid Recovery Group said. He has said he wants schools to reopen on March 8.
When Johnson announced the lockdown, he said the government would review the measures in mid-February based on their success in controlling the pandemic and progress in the vaccination effort.
The Ontario government reported that all long-term care residents across the province have been given an opportunity for their first dose of COVID-19 vaccine.
From 4 January people began receiving the Oxford University/AstraZeneca vaccine.
The government set Monday as the deadline to deliver a first vaccine dose to everyone in its top four priority segments, including all those aged 70 and over, a goal it looks likely to reach.
Britain, which has been the hardest hit in Europe by the pandemic, registering almost 117,000 deaths within 28 days of a positive Covid-19 test, has led the world in promptly approving the use of several vaccines.