LONDON (REUTERS) - Britain expects to have 10 million doses of Pfizer-BioNTech's candidate Covid-19 vaccine available by the end of the year, but Prime Minister Boris Johnson on Monday (Nov 9) said there were several hurdles to go after its positive clinical trial results.

Pfizer said its experimental Covid-19 vaccine was more than 90% effective, a big gain in the fight against a pandemic that has killed more than a million people and battered the world's economy.

Johnson said Britain had now ordered 40 million doses of the candidate vaccine developed by Pfizer Inc and BioNTech, enough to give protection to a third of the population. His spokesman said 10 million doses were due by the end of 2020 pending regulatory approval.

Johnson has said that the prospects of a vaccine are one cause for optimism that the situation will improve by spring, but said new lockdown rules in England still needed to be obeyed even though scientific progress was on the horizon.

"We have cleared one significant hurdle but there are several more to go before we know the vaccine can be used," Johnson said, referring to the need to see safety data and for approval by regulators.

"We absolutely cannot rely on this as a solution. The biggest mistake we could make now would be to slacken our resolve at such a critical moment."

Johnson - who himself was hospitalised earlier this year after being stricken with Covid-19 - said that once the vaccine was approved, Britain would be ready to start using it. The health service woud lead a programme of distribution, with the elderly and frontline workers first in line.

"I am hopeful, but not yet certain, that we could begin to see some vaccine by Christmas," said England's Deputy Chief Medical Office Jonathan Van-Tam.

In all, the government has signed six supply deals for vaccine candidates, including Pfizer/BioNTech's and one developed by Oxford University and AstraZeneca.

John Bell, regius professor of medicine at University of Oxford who sits on the government's vaccine taskforce, told BBC Radio he could not see "any reason now why we shouldn't have a handful of good vaccines available to this disease".

He added that he was "optimistic that given this result, we'll have a reasonably good chance of getting a good result as well," referring to Oxford's own late-stage trial results he said would come in "weeks not months".

Asked if life would be returning to normal by the Spring, Oxford's Bell said: "Yes, yes, yes." "I'm probably the first guy to say that but...I will say that with some confidence."