Boris Johnson has urged the public to stick to Covid-19 rules despite pharmaceutical giant Pfizer announcing it has made a coronavirus vaccine breakthrough.

The Prime Minister said the Pfizer vaccine news was a sign the “scientific cavalry” was on its way, but stressed it was “very, very early days”.

He told a Downing Street press conference that the vaccine had cleared a “significant hurdle” but there were more to cross before it could be used.

He said: “The Pfizer/BioNTech Vaccine has been tested on over 40,000 volunteers and interim results suggest it is proving 90% effective at protecting people against the virus.

“But we haven’t yet seen the full safety data, and these findings also need to be peer-reviewed.

“So we have cleared one significant hurdle but there are several more to go before we know the vaccine can be used.”

The UK Government has ordered 40 million doses of the Pfizer vaccine – enough for about a third of the UK population.

“If the Pfizer vaccine passes all the rigorous safety checks and is proven to be effective then we will begin a UK-wide NHS led programme of vaccine distribution,” the Prime Minister said.

But he added: “We have talked for a long time, or I have, about the distant bugle of the scientific cavalry coming over the brow of the hill

“And tonight that toot of the bugle is louder, but it is still some way off.

“And 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.”

The Prime Minister said the reproductive rate of the virus – the R value – was still above 1 and death numbers were rising.

“Irrespective of whether there is a vaccine on the way or not we must continue to do everything possible right now to bring the R down,” he said.

Earlier, Pfizer and its vaccine partner BioNTech said interim data showed their jab was more than 90% effective in preventing Covid-19 infection.

Scientists around the globe welcomed the news, with many saying it was a significant breakthrough in the fight against Covid-19.

Peter Horby, professor of emerging infectious diseases and global health at the University of Oxford, said: “This news made me smile from ear to ear.”

England’s chief medical officer, Professor Chris Whitty, said the findings demonstrated “the power of science against Covid”, adding: “We must see the final safety and efficacy data, but it is very encouraging.

“It is essential we continue to suppress Covid but it is a reason for optimism for 2021.”

The FTSE 100 jumped more than 5.5% following the news, adding £82 billion to the value of its shares in the market’s best day since March.

The vaccine has been tested on 43,500 people in six countries and no safety concerns have been raised.

Pfizer and BioNTech plan to apply to the US Food and Drug Administration – the US medicines regulator – by the end of the month for emergency approval to use the vaccine.

About a dozen Covid-19 vaccines around the world are currently in the final stages of testing but Pfizer’s is the first to report any results.

Deputy chief medical officer Professor Jonathan Van-Tam told the Downing Street press conference: “This is really a very important scientific breakthrough – I’m certain of that.”

He described it as a “huge milestone” and said it was good news for other future vaccines, but said more data would need to be collected on whether it stops transmission of the virus.

He also urged caution, saying “we don’t yet know” when life can get back to normal or when coronavirus restrictions can start to be lifted.

“Frankly, we’re in the middle of the second wave, and I don’t see the vaccine making any difference for the wave we are now in,” he said.

“I’m hopeful that it may prevent future waves, but this one we have to battle through to the end without a vaccine.”

However, he said he was “hopeful” the first coronavirus vaccine could be seen by Christmas.

He said if and when vaccines are approved “we might be able to look towards the end of spring for a much better horizon than we have in front of us right now.”

Prof Horby said the news “bodes well for Covid-19 vaccines in general”, adding: “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.”

Pfizer and BioNTech expect to be able to produce up to 50 million vaccine doses globally in 2020 and up to 1.3 billion doses in 2021.

The data from the full trial will be submitted for scientific peer-review.

The figures presented so far are based on the first 94 volunteers to develop Covid-19 and the overall effectiveness of the vaccine may change when the full results are released.

Some experts have urged caution over the results, saying there are many unanswered questions including how effective the jab is for the elderly, how long it may provide protection for and whether it can prevent people passing the virus on to others.

The vaccine could also pose some logistical issues if it is approved for widespread use because it needs to be stored at ultra low temperatures of around -70C or -80C.

David Nabarro, co-director of Imperial College London’s Institute of Global Health Innovation, said “any promising news about a vaccine is great news”.

But he added: “Even if a vaccine arrives in the near future, we’ve got many months of still dealing with the virus as a constant threat… We’ve got to make certain that we continue to do all that is necessary to solve the virus causing major problems.

“The vaccine will help, but it’s not going to be a complete game changer.”

But Sir John Bell, regius professor of medicine at Oxford University and a member of the Government’s vaccine taskforce, said: “Ninety percent is an amazing level of efficacy.

“It rolls the pitch for other vaccines because I can’t see any reason now why we shouldn’t have a handful of good vaccines.”

Asked if people could look forward to a return to normal life by the spring, Sir John told the BBC: “Yes, yes, yes, yes. I am probably the first guy to say that but I will say that with some confidence.”