'Obviously I'm hopeful — I've got my fingers crossed — but to say I'm 100 per cent confident that we'll get a vaccine this year, or indeed next year is, alas, just an exaggeration — we're not there yet.
'If you talk to the scientists they think the sheer weight of international effort is going to produce something. They're pretty confident that we'll get some sort of treatments some sort of vaccines that will really make a difference.
'But can I tell you that I'm 100 per cent confident? No.
'That's why we've got to continue with our current approach - maintaining the social distancing measures... we've got to continue to do all the sensible things; washing our hands. All those basic things.'
Mr Johnson added: 'It may be that the vaccine is going to come riding over the hill like the cavalry but we just can't count on it right now.'
It is not clear exactly how much the Department of Health has paid for the vaccines, but it announced in May a £131million fund to develop vaccine-making facilities.
And it has given Valneva — the French company supplying 90million doses — an undisclosed amount of money to expand its factory in Livingston, Scotland.
Kate Bingham, chair of the UK's Vaccine Taskforce, revealed she was still 'hopeful' it would be ready by the end of 2020 but admitted that academics are unlikely to get enough data to prove it works until the end of the year.
Ms Bingham, who is a high-profile health technology investor and has a degree in biochemistry, explained on BBC Radio 4 that the deals with BioNTech and Valneva was part of a spread-betting approach to make sure the UK has stocks of the working vaccine if one is found.
She said: 'The announcements show that the UK is on the forefront of global efforts to source and develop vaccines and we are doing so across a range of different technologies with a range of different companies around the world.
'It's important because we have no vaccines against any coronavirus, so what we're doing is identifying the most promising vaccines across the different types of vaccine so that we can be sure that we do have a vaccine, if one of those proves to be safe and effective...
'We just need to wait and see what the clinical trials tell us but I think again it's important to recognise that it's unlikely to be a single vaccine for everybody. We may well need different vaccines for different groups of people.'
Ms Bingham said that although it would be ideal to have a 'sterilising' vaccine that would completely stop the virus, that could be too ambitious.
'I think we may need to start off with and recognise is that the vaccines may just stop the level of mortality so we may just find a vaccine that helps reduce the symptoms rather than provides a full sterilising immunity
'If I look with my rose-tinted specs, I think they [Oxford University] could get data by the end of the year. They're running studies in the UK and in Brazil and South Africa.
'What I would hope we would see is strong safety data in all of the relevant populations who are most at risk which will come out in the UK trial. And I would hope to see efficacy signals which means data that shows the vaccine prevent infection or reduce the symptoms coming out of those two studies.
'It all depends on how much infection we see. The regulators will need to look at the data and every aspect of the vaccine and then they will take a view on whether this is a vaccine that should be licensed for use more broadly.'
Ms Bingham added that she was hopeful getting a vaccine by the end of the year was 'still possible' but such low numbers of coronavirus cases make it difficult to test one in Britain.
Valneva, the French company developing a vaccine that the UK has bought 60m doses, it creating a jab based on injecting people with dead versions of the coronavirus.
This is called an inactivated whole virus vaccine and works by injecting the virus itself but versions that have been damaged in a lab so that they cannot infect human cells. They can be damaged using heat, chemicals or radiation.
Even though the viruses are inactivated the body still recognises them as threats and mounts and immune response against them which can develop immunity.
The BioNTech vaccine, on the other hand, is one which injects RNA - genetic material - which codes the body to produce proteins that look like the spike proteins that would be found on the outside of the real coronavirus.
AstraZeneca, which is working in partnership with Oxford University, is already manufacturing a vaccine in the hope that it will work. The UK Government agreed to buy them without seeing results of clinical trials, which have not been completed yet.
But the team from Oxford, and another working on a different jab made by the American pharmaceutical company Moderna, have both revealed people in their studies are showing signs of immunity.
Each has been working on separate experimental jabs for months to try to protect millions of people from catching the coronavirus in future.
People being given the Oxford vaccine have been developing antibodies and white blood cells called T cells which will help their bodies fight off the virus if they get infected, it is reported.
And experts at Moderna, based in Cambridge, Massachusetts, said participants in their trial - of a different type of vaccine - all successfully developed antibodies.
The vaccines work by tricking the body into thinking it's infected with Covid-19 and causing it to produce immune substances that have the ability to destroy it.
While early research focused on antibodies, scientists are increasingly turning to a type of immunity called T cell immunity — which is controlled by white blood cells — which has shown signs of promise.
One source on the Oxford project told ITV News: 'An important point to keep in mind is that there are two dimensions to the immune response: antibodies and T-cells.
'Everybody is focused on antibodies but there is a growing body of evidence suggesting that the T-cells response is important in the defence against coronavirus.'
Oxford's phase 3 trial is involving around 8,000 people across the UK and also up to 6,000 people in Brazil and South Africa, where the jab may be easier to test because more people are infected with the coronavirus.
The vaccine is being manufactured by AstraZeneca, based in Cambridge, England, and millions of doses have already been ordered by Number 10 in the hope that it will work.
In the early stages researchers will want to see that the jab is safe for people to take and doesn't cause serious side effects, and also that it seems to be stimulating the immune system in the right way.
If it passes these checkpoints researchers are expected to move on to even larger tests with thousands more members of the public.
In its own tests Moderna, the US pharmaceutical company, reports that its vaccine has passed these early milestones and now plans to move on to bigger trials.
Researchers at the company last night announced that all 45 volunteers in its early phase had developed immune responses after being given the vaccine.
They also found the jab — one of the front-runners in the global coronavirus vaccine race — was safe and no participants suffered any serious side effects.
But more than half reported mild or moderate reactions such as fatigue, headache, chills, muscle aches or pain at the injection site.
Scientists said side effects were a 'small price to pay' for protection against Covid-19.
Dr Anthony Fauci, the US government's top infectious disease expert, said: 'No matter how you slice this, this is good news.'
Moderna was the first US company to start human testing of a vaccine for the novel coronavirus on March 16, 66 days after the genetic sequence of the pathogen was released by China.
It's now preparing to start a 30,000-person trial later this month to prove the vaccine really is strong enough to protect against the coronavirus.
The share price of the company surged on the news as it stoked hopes of progress in the global battle against Covid-19.
The US federal government is supporting Moderna's vaccine with nearly half a billion dollars in funding.
Speaking in a television interview this morning the Prime Minister said he has his 'fingers crossed' but isn't 100 per cent confident that a coronavirus vaccine will be found
Its vaccine, known as mRNA-1273, works using ribonucleic acid (RNA), which is a chemical messenger in human bodies that contains instructions for making proteins.
The jab introduces RNA which programmes the body to make proteins that look like those found on the surface of the coronavirus, which triggers the immune system to react because it recognises those proteins as a danger - even though they aren't actually attached to a virus and can't cause any harm.
This then trains the body to recognise these as a foreign invader, and mount an immune response against it.
The results, published in the prestigious New England Journal of Medicine, involved three groups of 15 volunteers aged 18-55.
The groups tested 25, 100 or 250 micrograms of the vaccine. Everyone got two doses, 28 days apart.
The team reported a dose-dependent effect, whereby the participants grew a larger antibody response the higher their vaccination dose was.
In comparison, the University of Oxford team's jab works by injecting a damaged part of the real coronavirus that has been attached to another, harmless, virus.
It's a type of immunisation known as a recombinant viral vector vaccine.
Researchers place genetic material from the coronavirus into another virus that's been modified. They will then inject the virus into a human, hoping to produce an immune response against SARS-CoV-2.
The carrier virus, weakened by genetic engineering so it doesn't make people ill, is a type of virus called an adenovirus, the same as those which cause common colds, that has been taken from chimpanzees.
If the vaccines can successfully mimic the spikes that are found on the outside of Covid-19 inside a person's bloodstream, and stimulate the immune system to create special antibodies to attack it, this could train the body to destroy the real coronavirus if they get infected with it in future.
One of the other leading vaccine candidates, being made by Imperial College London, works in a similar way to Moderna's, instead of Oxford's.
It will try to deliver genetic material (RNA) from the coronavirus which programs cells inside the patient's body to recreate the spike proteins. It will transport the RNA inside liquid droplets injected into the bloodstream.
The UK has ordered 30million doses of a vaccine created by the German company BioNTech
Oxford's vaccine could be developed so rapidly by Professor Sarah Gilbert, a vaccinology expert, and her team because they already had a base vaccine for similar coronaviruses.
Professor Gilbert said earlier this month that protection from a jab against coronavirus should last for several years at least.
She told MPs she was optimistic that a vaccine would provide 'a good duration of immunity'.
Professor Gilbert is the world-renowned expert leading an Oxford University team devising a vaccine, so her claim could help to dispel the fears over how long protection against Covid-19 might last.
Concerns had been raised after those with other types of coronavirus – which are less dangerous and cause the common cold – were able, in tests, to be reinfected within a year.
But Professor Gilbert told the Commons science and technology committee there may be a better result from a vaccine than the natural immunity acquired when individuals simply recover from a virus.
She said: 'Vaccines have a different way of engaging with the immune system, and we follow people in our studies using the same type of technology to make the vaccines for several years, and we still see strong immune responses.
'It's something we have to test and follow over time – we can't know until we actually have the data.
'But we're optimistic based on earlier studies that we will see a good duration of immunity, for several years at least, and probably better than naturally-acquired immunity.'
The key question is whether the vaccine will protect them from becoming infected, or simply make them less ill. It may also work less well in older people because their immune systems are weaker.
Sir John Bell, regius professor of medicine at Oxford University, also gave evidence to the committee, warning that the UK must 'prepare for the worst' this winter, rather than rely on the development of a vaccine.
Scientists at Moderna in Cambridge, Massachusetts, are developing a vaccine which uses RNA to force the body to produce proteins that look like the coronavirus and spur the immune system into action
Experts in an Oxford Vaccine Group lab in England have developed a vaccine which injects a damaged and harmless section of the coronavirus in order to provoke the immune system
But he said he has now seen tests for coronavirus of a good standard which can produce a result in a 'few minutes'.
Sir John said: 'That would be transformative because we could all test ourselves regularly and test our kids after they've been off to a rave and all that stuff.'
He also urged Britons to have the flu jab to 'avoid pandemonium in A&E departments'.
Trials of a potential antibody treatment that could protect older people from coronavirus have also started.
Instead of a traditional vaccine the proposed treatment would see patients given a three-minute infusion of antibodies to the virus that could provide protection for up to six months.
For people whose immune systems do not respond to a vaccine, including those taking immunosuppressant drugs or undergoing chemotherapy, it could provide alternative way of developing resistance to the virus.
Older people also have less of a response to vaccines so the antibody infusion could help give extra protection for older people who are more at risk from coronavirus, reported The Times.
Pharmaceutical firm AstraZeneca are trialling the treatment and the drug maker is also working with Oxford University on a potential vaccine.
As well as preventing people catching the disease antibody therapy can help people who have caught it recover more quickly.
Sir Mene Pangalos, who heads the company's research into treating respiratory diseases told The Times: 'There's a population who are elderly that [may not] get a particularly good immune response to the vaccine,
'In those instances you might want to prophylactically treat those patients with an antibody to give them additional protection.'
It is not yet clear if the treatment will work and the first human trial will only have around 30 participants.
If no safety issues arise larger scale testing could begin in the autumn.
Sir Mene added: 'We're going to do this as fast as we can. Obviously we've got to show that you're safe but antibodies are well known entities - it should be safe.
The trial comes following initial research at Vanderbilt University in the United States which looked into monoclonal antibodies, which can imitate the antibodies created by the body after being infected by coronavirus.
However the antibody therapy is not expected to be an alternative to a vaccine as it will cost a much while not providing protection for as long a period of time.
The quest for a coronavirus vaccine took a strange turn last week when Russian hackers were accused of trying to steal data from the UK, US and Canada.
Dominic Grieve, former chair of the Intelligence and Security Committee, who prepared a report on Moscow's interference in the UK told The Telegraph: 'The Russians are masters at disinformation, and what they say cannot ever be taken at face value.
'I have no reason to think the UK Government is misleading the public and every reason to suppose that our security services have been categorically professional in tracking down where this hack came from.'
The Russian government has denied the allegations that it hired hackers to try and get information about Covid-19 vaccines and treatments.
Kirill Dmitriev, head of the Russian Direct Investment Fund (RDIF), said in an interview on Friday that Moscow did not need to steal secrets as it already had a deal with AstraZeneca - a British company - to manufacture its vaccine in Russia.
Dmitriev said: 'AstraZeneca already has an agreement.... with R-Pharm (an RDIF portfolio company) on the complete localisation and production of the Oxford vaccine in Russia'.
Alexey Repik, R-Pharm's board chairman, said on Friday his company had signed the deal.
'There's nothing that needs to be stolen,' Dmitriev, who is involved in coordinating Russia's own pursuit of a vaccine, told Reuters. 'It's all going to be given to Russia.'