The UK's chief scientific adviser has revealed that around 40million people could be left to catch the coronavirus as part of a controversial Government plan for the country to develop herd immunity.

Sir Patrick Vallance said around 60 per cent of the population will need to catch the virus to build up a national tolerance strong enough to stop the virus circulating.

Chief medical officer, Professor Chris Whitty, did not say how many people could die during the time this takes to work, but the current global death rate – around three per cent – suggests it could be 1.2million people in a worst-case scenario.

But officials expect it to be considerably lower – Professor Whitty said even one per cent – 400,000 people – would be an overestimate.

The Government is now aiming to control the speed of the UK's epidemic and predicts the worst of it could come in around 12 weeks' time – mid-June – suggesting it could taper off to levels similar to the current situation by September.

It will attempt to slow down the spread by gradually introducing stricter measures, such as potentially banning large gatherings or closing schools in future, as the outbreak speeds up.

Sir Patrick admitted trying to stop the virus completely would be more dangerous in the long run.

He said: 'All of the evidence from previous epidemics suggests that when you do that and then you release it, it all comes back again... this is to make sure we don't end up with a sudden peak again in the winter, which is even larger, which causes even more problems.'

Sir Patrick Vallance said millions fighting off the virus that has killed ten in Britain and almost 5,000 worldwide will 'help' in the long run because it is likely to become an 'annual virus'

Former Tory Minister Rory Stewart has described the concept of herd immunity as ‘eccentric’, having helped tackle the Ebola outbreak in Africa as International Development Secretary in 2019.

He told CNN last night: 'This is a very eccentric policy and I am troubled by it on a number of different levels.

'One of them is if the UK, with all its resources as a major economy is one of the only countries to actually allow this virus to spread quickly that will pose huge strains for the rest of the world system.

WHAT IS HERD IMMUNITY AND WILL IT EVEN WORK? Herd immunity is a situation in which a population of people is protected from a disease because so many of them are unaffected by it that it cannot spread. To cause an outbreak a disease-causing bacteria or virus must have a continuous supply of potential victims who are not immune to it. Immunity is when your body knows exactly how to fight off a certain type of infection because it has encountered it before, either by having the illness in the past or through a vaccine. When a virus or bacteria enters the body the immune system creates substances called antibodies, which are designed to destroy one specific type of bug. When these have been created once, some of them remain in the body and the body also remembers how to make them again. This provides long-term protection, or immunity, against an illness. If nobody is immune to an illness – as was the case at the beginning of the coronavirus outbreak – it can spread like wildfire. However, if, for example, half of people have developed immunity – from a past infection or a vaccine – there are only half as many people the illness can spread to. As more and more people become immune the bug finds it harder and harder to spread until its pool of victims becomes so small it can no longer spread at all. The threshold for herd immunity is different for various illnesses, depending on how contagious they are – for measles, around 95 per cent of people must be vaccinated to it spreading. For polio, which is less contagious, the threshold is about 80-85 per cent, according to the Oxford Vaccine Group. Advertisement

'Secondly this problem is this theory is based on very, very complex modelling and they are putting a lot of faith in the mathematical modelling to be able to land their peak in the summer.

'And thirdly I think they underestimating the impact that I think is going to hit our health system if they allow it to go in this direction.'

Today, when asked about the lock-down measures deployed by Italy and Ireland, Sir Patrick said that it was 'impossible' for a country to attempt to self-isolate its entire population.

He said: 'We want to suppress it, not get rid of it completely, which you can't do anyway, and also allow enough of us who are going to get mild illness to become immune to this to help with the sort of whole population response which would protect everybody.'

Sir Patrick continued: 'We think that this virus is likely to be one that comes back year on year, become like a seasonal virus, and communities will become immune to it, and that's going to be an important part of controlling this longer term.

Controversially the British Government appears to be pursuing a policy of 'herd immunity' - where the majority of the healthy population will get and fight off the virus so that the disease cannot be spread easily any more. The elderly and ill will be 'cocooned' from the outside world to protect them for weeks or even months.

Critics have warned the policy is high risk because there have been multiple reports in China of people getting it twice - and concerns from experts that coronavirus could mutate and infect hundreds of thousands again in the coming years unless there is a vaccine.

He told BBC Radio 4's Today programme that the Government's approach is about flattening the 'broader peak' of the epidemic and developing an immunity among the population.

'What we don't want is everybody to end up getting it in a short period of time so we swamp and overwhelm NHS services - that's the flattening of the peak,' he said.

'You can't stop it, so you should end up with a broader peak during which time you'd anticipate that more people would get immunity to this. That in itself becomes a protective part of this process.

'This is quite likely, I think, to become an annual virus, an annual seasonal infection.'

The UK now has 798 confirmed cases of the coronavirus, 11 people have died and more than 31,000 people have been tested (Figures accurate on Friday March 13)

The official number of coronavirus cases in the UK yesterday jumped by 134 to 596 with 10 deaths - but the Government's chief scientific adviser Sir Patrick said yesterday the true number was probably up to 10,000 cases and growing rapidly.

But Mr Johnson has still held off drastic measures enforced across Europe such as banning mass gatherings and closing schools, telling Britons that the most important measure remains to wash hands for 20 seconds - and told people to stay at home for seven days if they have a new persistent cough or a high temperature. A Trump-style ban on flights to and from Europe was also dismissed by the Government yesterday.

Asked why the UK has delayed shutting down schools, Sir Patrick said a closure now would see educational establishments shut down for 'many months'.

He said it is not yet clear if any children have transmitted the illness and that most experience only mild Covid-19 symptoms.

'Because of the nature of this disease and the way it spreads and its duration, we would have to close schools for many months, not just a few weeks,' he told LBC.

'Schools would need to be shut for a very prolonged period.

'Children, of course, aren't going to separate for long periods, they are going to go and do other things together.

'And they may go and stay with grandma, one of those in the most vulnerable group.'

He said the Government had issued new advice on dealing with coronavirus that would have a 'big impact' on the public.

He told LBC radio: 'This isn't an epidemic that is going to last a couple of weeks, it is going to go on for months.

'We need to make sure we do the right thing at the right stage to have the biggest impact.

'And the measures that were announced yesterday, which were that anybody with mild symptoms, which are a fever, a raised temperature or a cough, should stay at home. That's not a trivial change.

'That is going to have a big impact of many people staying at home for a week.

'It is one of the things that can have the biggest impact in terms of reducing the transmissability and ability of the virus to spread quickly across the population.'