Britain could already have herd immunity against Covid-19 because so many people have had similar illnesses in the past, a study claims.
Experts have noticed the infection looks extremely similar to other, milder strains of coronaviruses which cause coughs and colds and circulate regularly.
Brits who have had these in the past may have some level of ‘cross-protection’, they suggest, which means they aren’t seriously harmed by Covid-19.
While it remains unlikely that people will be totally protected from any infection at all, ‘background’ immunity could make their illness less severe and death less likely.
Theories that even exposure to common colds may protect people from the coronavirus have been floating around for months and raise hopes for a milder second wave.
Combined with the fact millions of people have been infected in the pandemic’s first wave, it may mean the UK is already protected against another deadly surge.
The concept of herd immunity – in which so many people are immune to a virus that it cannot spread – is controversial because there is no scientific proof that people who have had Covid-19 once can’t get it again.
Scientists have claimed, however, that if immunity does develop, the proportion of people who need to have had it could be as low as 20 or even 10 per cent.
And Britain may already be reaching this level, the Oxford University paper said, adding: ‘[Immunity] measures of 10-20 per cent are entirely compatible with local levels of immunity having approached or even exceeded the [herd immunity threshold], in which case the risk and scale of resurgence is lower than currently perceived.’
Scientists say if a vaccine was developed it would need 60-70 per cent coverage to work — but this threshold could be significantly lower for natural immunity because the most at-risk people will always be the first to get exposed to the virus and, if it can’t infect them, it can’t spread through them to the less at-risk groups
A study by Oxford University said the threshold needed to achieve herd immunity could be lower than expected – scientists had thought it would be around 60 per cent if a vaccine was used – because coronaviruses are common.
There are four other types of coronavirus known to infect humans regularly, which are named NL63, 229E, OC43, and HKU1.
The fifth, known as SARS-CoV-2, is the one that causes Covid-19.
If people have had these in the past, their bodies may have developed some immunity to coronaviruses, the Oxford researchers suggest.
Professor Suneptra Gupta and colleagues wrote: ‘It is widely believed that the herd immunity threshold (HIT) required to prevent a resurgence of SARS-CoV-2 is in excess of 50 per cent.
‘Here, we demonstrate HIT may be greatly reduced if a fraction of the population is unable to transmit the virus due to innate resistance or cross-protection from exposure to seasonal coronaviruses…
‘Significant reductions in expected mortality can also be observed in settings where a fraction of the population is resistant to infection.
‘These results help to explain the large degree of regional variation observed in seroprevalence [how many people have signs of immunity] and cumulative deaths and suggest that sufficient herd-immunity may already be in place to substantially mitigate a potential second wave.’
The way cross-protection might develop lies in the fact that coronaviruses all have similar structures – that is, they have spike-shaped proteins on the outside.
These spikes may look similar to the body’s immune system and be recognised as a threat even if someone has not been infected with that particular one before.
When the body recognises a protein as a danger it can stoke the immune system into life and immediately send white blood cells and antibodies to destroy the viruses, thereby either preventing illness or making it less severe.
The body stores memories of how to fight viruses it has seen in the past and, if it encounters one that looks a lot like another one it has attacked, it may attack that more quickly than usual, too.
WHAT IS HERD IMMUNITY?
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 – because they’ve already had it or have been vaccinated – 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.
Immune cells are highly specific and only attack the bugs they are designed to, but if coronaviruses are extremely similar there is a chance that immunity developed to one virus may be compatible with another.
While this might not stop infection completely, the fast immune response could make the illness less severe and make it more likely that people will survive.
Research from scientists in Germany last month found that 81 per cent of people who had never even had the coronavirus produced some kind of immune response to it – which they put down to infection with common colds.
Researchers at the University Hospital Tübingen, who studied the immune reactions of 365 people, wrote: ‘Similarity to common cold human coronaviruses provided a functional basis for… immunity in SARS-CoV-2 infection’.
And Professor John Bell, another researcher at Oxford, recently said a significant number of people may have ‘background immunity’ to Covid-19.
He explained to MPs on the Science and Technology Committee that people were showing signs of a type of immunity called T cell immunity – T cells are ones that trigger the production of antibodies, which fight viruses.
Professor Bell said: ‘What seems clear is you do have cross-reaction from T-cells that are activated by standard endemic coronaviruses,’ The Telegraph reported.
‘I think they are present in quite a significant number of people.
‘So there is probably background T-cell immunity in people before they see the coronavirus, and that may be relevant that many people get a pretty asymptomatic disease.’
The Oxford team’s latest study – which did not involve Professor Bell – has not been published in a journal but on the website medRxiv without being reviewed by independent scientists.
In Britain’s first wave of the coronavirus pandemic, there have been almost 300,000 confirmed infections and 45,000 deaths.
Separate data suggests there have likely been more than 3.5million infections – most of them untested – and more than 60,000 fatalities.
Britain as a whole is not close to a high level of herd immunity, with Government testing surveys suggesting between five and six per cent of the population have had Covid-19 so far – about three million people.
London, however, has a much higher past infection rate at an estimated 17.5 per cent, so could be approaching a low level of protection.
The NHS is now preparing for a second wave of the disease but experts say they do not expect another one to be quite as devastating.
Future outbreaks will likely be confined to local areas and be able to be controlled with local lockdowns, they suggest.
While scientists around the world are racing to try and create a vaccine for the coronavirus, herd immunity may be vital as a long-term solution to the disease.
Sir Patrick Vallance, the UK’s chief scientific adviser, said yesterday: ‘It’s important to recognise that the chances of having a totally sterilising vaccine, i.e. one that 100 per cent protects you from this, I think, are low.
‘Much more likely that you have a vaccine that reduces the severity of illness and reduces spread a bit. I think that’s the more likely outcome on vaccines.’
Natural herd immunity – if lasting immunity develops from infection – can arise from the virus spreading through a large part of the population.
Immunity from a vaccine was expected to have to include at least 60 per cent of the population to effectively stop the virus from spreading.
CHILDREN ‘MAY BE PROTECTED FROM COVID-19 BECAUSE THEY GET SO MANY COLDS’
Children may be better protected against coronavirus because they get so many colds, some scientists believe.
There are four coronaviruses known to cause coughs and colds, with adults averaging between two to four colds a year.
But children are believed to attract up to 12 colds a year, and scientists say this could provide youngsters with a resistance to the virus that adults lack.
Professor Sir John Bell, a medicine professor at Oxford University, told the House of Lords Science and Technology Select Commitee: ‘How you respond may be due to the state of your existing immunity coronaviruses generally.
‘There is an interesting speculation at the moment, that many people in young or middle age groups may have T-cells that can already see coronaviruses.
‘It may well be able to provide some protection against this pathogen when it arrives.’
Professor Adrian Hayday added: ‘All adults past a certain age – 30 to 35 – eventually have no thymus so their T-cells work by looking at whether they have seen something before, whereas children are very good at seeing things that are unknown.
‘The issue may be that children are able to see this as something fresh.’
Coronaviruses are thought to cause up to 30 per cent of all colds but it is not known specifically how many are caused by the betacoronavirus types, which also cause severe chest infections in the oldest and youngest patients.
But developing it through natural infection, meaning that the people most likely to spread the disease would get it first, may dramatically reduce that threshold.
A recent study claimed it could work to some extent if only one or two out of 10 people have been infected naturally and become immune to the disease.
They said higher estimates worked on the basis that immunity is given to everyone by a vaccine, but in reality the people who first get infected are likely to continue to be the ones most at risk, so if they develop immunity, the less-at-risk will also benefit.
These could include health workers, people who live in cities and those in people facing jobs like drivers, shop workers and schoolchildren and teachers, for example.
Immunity among the most socially active people, scientists say, could protect those who come into contact with fewer others.
The study led by Dr Gabriela Gomes, a mathematician at the Liverpool School of Tropical Medicine and the University of Strathclyde, said: ‘In idealized scenarios of vaccines delivered at random and individuals mixing at random, herd immunity thresholds are given by a simple formula which, in the case of SARS-CoV-2, suggests that 60-70 per cent of the population would need be immunized to halt spread considering estimates of R0 between 2.5 and 3.
‘A crucial caveat in exporting these calculations to immunization by natural infection is that natural infection does not occur at random.
‘Individuals who are more susceptible or more exposed are more prone to be infected and become immune, which lowers the threshold.
‘In our model, the herd immunity threshold declines sharply… and remains below 20 per cent for more variable populations.’
Another study has taken a similar line and suggested herd immunity could develop at around 43 per cent of the population getting infected.
Professor Frank Ball, Professor Tom Britton and Professor Pieter Trapman — three authors of the study from the universities of Nottingham and Stockholm — wrote in the journal Science: ‘Our application to Covid-19 indicates a reduction of herd immunity from 60 per cent… immunization down to 43 per cent in a structured population, but this should be interpreted as an illustration, rather than an exact value or even a best estimate.’
They said that immunity would be stronger in cities, large households and big workplaces.
THE MYSTERY OF COVID-19 IMMUNITY
Scientists still do not know for sure whether people can catch Covid-19 more than once or if they become immune after their first infection.
With some illnesses such as chickenpox, the body can remember exactly how to destroy it and becomes able to fend it off before symptoms start if it gets back into the body.
But it is so far unclear if people who have had coronavirus can get it again.
Tests have shown that many people who recover have antibodies – which can produce future immunity – but it is not known whether there are enough of them.
One doctor, Professor Karol Sikora, said he had found that only 10 per cent of people known to have had Covid-19 actually developed antibodies.
This means it is hard to measure whether they could fight it off immediately if infected again.
Another study, by the University of Melbourne, found that all patients in a group of 41 developed antibodies but, on average, they were only able to fend off 14.1 per cent of viruses if they were exposed a second time.
Research into other similar coronaviruses, which also infect humans but usually only cause mild illnesses, found that people did tend to develop protective immunity but their antibody levels dropped off within months and they could get reinfected again after around six months.
However, antibodies are only one type of substance that can produce immunity.
Others, including white blood cells called T cells and B cells, can also help the body to fight off disease but are more difficult to discover using currently available tests.
The Melbourne study found signs of elevated numbers of coronavirus-specific B cells and T cells in recovered patients, suggesting those types of immunity may be stronger than antibodies.
They called for more research on the subject.
A promising study done on monkeys found that they were unable to catch Covid-19 a second time after recovering from it, which led scientists to believe the same may apply to humans.
The rhesus monkeys were deliberately reinfected by scientists in China to test how their bodies reacted.
Because the coronavirus has only been known to scientists for seven months there has not been enough time to study whether people develop long-term immunity.
But, so far, cases of people getting infected more than once have not been numerous nor convincing.