What is herd immunity and will it stop coronavirus in the UK?

The UK's Chief Scientific Adviser has said a degree of herd immunity will help the UK population as Covid-19 spreads.

Sir Patrick Vallance acknowledged there are fears that clamping down too hard on the spread of the virus through tight social distancing measures could see it return in the future.

He said the aim is to "reduce the peak, broaden the peak, not suppress it completely; also, because the vast majority of people get a mild illness, to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission".

In Sir Patrick's opinion, almost two thirds of Britain's population would need to contract coronavirus in order for herd immunity to stave off the disease in future.

Otherwise, he says, this outbreak could become an annual plague on our communities.

So, what exactly is herd immunity?

Herd immunity is when enough people become resistant to a disease - through vaccination or previous exposure - that it can no longer significantly spread among the rest of the population.

With no vaccine available for Covid-19, herd immunity relies on enough people in the population becoming infected to lessen the impact of the disease.

Sir Patrick told Sky News that around 60% of the UK population will need to become infected with coronavirus in order for society to have herd immunity from future outbreaks.

For now, there is no doubt that thousands of people in the UK have already been exposed to Covid-19 and will have built up immunity, and that that will continue as people continue to move around freely.

While this exposure continues, the Government hopes the spread of Covid-19 can still be slowed and reduced through telling people to stay at home for seven days if they are ill.

And at some point, the elderly and those with serious health problems will be "cocooned" from the virus by being told to stay at home as much as possible and reduce social contact, regardless of whether they are ill or not.

Creating a 'ring of immune people' to protect the vulnerable

Professor Beate Kampmann, director of the vaccine centre at the London School of Hygiene and Tropical Medicine, said the Government appears to be carrying out a two-pronged approach.

She said cocooning means that a "ring of immune people" protect the vulnerable people in the middle.

"If you cocoon the elderly and you increase herd immunity, then the spread of the virus in the community in a few months is going to be lower," she said.

Asking people to self-isolate at home for seven days if they are ill is a way of slowing down transmission and "flattening the curve", she said.

But she added that she believes those at most risk of severe illness - such as those with serious underlying health conditions - should already be practising self-isolation.

"The people who are not as vulnerable will be the first to acquire immunity I expect, but that will never be 100%," she added.

"Allowing free spread to nursing homes and so on would be a disaster."

But, not everyone agrees with herd immunity

World Health Organization spokesman Margaret Harris has questioned the UK’s approach.

Dr Harris told BBC Radio 4’s Today: "We don’t know enough about the science of this virus, it hasn’t been in our population for long enough for us to know what it does in immunological terms.

"Every virus functions differently in your body and stimulates a different immunological profile. We can talk theories, but at the moment we are really facing a situation where we have got to look at action."

And she's not alone.

Anthony Costello, professor of international child health and director of the University College London Institute for Global Health, questioned the tactics and argued they looked like they were against the policy set down by the WHO, where he used to be a director.

On Twitter, Prof Costello said: "Doesn't this herd immunity strategy conflict with WHO policy? After the announcement of this being a pandemic, Dr Tedros, Director General WHO, said 'The idea that countries should shift from containment to mitigation is wrong and dangerous'."

Prof Costello said the Government was arguing that allowing a proportion of the population to catch the virus and gain immunity "will block a second peak in several months' time".

But he tweeted a series of questions showing scepticism for the policy, including: "Will it impair efforts to restrict the immediate epidemic, and cause more infections and deaths in the near term? Evidence suggests people shed virus early, and those without symptoms may cause substantial spread..."

He also questioned whether "coronavirus cause strong herd immunity or is it like flu where new strains emerge each year needing repeat vaccines? We have much to learn about Co-V immune responses."

He said there was also an argument to see what happened in China, where the epidemic there has been contained "after 7 weeks of intense national effort".

He added: "Without an all-out national mobilisation plan for social distancing, are the UK government behavioural and nudge strategies really evidence-based to flatten the peak? Or simply based on models?"

Prof Costello suggested that "shouldn't we go all-out to snuff this UK epidemic out, with national mobilisation at all levels, using all possible preventive measures (whether evidence is strong, uncertain or weak) and worry about herd immunity when we have more evidence?"

He continued: "Vaccines are a safer way to develop herd immunity, without the risks associated with the disease itself. Is it ethical to adopt a policy that threatens immediate casualties on the basis of an uncertain future benefit?"

In a separate tweet, Devi Sridhar, professor of global public health at Edinburgh University, said: "Part of my job is speaking truth to power. And the UK govt is (in my view) getting it wrong.

"Other countries have shown speed is crucial. There is a middle path between complete shutdown & carrying on as normal."

Jeremy Rossman, honorary senior lecturer in virology at the University of Kent, added: "A delay strategy when combined with surveillance and containment, as recommended by the WHO, could be very effective in combating the spread of COVID-19.

"Yet if we slow the spread of the virus but are relying on herd immunity to protect the most vulnerable people, we would still need 47 million people to be infected."

However, Martin Hibberd, professor of emerging infectious disease at the London School of Hygiene and Tropical Medicine, said it was unclear how the UK policies will work compared to other European policies "but I suspect they will be similar".

He added: "The Government plan assumes that herd immunity will eventually happen, and from my reading hopes that this occurs before the winter season when the disease might be expected to become more prevalent.

"However, I do worry that making plans that assume such a large proportion of the population will become infected (and hopefully recovered and immune) may not be the very best that we can do.

"Another strategy might be to try to contain longer and perhaps long enough for a therapy to emerge that might allow some kind of treatment."