Sabrina Barr, The Independent

The global spread of the coronavirus has left many people wondering when and how the outbreak may subside. In March it was claimed that the British government was hoping to reduce the impact of the virus by allowing it to “pass through the entire population so that we acquire herd immunity”.

Although Matt Hancock later denied this was the case, a spokesperson for the Department of Health and Social Care said that “herd immunity is a natural by-product of an epidemic”.

In a study published in medical journal The BMJ on 3 September, scientists claimed that tests currently being used to ascertain levels of antibodies among members of the public who have had COVID-19 may underestimate the number of people who have had the virus, as some people who have experienced mild symptoms may not test positive.

This could in turn have “major implications for epidemiological modelling of disease transmission and herd immunity”, they said. But what exactly is herd immunity and is it a possibility amid the coronavirus outbreak?

What is herd immunity? When enough people in a community are vaccinated against a disease, this can make it more difficult for the disease to spread to susceptible individuals who have not yet been or cannot be vaccinated. This, the NHS outlines, is called “herd immunity”.

The Vaccine Knowledge Project at Oxford University explains in greater detail, using the analogy of a person being infected by measles.

“If someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone, and it will quickly disappear again,” the organisation states. “This is called ‘herd immunity’, ‘community immunity’ or ‘herd protection’, and it gives protection to vulnerable people such as newborn babies, elderly people and those who are too sick to be vaccinated”.

However, the organisation stresses that herd immunity “only works” if the majority of a population have been vaccinated against a condition, adding that it “does not protect against all vaccine-preventable diseases”.

“Unlike vaccination, herd immunity does not give a high level of individual protection, and so it is not a good alternative to getting vaccinated,” the Vaccine Knowledge Project says.

Professor Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh, told The Independent that the concept of herd immunity is “the basis of all vaccination programmes”.

However, it can also occur naturally, he explained, stating: “If you’ve been exposed to any infection, enough people have already been exposed to it, have developed antibodies and they’re immune to it, you can have natural herd immunity, and that particular virus will not be able to cause an epidemic in the population. “It doesn’t mean it won’t be able to spread as there’ll still be some susceptible people, but it won’t take off and cause an epidemic,” he said.

Professor Paul Hunter, a professor in medicine at the University of East Anglia, added that herd immunity is “an indication of the proportion of people who are immune in a population”.

“The relationship of the proportion of people that are immune that you need to prevent an epidemic varies from infection to infection,” he outlined. “With something like measles that is very infectious, you need something like 90 per cent of people immune, but with other infections you can get away with much less.”

According to a 2011 paper published in medical journal Clinical Infectious Diseases, while the term herd immunity is “widely used”, it carries “a variety of meanings”.

Academics from the London School of Hygiene and Tropical Medicine wrote that while some authors use the term to describe the proportion of individuals in a community who are immune to a condition, others use it in reference to “a particular threshold proportion of immune individuals that should lead to a decline in incidence of infection”.

“A common implication of the term is that the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals,” they added, in line with the definition of herd immunity outlined by the Vaccine Knowledge Project.

Tonia Thomas, Vaccine Knowledge project manager, explained that the prospect of developing immunity to a condition through infection, rather than through vaccination, could be harmful, as people may “risk developing complications from the disease”.

“Vaccines are a safer way to develop immunity, without the risks associated with the disease itself,” Thomas stated.

Is herd immunity a possibility amid the coronavirus outbreak? In March, it was reported that the British government was hoping to achieve herd immunity across the nation by allowing the virus to make its way through the population.

It was said that the government wanted to ensure the coronavirus passes through the nation “at a much delayed speed so that those who suffer the most acute symptoms are able to receive the medical support they need”, in addition to ensuring health services are “not overwhelmed”.

On 28 August, WHO’s chief scientist Dr Soumya Swaminathan explained the implications of allowing COVID-19 to spread around the population, stating that it is believed that at least 60 to 70 per cent of the population would need to be immune “to really break the chain of transmission”.

“If you allow this to happen naturally, it will take a long time, of course, but more importantly, it’s going to do a lot of collateral damage. So even if 1 per cent of people who get infected are ultimately going to die, then this can add up to a huge number of people, if we look at the global population,” Dr Swaminathan said. “And that is why we believe it’s not a good idea to try to achieve herd immunity by just letting the infection run wild in the population and infect a lot of people and that we should talk about herd immunity in the context of a vaccine.”

On a section on the WHO website about coronavirus myths, the organisation clarifies that there is not yet a vaccine to treat coronavirus, adding that vaccines used against pneumonia do not provide protection against the virus.

“The virus is so new and different that it needs its own vaccine,” WHO states. “Researchers are trying to develop against 2019-nCoV [novel coronavirus], and WHO is supporting their efforts.”

WHO also states that while there are not currently vaccines available that are effective against the coronavirus, “vaccination against respiratory illnesses is highly recommended to protect your health”.

In March, The Independent’s health correspondent Shaun Lintern said that there was “no chance of herd immunity with coronavirus”.

“As a brand new virus, no one has immunity to it, so every human being is susceptible to the virus,” he explains. “Herd immunity will only come into effect once a huge majority of people have had it and survived so their bodies create antibodies to the virus.”

According to Professor Woolhouse, as coronavirus is a viral infection that causes an antibody response “we would expect” herd immunity to occur.

Professor Woolhouse said that while the long-term antibody response to the virus still requires further study, “it is a reasonable expectation that people who have recovered from infection do have some degree of immunity to subsequent infection”.