The global spread of the coronavirus has left many people wondering when and how the outbreak may subside.
In a study published in medical journal The BMJ on Thursday 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? Here is everything you need to know.
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”.
“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 15 March, a statement released by health secretary Hancock on the government’s website denied achieving herd immunity was its intention.
“We have a plan, based on the expertise of world-leading scientists. Herd immunity is not a part of it,” the statement read.
“That is a scientific concept, not a goal or a strategy. Our goal is to protect life from this virus, our strategy is to protect the most vulnerable and protect the NHS through contain, delay, research and mitigate.”
On Tuesday 3 March, a statement released by the World Health Organisation (WHO) said that while many people around the world have developed immunity to seasonal flu strains, the same cannot be said for the coronavirus.
“Covid-19 is a new virus to which no one has immunity,” the statement read. “That means more people are susceptible to infection, and some will suffer severe disease.”
In a scientific brief published by WHO in April, it further stressed that while antibodies may be detected among people who have previously tested positive for the virus, “there is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection”.
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.”
Lintern affirms that in order to achieve “a good herd immunity”, approximately 90 to 95 per cent of the population would need to have recovered and become immune to the infection, such as in the case of measles, which could subsequently result in a rise in the number of deaths and people in intensive care.
“There is also a risk coronavirus becomes seasonal and like the flu will mutate each season and therefore again herd immunity can’t come into play.”
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”.
“But it is a reasonable expectation that people who have recovered from infection do have some degree of immunity to subsequent infection,” he says.
Professor Woolhouse added that herd immunity would be more likely to happen if we were to experience “an uncontrolled epidemic”. However, as there are “lots of measures being put in place to stop that happening”, this could reduce the likelihood of herd immunity occurring.
“So there is this balance between the better you control this infection, at the same time the less you build up herd immunity,” he said.
In Professor Hunter’s opinion, herd immunity is unlikely to happen until next year “when we hopefully will have a vaccine”.
“I am just making effectively educated guesses at this point, but I suspect we won’t see the last of it this year,” he stated.
“I suspect it will become what’s called endemic where the virus circulates forever into the future, and in those circumstances in the future herd immunity will have a big impact,” the professor said.
The US-based Centres for Disease Control and Prevention states on its website that the immune response for Covid-19 “is not yet understood”, and so it is not completely certain whether patients can be infected again.
In July, a study conducted by researchers at King’s College London concluded that coronavirus patients may lose immunity to the disease within months.
The researchers found that 60 per cent of patients had a “potent” level of antibodies in the two weeks after their symptoms first showed. However, after three months, their antibody levels dropped to less than 17 per cent.
“One thing we know about these coronaviruses is that people can get reinfected fairly often,” said Professor Stuart Neil, co-author of the study.
“What that must mean is that the protective immunity people generate doesn’t last very long. It looks like Sars-Cov-2, the virus that causes Covid-19, might be falling into that pattern as well.”
In August, scientists at Hong Kong University claimed they had discovered the first human being reinfected with Covid-19, stating that the man had been infected twice by different versions of the coronavirus months apart.
If the research proves correct, it may mean that patients may not necessarily be immune to Covid-19 after recovering from the virus.
The Independent contacted the government’s Department of Health for comment when this article was first published in March.