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, tells The Independent that the concept of herd immunity is “the basis of all vaccination programmes”.
However, it can also occur naturally, he explains, 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 says.
Professor Paul Hunter, a professor in medicine at the University of East Anglia, adds 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 outlines.
“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, explains 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 states.
Is herd immunity a possibility amid the coronavirus outbreak?
Earlier this month, 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.”
During an appearance on BBC Radio 4’s Today show on 14 March, WHO spokesperson Margaret Harris stated that as “we don’t know enough about the science of the virus” and “it hasn’t been in our population for long enough”, scientists cannot yet be certain of “what it does in immunological terms”.
“Every virus functions differently in your body and stimulates a different immunological profile,” Ms Harris said.
“We can talk theories, but at the moment we are really facing a situation where we have got to look at action.”
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”.
According to The Independent’s health correspondent Shaun Lintern, there is currently “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.
However, as the coronavirus is “not well-studied” as of yet, “we don’t know how protective the antibody response is long-term, we don’t know how long it lasts, we don’t know any of these things”.
“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 adds 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 says.
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 states.
“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 says.
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.
The British government is currently in the process of trialling an antibody test, which could indicate whether a person has previously suffered and recovered from the coronavirus.
During the daily press conference on the coronavirus on Tuesday 24 March, health secretary Hancock said the test “will allow people to see whether they have had the virus and are immune to it and then can get back to work”.
“We expect people not to be able to catch it, except in very exceptional circumstances, for a second time,” he said.
The Independent has contacted the government’s Department of Health for comment.