Coronavirus: What is herd immunity and could it work in the UK?

When enough people are immune to a disease, this can reduce the likelihood of it spreading, resulting in herd immunity

As we reach six months since the initial UK lockdown, many are still wondering how and when the global spread of the coronavirus may subside, especially as a vaccine is still a long way from rollout ready.

What about other approaches? Is there a way we could avoid constant lockdown until a vaccine is ready? 

In March it was claimed that the British government was hoping to allow the virus to “pass through the entire population so that we acquire herd immunity” like other illnesses.

Matt Hancock later denied this was the case, and emails seen by the BBC on 23 September, reportedly reveal alarm among the government’s top scientific advisers at references made to herd immunity by scientists in media interviews in March.

A spokesperson for the Department of Health and Social Care separately said that “herd immunity is a natural by-product of an epidemic” but the government denied herd immunity was ever part of government policy. 

On 15 March, a statement read: “We have a plan, based on the expertise of world-leading scientists. Herd immunity is not a part of it…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.” 

But what exactly is herd immunity and was it ever a possible strategy for the UK? 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”.

Academics from the London School of Hygiene and Tropical Medicine wrote in a 2011 paper 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.

The Vaccine Knowledge Project at Oxford University uses 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”.

But what happens if there isn’t yet a vaccine?

Herd immunity can also occur naturally. Prof Woolhouse explained: “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.

But this approach for coronavirus would have required a large proportion of the population becoming ill first making it a non-starter.

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.

 And, as time as gone on, there is still questions about how long-term Covid antibodies are anyway. Chris Witty said evidence suggests in some Covid-patients that these antibodies “fade” after a period of time.

How many people need to have immunity to develop herd immunity?

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.”

Is herd immunity a possibility amid the coronavirus outbreak?

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 could not 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.”

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.”

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.

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.