What is herd immunity - and can it stop coronavirus?

It's been a theory bandied about since the start of the pandemic to help lessen the impact of coronavirus on the country - but what is herd immunity?

Over six months into the pandemic, the concept of herd immunity has been suggested by many nations around the world as a solution to the pandemic. One of the first was Sir Patrick Vallance, the UK's Chief Scientific Advisor, who said back in March that the aim of herd immunity is to "reduce the peak [of the coronavirus], 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". The idea immediately received huge amounts of backlash, when the concept of herd immunity was explained, because it naturally leads to huge amounts of illness and by consequence, death.

So the government soon denied that herd immunity had ever been part of the plan. Writing in the Sunday Telegraph, Health Secretary Matt Hancock clarified the government's initial plan. He said it was "based on the expertise of world-leading scientists and added, "herd immunity is not 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."

Part of this included over 70s staying at home and shielding for some months and some months later, local lockdowns where introduced so people knew when to self-isolate after cases rose across the country. Since the government's confirmation that herd immunity was not part of the response, research has come on leaps and bounds around what herd immunity actually is and whether it could stop coronavirus.

What is herd immunity?

Herd immunity is when a population becomes resistant to a contagious disease because they have become immune to it. While this happens predominantly through vaccines, it can also theoretically happen when almost everyone in the area contracts the disease and develops antibodies. The more people who are immune to the disease, the harder it is to spread, as fewer people catch it and pass it on.

The benefits of the system were first realised when it was successfully used to wipe out small pox, whooping cough bacterial meningitis and measles. Yet in these cases, it was the creation of the vaccine (which we currently do not have for coronavirus) that helped to create full herd immunity. Similar vaccines, like the flu jab which has been made available to 30 million people in 2020, have also helped to slow the spread of common viruses that appear yearly.

Whatever the method of achieving herd immunity, whether by vaccine or letting the disease pass through the population, in order for it to be effective there needs to be a greater proportion of people infected and immune to the disease than not. For highly contagious diseases, such as measles, about 94% of the population would have to be immune for herd immunity to be effective.

This means that almost all of the population would have to catch coronavirus in order for herd immunity to possibly work. But aside from a dismissal of the mass loss of human life that it would involve, there are other barriers that prevent herd immunity from becoming an effective way to stop coronavirus.

How does herd immunity work?

Herd immunity works on the basis that the more people infected with the disease, the higher the chance of success for herd immunity. This happens as the immune system builds antibodies against the virus so if it comes into contact with it again, the body can successfully fight it off without the person becoming ill.

Jeremy Rossman, Honarary Senior Lecturer in Virology and President of Research-Aid Networks at the University of Kent, says that in order for us to achieve herd immunity for the coronavirus, we would need 60 per cent of the population to be affected, which is about 47 million people. This is because the coronavirus is less contagious than measles for example, but more contagious than the common flu - which requires 40 per cent immunity in the population to prevent it from spreading.

He goes on to write in the World Economic Forum that, “Slowing the spread of COVID-19 is a promising strategy, especially when combined with enhanced measures to protect the elderly and those with underlying health conditions. By slowing the spread of the disease, the NHS might have more time to prepare, we might be able to develop treatments or vaccines and we will be closer to the summer when we have lower incidences of other diseases that burden the NHS, such as the flu.”

However, many have criticised the policy to just let the virus pass through the community as an ineffective - and even dangerous - public health strategy. This is because the immunity would be mainly generated in younger people, as they are considered to be capable of handling the virus. So in other words, younger people (aged 20-40) would get the virus, ideally becoming immune and therefore not able to pass it on to anyone older. Naturally, this doesn't include those younger people with weakened immune systems through conditions such as diabetes. The criticism also comes as those between the ages of 20 and 40 are also often in contact with those who are vulnerable to the coronavirus. In some cases this will be someone who is caring for a family member, or those who work in healthcare and care homes.

This useful diagram from the National Institute of Allergy and Infectious Diseases shows how herd immunity works:

As we have seen, herd immunity through vaccines is widely acknowledged as a successful way to prevent a disease from spreading. By and large, there has been an eradication of life-threatening diseases through herd immunity, such as measles, mumps and rubella, which is why young children are encouraged to have the MMR vaccine as soon as they're eligible. But right now, we don't have a COVID-19 vaccine and coronavirus vaccine trials are really only just getting started in the UK. So could herd immunity work to prevent the spread of coronavirus?

Would herd immunity work to prevent the spread of the coronavirus?

The race to find a vaccine for coronavirus has been so intense as now six months down the line, we recognise that a COVID-19 vaccine would be a promising step in suppressing the virus around the world. But herd immunity without a vaccine is not only accepting mass loss of life, but it's also been proven recently to be an ineffective way to stop the spread.

Back in August, Michael Ryan of the the World Health Organisation (WHO) warned that "we are nowhere close to the levels of immunity required to stop this disease transmitting. We need to focus on what we can actually do now to suppress transmission and not live in hope of herd immunity being our salvation."

In the UK, some of these methods have included limiting the amount of people who can meet inside and outdoors by introducing a three tier system in England and a 4 tier system in Scotland. The new restrictions, which have been enforced alongside a new NHS contact tracing app to help the test and trace system, have come into place as even more research has concluded that herd immunity would be a dangerous and completely ineffective way to control the virus.

A new study by Imperial College London has revealed that antibodies, which fight off infection in our immune systems and are vital for herd immunity, for COVID-19 decline over time. The researchers tested 365,000 people in England between June and September to see if they had any antibodies for coronavirus. They found that the amount of people who tested positive for antibodies was 6% between June 20 and July 13, and by the end of September it was down to just 4.4%. Researchers, including one of the lead authors of the study, Helen Ward,  have suggested that the antibody response to coronavirus reduces over time.

She said, "This very large study has shown that the proportion of people with detectable antibodies is falling over time. We don't yet know whether this will leave these people at risk of reinfection with the virus that causes Covid-19, but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others."

While other scientists involved in the research have emphasised the need for a vaccine to help protect large numbers of the population as herd immunity is still a "long, long way" off.

Professor of infectious diseases at Imperial College London has said, "The big picture is that after the first wave, the great majority of the country still did not have evidence of protective immunity."

“So the need for a vaccine is still very large if you want to try and get a large level of protection in the population.”

Essentially, for the short term, the best way to prevent the spread of the virus is to follow the latest guidelines from the NHS. This includes washing your hands with soap and water often for at least 20 seconds, avoiding contact with people who are unwell and covering your mouth and nose with a tissue or your sleeve when you cough or sneeze.

Are people who recovered from COVID-19 immune from the disease?

In April, WHO confirmed suspicions that people who recovered from COVID-19 are not immune from the disease forever and can get it again. In a statement, they said, "There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection,"

This was backed up by another August study, where researchers identified a man in his 20s in the USA who was reinfected with the virus. He experienced only mild symptoms when he caught the virus back in April but in the second infection in May, he developed more serious symptoms of COVID-19. Now, Now following the antibody study by Imperial College London, it seems like there's even more evidence that those who recover from coronavirus can get it again as the antibodies they create diminish over time.

This might be because coronavirus, much like the common flu, has genetically different strains and can mutate over time. So the first infection was a different strain of COVID-19 than the second and so, there were not any present antibodies to fight off the infection.

So while some people may still hold out hope for herd immunity, research is being conducted more and more with results suggesting that it's not going to be an effective way forward to prevent the spread of COVID-19.