What About Herd Immunity?

Everybody talks about 'herd' immunity, but do we really know what we're talking about? The concept is simple enough--it's a bit like vaccination, only the vaccine is developed naturally through enough people acquiring the disease and recovering with an acquired immunity. If enough people develop that 'natural' immunity the disease is thwarted of new victims and stops spreading--or, ideally, dies out.

However, there are problems with using this as a deliberate strategy.

* The most obvious problem is calculating the percentage of a given population that must develop an immunity in order to protect the non-immune portion of the population. A good example of that balance would be the flu vaccine. 

* Another problem has to do with whether the vaccine is or is not highly immunogenic. If it's not, it may be relatively ineffective in building up immunity in the general population. A good example of that, again, is the flu vaccine. 

* Since a large percentage of the given population must become infected to develop a 'natural' immunity and produce a 'herd' immunity effect, the possibility that a large number of people will die becomes a consideration. Even a 'low' mortality rate translates into large numbers of dead people if the disease spreads widely enough. 

* The question also arises, How long will any acquired immunity last? If you forgo social distancing instead adopt the quest for 'herd' immunity as a deliberate strategy--in the absence of a vaccine or a specific medication, which is  the situation we're currently in--but the acquired immunity is short lived, then you'll have killed off a substantial number of people for no particular benefit.

These are the questions that experts in epidemiology and infectious disease are wrestling with. Back on March 13, 2020, Science Media Centre published the views of a fairly large number of UK experts--expert comments about herd immunity--seeking answers to just these questions.

Because so many people have advocated living life as 'normal' while waiting for 'herd' immunity to kick in, I wanted to address this issue so we all know more or less what we're talking about while evaluating policies for dealing with this pandemic. To accomplish this I've taken the liberty of pasting in some (not all) of the responses and editing them. There's a certain amount of repetition, but I think that's a good thing.

One reminder. Comparison is regularly made in these responses to the flu. But, as these scientists all know, This Is Not The Flu! Also, you will see that not all of these scientists are on the same page with one another.

Prof Willem van Schaik, Professor of Microbiology and Infection, University of Birmingham, said: 

“Herd immunity describes the phenomenon that at-risk individuals are protected from infection because they are surrounded by immune individuals.  The spread of the virus is thus minimised.  Currently, we talk mostly about herd immunity in the context of vaccines.  If a sufficiently high number of individuals in a population are vaccinated, they will provide herd immunity to the small number of people that are not vaccinated (e.g. for medical or religious reasons).  We have recently seen cases of measles outbreaks where herd immunity was not sufficiently high because children were not vaccinated out of completely unfounded fears against vaccination.

“Herd immunity exists for flu.  If large proportions of the population would get a flu vaccine that could protect non-immunised individuals.  The problem with flu is that it is difficult to know which strains of flu (think of strains as variations on a theme: it is flu but just a little bit different) will be causing infections at any given point in time and so that is why the flu vaccine is not always 100% effective.  The major problem with coronavirus is that this is a novel virus that has never spread before, which means that everyone is at risk for infection.  Herd immunity can only be reached by widespread vaccination (but there is currently no vaccine, and it may take a long time before an effective vaccine becomes available) or by individuals falling ill and recovering thereby developing natural immunity against the virus.

“Unfortunately, a very rough estimate suggests that we will only reach herd immunity to Covid-19 when approximately 60% of the population is immune (and remember that immunity is currently only reached by getting the infection as we have no vaccine!).  The major downside is that this will mean that in the UK alone at least 36 million people will need to be infected and recover.  It is almost impossible to predict what that will mean in terms of human costs but we are conservatively looking at 10,000s deaths, and possibly at 100,000s of death.  The only way to make this work would be to spread out these millions of cases over a relatively long period of time so that the NHS does not get overwhelmed.  Social distancing might contribute to this.  ...

...  Letting a potentially lethal infectious disease burn through a population is another way to reach herd immunity, but it comes with considerable risks and downsides (see above).  This is why we still need to all work together to try to slow down and minimise the spread of Covid-19 by practicing hand hygiene and social distancing. ...

Dr Bharat Pankhania, Senior Clinical Lecturer with the University of Exeter Medical School, said: 

“Trying to create herd immunity through Covid-19 brings in questions of safety.  You can’t control infection spread to “high risk” people.  Therefore, some people who become infected will develop very severe illnesses, and some of those would die.

“There is some herd immunity against flu, which is mediated by vaccination – but it’s not excellent as the majority of the population are not vaccinated, nor is the Influenza vaccine highly immunogenic.”

 

Prof Martin Hibberd, Professor of Emerging Infectious Disease, London School of Hygiene & Tropical Medicine, said: 

“The evidence is increasingly convincing that infection with SARS-CoV-2 leads to an antibody response that is protective.  Most likely this protection is for life, although we need more evidence to be sure of this, people who have recovered are unlikely to be infected with SARS-CoV-2 again.

“As more and more people become infected, there will be more people recovered who are then immune to future infection.  As these numbers build up, it will be harder for an infected person to transmit to other people, because some of the people that meet the infected person will be resistant (because they had the infection previously and are now immune).  When about 70% of the population have been infected and recovered, the chances of outbreaks of the disease become much less because most people are immune (resistant to infection), this is called herd immunity.

Prof Matthew Baylis, Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, said: 

“So what about COVID-19?  Estimates are that one person may infect as many as 2-3 others, on average, meaning herd immunity should kick in at 50 – 67% of the population immune.  And so in the absence of a vaccine, there would appear to be nothing to stop the spread of the virus until 50-67% of us have had it; and at that point herd immunity will kick in and transmission will decline or stop.  This is where the 60% of the population statistic has come from.  And this is deeply concerning – taking the low fatality rate estimate of 1%, even 50% of the UK population infected by COVID-19 is an unthinkable level of mortality.

“But it doesn’t have to be – and it won’t be – this way.  By reducing the number of people that one person infects, on average, then we lower the point at which herd immunity kicks in.  If we reduce it to 1.3, COVID19 becomes more like flu, and herd immunity kicks in when about one quarter of the population has had the disease and is now immune.  So, from an epidemiological point of view, the trick is to reduce the number of people we are in contact with (by staying more at home), and reduce the chance of transmission to those we are in contact with (by frequent hand washing) so that we can drive down the number of contacts we infect, and herd immunity starts earlier.  The sweet spot comes at the point where one infected person infects one, or less than one, person on average.  But, importantly, we will need to sustain this until we have a vaccine: only at that point can we return to normal behaviour patterns, with herd immunity now achieved by vaccination, not disease.”
 

Professor Peter Openshaw, Past President of the British Society for Immunology and Professor of Experimental Medicine at Imperial College London, said:

“SARS-CoV-2 is a novel virus in humans and there is still much that we need to learn about how it affects the human immune system.  Because it is so new, we do not yet know how long any protection generated through infection will last.  Some other viruses in the Coronavirus family, such as those that cause common colds, tend to induce immunity that is relatively short lived, at around three months.  However, these viruses have co-evolved with the human immune system over thousands of years meaning they may well have developed methods to manipulate our immune responses.  With the novel SARS-CoV-2, the situation may be very different but we urgently need more research looking at the immune responses of people who have recovered from infection to be sure.”

Dr Erica Bickerton, The Pirbright Institute, said:

“Immunity to SARS-CoV-2 is not yet well understood and we do not know how protective the antibody response to this new virus will be in the long-term.  This is a new coronavirus and there is a lot of work going on to understand immunity to this virus.  It is too early to say how long immunity lasts or how the virus will adapt to escape immunity.  There is still much to be learned.”

Dr Simon Gubbins, The Pirbright Institute, said: 

“For SARS-CoV-2 estimates for R0 are around 2.5, so the proportion of the population that needs to be immune to achieve herd immunity is around 60%.
“Herd immunity acts as an evolutionary pressure for a virus to adapt so that it can escape immunity and can spread more easily.  Influenza viruses are very good at this and frequently mutate to produce new strains to which people are not immune.  This is the reason the seasonal flu vaccine needs to be updated annually.  There is no information to show whether something similar will happen with SARS-CoV-2.”