Herd immunity is a phrase being used more and more in relation to COVID-19.
In its simplest terms it refers to a whole population having immunity to a disease but it is not easily achieved. Either an entire population has to succumb to the disease or a vaccine against it is needed.
Some Definitions
Endemic
When an infectious disease is always present in a community and causes a steady stream of infection it is endemic. Examples include chickenpox in the UK and malaria in many parts of Africa. An endemic disease is one people always live with.
Epidemic
If a disease spreads rapidly over an area and infects a large group of people it is epidemic. The key feature is that the disease is rapidly spreading. It can be within one country or spread across several countries. An example is Ebola Virus Disease in parts of Africa.
Pandemic
They key feature of a pandemic is that the disease has spread over most of the world.
Outbreak
An “outbreak” of disease is a looser term and signifies an increase in cases of a disease that is usually endemic.
Diseases can “move” between these categories.
For example, measles is endemic in many low income countries but every so often there is an explosion in case numbers and it becomes epidemic. However, because of routine vaccination available in developed countries, measles cannot become pandemic as a substantial proportion of the world’s population are immunised against it. In the UK we have occasional outbreaks of measles but these are usually in non-vaccinated individuals and do not spread into vaccinated people. In the UK we don’t see measles epidemics.
When COVID-19 first emerged in China it was an epidemic. Numbers were growing but initially it appeared to be contained within China. As it spread globally it became a pandemic.
Herd Immunity's Role in Infectious Disease.
There are many factors that determine the course an infectious disease follows of which herd immunity is only one. These factors can be split into categories:
1) The host-parasite relationship (with COVID-19, humans are the host and the coronavirus that causes COVID-19 is the parasite.)
This includes factors such as how the virus enters the body and the effects it has; how long the incubation period is and how infectious the person is. All of these will influence the outcome of the disease, how serious it is and how quickly it spreads.
Different infectious diseases have different R numbers, the number that indicates how many people will catch an infectious disease from one person with it. If the R number is 2 one infectious person infects two others; if it is 20 then one infectious person will infect twenty others. (click here to read more about R Numbers.)
Measles is highly infectious and has an R number of around 15 in an unvaccinated population. Vaccination has brought the measles R number down in many countries, in the UK the measles vaccination programme aims to keep it below 1. COVID-19’s R number has varied between 1 and 3.5 depending on lockdown restrictions. (3)
Not every new virus leads to an epidemic or pandemic. If a new virus emerges with a very low R number, say 0.2, it will simply die out before it has had time to infect too many people.
2) Environmental factors influence disease outcomes.
As is seen with COVID-19, the virus spreads more rapidly in crowded, indoor settings. By controlling the environment with measures such as lockdowns and associated restrictions we are aiming to keep the spread of the virus under control but are not achieving immunity against the virus.
3) The immunity the population as a whole possesses or “Herd Immunity”.
If herd immunity is high it can hinder the spread of a disease. In some diseases it is achieved by vaccination, for example babies in the UK receive diphtheria, polio and measles vaccines as part of their vaccination programme and these ensure the vast majority of the population grow up immune to these illnesses. Those who aren't vaccinated are at risk of the diseases but usually benefit from the fact the people around them are. This is only true for illnesses spread by people. Babies are routinely vaccinated against tetanus but herd immunity cannot protect an un-vaccinated person from tetanus because it is not spread human to human, it is contracted directly from soil.
The other source of herd immunity is from people naturally acquiring the illness, for example chickenpox. Chickenpox is highly infectious but rare in adults because most people acquire it in childhood and go on to have lifelong immunity.
When herd immunity is achieved via people acquiring the infection, case numbers can only go down when sufficient people have had it and the reservoir of susceptible people has been severely depleted. This can take years to achieve.
Is Herd Immunity a Possibility in COVID-19?
The short answer is that this side of a vaccine, no.
It is easy to see why herd immunity sounds appealing but to achieve it through allowing everyone to acquire the infection would mean huge numbers of serious ill people and deaths along the way. We do not yet know if herd immunity via the infection route is even possible for COVID-19.
Not all viruses can be contained with herd immunity. HIV (Human Immunodeficiency Virus) is one such example. Infection with HIV does not lead to immunity, the virus does not clear the body and the aim of subsequent treatment is to keep it suppressed for the lifetime of the infected person. If an entire population had HIV infection no herd immunity would be achieved, HIV simply does not behave in that way.
It is not clear yet if people who have had COVID-19 become immune to further infection or not. There have now been documented cases of second infections in the same person so allowing it to infect an entire population may not achieve anything in terms of immunity. (4)
If herd immunity is achievable current estimates are that at least 80% of the population will need to have COVID-19 first. (5)
The case fatality rate for COVID-19 (the proportion of cases that die from it) is still not fully known. It varies within and across different countries and will alter as new treatments for the disease help bring the death rate down. At the time of writing the rate varies between 0.25% and 10%. (6)
To translate those percentages into figures, the population of the world is 7.8 billion people; if 80% need to be infected with COVID-19 to achieve herd immunity that is 6.2 billion people; even with the lowest fatality rate of 0.25% over 15 million people worldwide would die from COVID-19.
So far there have been 32 million cases and 984,000 deaths worldwide, high numbers but way below the levels that could be reached if we allow the virus to continue to spread unabated in the hope herd immunity will eventually protect us. (7)
Herd immunity via the infection route for COVID-19 could only be achieved safely by the implementation of impossible parameters. We would have to ensure that only those who are highly unlikely to be seriously ill or die from COVID-19 are exposed to the virus whilst totally shielding all those considered vulnerable and keep these two groups apart at all times, every minute of the day and night for several months. A completely unworkable situation in practice. (8)
Scientists speaking to The Lancet made it clear that “any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable.” (9)
Herd Immunity and a Vaccine
The best hope for achieving herd immunity rapidly will be when a safe and effective vaccine can be manufactured against COVID-19. Despite much political hype over vaccines (click here to read more) the reality is that we are still months away from having a vaccine available for the general population.
In the meantime, efforts to control the virus have to continue in order to protect the most vulnerable. Smallpox was eradicated by vaccination and polio is on the cusp of eradication too. Diseases such as diphtheria, pertussis and tetanus are rarely seen in vaccinated populations. In the fight against COVID-19, vaccination is likely to be the surest way out of the pandemic.
