How does herd immunity benefit us?
By Safiya Zaloum
Herd immunity benefits us, but what exactly is herd immunity? Herd immunity is when a large enough proportion of the population is immune to an illness so that it cannot spread within that population. This can only be created for diseases that can be passed between people such as measles.
First, let’s start off by looking at what immunity is. Once you have had certain illnesses once, you don’t get them again – chicken pox for example. Why is this? The answer is that your immune system remembers. Next time you get infected, your body fights off the infection before it has a chance to make you ill.
How does the immune system work?
The first time you encounter a particular infection, your body takes a long time to work out how to fight it off. Diseases are caused by pathogens, which are bad microorganisms that cause you to become ill. Viruses are one example of a type of pathogen. The first time your immune cells see a virus, they don’t know how to destroy it. It takes them a long time to make the correct antibodies: these attach to those red spiky bits on the virus, called antigens, and help destroy it. The antigens from the virus let your body’s immune system know that something foreign is in your body and stimulates a response. You also have antigens on your own cells too – these are called autoantigens. This is so that your immune system knows what to attack – only things with foreign antigens.
It isn’t very nice becoming ill, but after the illness when most of the immune cells die, some B cells remain and become memory cells. They float around in your bloodstream and if you encounter that same virus, they remember how to fight it! This virus has a certain shaped spike and the B cell recognises this shape and can make an antibody that fits onto these spikes. Because the B cell remembers, your body can destroy the virus much quicker than the first time. Your immune system destroys the pathogen before it has time to make you feel unwell, so you don’t feel ill. You are immune to a pathogen once you have created these memory cells.
How is herd immunity created?
Herd immunity occurs when enough of the population is immune to a disease. This is about 95% for any given disease that is caused by pathogens. This stops the disease spreading in a population, or if one person gets it, they cannot pass it on as the other people around them are immune.
The image below demonstrates how a disease will spread rapidly through a population if no one is immune to it. COVID-19 is an example of this. The red dot represents one person with the infection and the blue dots healthy individuals who have no immunity. See how most of the blue dots turn into red dots – almost everyone catches the disease, maybe even everyone over time.
Herd immunity usually comes from vaccination. Vaccines come in different forms but many work by showing your immune system antigens from the pathogen. These spikes alone cannot make you ill. Your immune system springs into action and works to make antibodies – those complementary shapes to the spikes that help to destroy the pathogen. Just like when you catch the virus for real, some immune cells remain as memory cells. Then when you catch the real pathogen, your immune system can fight it before it has a chance to make you ill.
Vaccinations work for diseases that can be passed between people like measles and mumps but also against other pathogens that you can catch from the environment but not from other people. One example of this is tetanus.
Shown below is when about 80% of the population is vaccinated against a disease. Green dots represent those vaccinated and blue those who are healthy but unvaccinated. The red dot is someone infected – see how most of the blue dots (healthy unvaccinated people) can still be infected. Nowhere near as many people get ill as before but the pathogen can still spread.
The aim is to reach 95% vaccination. The diagram below shows this: one person is infected but cannot infect the other 2 unvaccinated people as there are so many people who are vaccinated and therefore immune. This is how herd immunity works in practice.
Those who are immune protect those who aren’t. There are many people who cannot be vaccinated because they have weakened or under-developed immune systems. For example, newborn babies, the elderly, those with cancer or an organ transplant and those who do not respond effectively to vaccines. These groups are kept safe from many diseases by herd immunity.
This is the main benefit that herd immunity offers to us as a society. If a young healthy person catches a virus such as COVID-19, they are much more likely to recover than someone with a weakened immune system. This is the case with many illnesses and is why seemingly fit and healthy people should get vaccinated – to protect themselves and those around them. Once a certain level of herd immunity has been reached, a disease can even be eradicated from a population, as is the case with smallpox.