How Do Vaccines Work?

A straightforward guide…

Rosie Alderson, PhD
May 11 · 5 min read
Photo by CDC on Unsplash

2020: a year in which we’ve all had to get on board with key concepts in science and medicine. The buzzword for 2021, it seems, is ‘vaccine’.

In this pandemic, we now hear doctors, researchers and other health professionals talk at length about this crucial topic, citing terms such as: ‘herd immunity’, ‘antibodies’, ‘transmissibility’ and the like.

Understanding the fundamental science behind vaccines and immunology can help us take care of our own health, as well as that of others.

Let’s start with some fundamental immunology: what are antibodies?

Imagine a scenario where you become ill, say, with the common cold. You feel rubbish: sneezing, coughing, maybe some body aches. Whilst you’re (hopefully) sat with you feet up, perhaps watching your favourite box set, your body is hard at work.

Photo by Isabella and Louisa Fischer on Unsplash

Your body is generating antibodies that are specific in shape to, and can therefore lock onto, structures on the outside of the cold virus.

Antibodies are tiny, y-shaped proteins that your immune system generates in response to infection. The antibodies bind to the outside of a particular pathogen and make it much easier for your immune system to destroy it. Any substance which provokes the production of antibodies is called an antigen.

Fvasconcellos, Public domain, via Wikimedia Commons

Amazingly, your immune system is capable of recognising billions of types of antigens and producing huge numbers of antibodies that have the right shape to ‘lock’ onto the specific antigen. However, this process takes time (sometimes weeks, or even months). So it takes time for your body to mount an effective, specific immune response and fight the infection.

Over time, the numbers of antibodies specific to a particular antigen start to decrease. However, special cells, called ‘memory cells’, ‘remember’ the pathogen. If you encounter that same pathogen again you will be able to mount an antibody response much more quickly — preventing you from becoming ill with that particular disease a second time.

We call this: ‘immunity’.

Remember, antibodies work by binding onto specific parts of the pathogen — they need to have the correct shape. A vaccine is a substance which allows the body to make antibodies against a specific pathogen. This is done by introducing a small amount of a specific antigen into the body.

For example, a vaccine might contain:

→ Just one part of the pathogen structure, for example, a protein found on the outside of the virus.

→ An ‘attenuated’ (weakened) form of the ‘live’ pathogen.

→ Part of the pathogen’s genetic material. This can then be translated into a specific viral protein by the patient’s cells and can trigger an antibody response.

The body recognises the antigen as foreign and, over a few weeks, generates antibodies against it. If successful, the vaccinated person will develop memory cells and will have some level of immunity to the pathogen.

Vaccines have helped us to overcome devastating illnesses such as polio, smallpox and measles: they have revolutionised medicine.

The word ‘vaccine’ is derived from the word ‘vacca’ which is Latin for ‘cow’ and gives us a big clue to the nature of its discovery.

In the 1800s, smallpox was raging across Europe — with 400,000 perishing each year. Even if one were to survive smallpox, it left many blind and with terrible scars. In England, the devastating disease was nicknamed the ‘speckled monster’ due to the sudden appearance of skin lesions in the infected.

It had long been held that dairymaids, who often contracted cowpox (a much milder disease), were protected against becoming ill with smallpox. In 1796 Edward Jenner, an English trained physician and scientist, went on to test the idea that purposefully inoculating people with cowpox would protect against smallpox.

Jenner inoculated a young boy (see depiction below), James Phipps, with material from a cowpox lesion from the dairymaid Sarah Nelms. A couple of months later, Jenner tested Phipp’s immunity to smallpox by inoculating him with fluid from a smallpox lesion — Phipps did not become ill with smallpox and Jenner concluded he was immune from the disease.

Jenner’s vaccine had worked.

Ernest Board,, Public Domain

If a large proportion of the population becomes immune to a disease, either by contracting the disease or by being vaccinated, it is possible for ‘herd immunity’ to develop.

Herd immunity occurs when the pathogen can no longer transmit effectively through a given population. Essentially, as more and more members of a population become immune to a disease, it gets harder and harder for the pathogen to spread between individuals.

This is important because, for a variety of reasons, not everyone in a population can be vaccinated — for example, those with compromised immune systems. However, if herd immunity is achieved, with most people being immune, the chances of a non-immunized person contracting the infection are greatly decreased.

By Tkarcher, Wikimedia Commons, CC BY-SA 4.0

There is no ‘magic’ percentage of those needed to be immune to achieve herd immunity. The ability to achieve herd immunity depends on the particular disease in question and the ability of any vaccine to lower how infectious people are (transmissibility).

Okay, back to you and your cold….

If vaccines allow the human race to avoid so many diseases why are do billions of people still get ill? For example, with colds and flu?

Let’s think about those antibodies again. Remember antibodies are a specific shape, they are complementary to the particular pathogen. If the pathogen manages to change its structure then the antibodies may not be able to bind it so well.

There are over 200 strains of cold virus, so it is very difficult for scientists to engineer a vaccine that will build immunity to every single one. Flu viruses tend to mutate quite rapidly, changing the shapes of their antigens frequently. This is why new flu vaccinations are given every year.

Photo by Elena Mozhvilo on Unsplash

The exact technology and formulation behind any particular vaccine is a complex and fascinating area of science. Many vaccines are the result of months, years, even decades of hard work. Therefore, It can be easy, especially in the current climate, to be overwhelmed by the myriad of vaccine-related terminology.

I hope this article has gone some way in demystifying them.

Stay well.

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