Herd immunity through vaccination

An individual person gains immunity to an infectious disease through their adaptive immune system developing an active immunological memory of a pathogen. This immunisation can occur naturally through exposure to a pathogen or artificially through vaccination with a pathogen. Herd immunity refers to the protection provided to non-immunised individuals living in a population of mostly immunised individuals. There is less spread of a contagious disease in a highly immunised population, therefore limiting the exposure of non-immunised individuals. Herd immunity can occur through a population of humans naturally acquiring immunity through experiencing and surviving an infectious disease. It can also be achieved safer, faster, and preventatively, using vaccination.

The image below shows the varying spread of a disease in a population, in three different scenarios:

  1. No one has immunity to the pathogen
  2. Some people have immunity to the pathogen
  3. Most people have immunity to the pathogen

A graphic depicting the process of herd immunity, divided into 3 vertical sections. At the top of the graphic, in the first section, on the left side there is a group of people depicted with most coloured blue and two coloured red. To the right of this group is an arrow pointing to the right with the label “no one is immunized” and “contagious disease spreads through the population”. To the right of this arrow is another group of people, with most coloured red and only a few coloured blue, indicating the spread of disease in a population. In the second section, in the middle of the graphic, on the left side there is a group of people depicted with most coloured blue, two coloured red, and six coloured yellow. To the right of this group is an arrow pointing to the right with the label “some of the population gets immunized” and “contagious disease spreads through some of the population”. To the right of this arrow is another group of people, with most coloured red, some coloured blue and some coloured yellow, depicting less spread of the disease in the population compared with the image above. In the final section, at the bottom of the graphic, on the left side there is a group of people depicted with most coloured yellow, a few coloured blue, and two coloured red. To the right of this group there is an arrow pointing to the right and the label “most of the population gets immunised” and “spread of contagious disease is contained.” To the right of this arrow is another group of people with most coloured yellow, and a minority coloured blue and red, depicting very limited spread of the disease in the population.

Source: National Institute of Allergy and Infectious Disease (2010). Adapted under CC BY 4.0

Vaccination

Vaccination refers to being given a vaccine, either orally or via an injection. A vaccine usually consists of a weakened pathogen, or parts of pathogens that include antigens for that pathogen. Some newer vaccines, such as those against Covid-19, contain genetic messages that allow the body to create and respond to the pathogen’s antigens, without delivering the pathogen itself. Taking any vaccine introduces antigens that trigger our adaptive immune system to recognise and remember that pathogen.

Creating our own memory cells that recognise that pathogen enables us to respond faster and more effectively if we are exposed to that pathogen again in our environment. This limits both the impact of that pathogen within our body as well as our transmission of that pathogen to others.

Some vaccines, such as for polio, prevent you from developing any symptoms from a disease if you are exposed to it again naturally. Other vaccines, such as those for COVID-19, reduce your risk of serious symptoms. Vaccines do not guarantee immunity, as some recipients may not produce enough of an immune response and so do not become immunised. Further, some individuals are not able to receive certain vaccines due to their age, pregnancy, taking immunosuppressant drugs, or they are allergic to preservatives in the vaccine. Herd immunity is especially important in protecting these individuals.

The level of population immunity required to reach herd immunity varies widely for different diseases and can depend on how contagious that disease is. Measles is highly contagious, with one infected person able to infect up to nine out of 10 unvaccinated close contacts. Therefore, vaccination rates of 95% of the population in an area are required to achieve herd immunity. Polio is still contagious, but less so than measles, and therefore requires a  vaccination rate of at least 80% of the population for herd immunity (World Health Organisation, 2020).