It’s a term used (sometimes known as population immunity) to describe a population that is protected or immune from a disease. Herd immunity is achieved through vaccination or by people developing antibodies by previously having the disease. Herd immunity means enough people have achieved immunity to disrupt person-to-person transmission in the community, meaning that those who are not immune are still nonetheless somewhat protected.
It is important to note that The World Health Organization has indicated the preferable way to reach herd immunity is through vaccination, and not through the community contracting the disease. WHO says that “attempts to reach ‘herd immunity’ through exposing people to a virus are scientifically problematic and unethical”.
There is a percentage of the population that needs to be immune to the disease in order to reach herd immunity. This percentage changes depending on the disease/virus. For example, 95% of a population needs to be vaccinated against measles to reach herd immunity. For polio, the threshold is at around 80%. The WHO is yet to set a percentage threshold for COVID-19. This area is still being researched. However, some experts are estimating the threshold to sit between 60-90% of the population needing to be vaccinated.
Why is there currently a wide percentage range for the threshold?
COVID is still relatively new, and continues to mutate. Plus, there are many defining factors that will influence the threshold for herd immunity including:
- How well the vaccination and prior infection prevents illness and transmission
- Variants that have a higher transmission rate
- Use of measures to interrupt transmission (like face masks and social distancing)
- Population sizes and density
- Other environmental factors
Overall, what we do know now is that achieving herd immunity with safe and effective vaccines makes diseases rarer and saves lives.