What is Herd Immunity?
Scientific article summary by Niamh Mortimer
What is herd immunity and is it achievable for coronavirus disease 2019?
Acquired immunity is at the level of the individual through natural infection with a pathogen or through immunisation with a vaccine. Herd immunity, on the other hand, stems from the effects of individual immunity scaled to the level of the population. It is the indirect protection from infection for susceptible individuals when a large enough proportion of the population are immune.
Vaccination programs aim to establish herd immunity so that those who cannot be vaccinated – the very young or immunocompromised – are still protected against the disease.
The point at which the proportion of susceptible individuals falls below the threshold needed for transmission is known as the herd immunity threshold.
The herd immunity threshold depends on the R0 (R number) – which refers to the average number of secondary infections caused by a single infectious individual introduced into a completely susceptible population. Find out more about what an R number is in Niamh's previous blog post.
Once the herd immunity threshold is reached, how well it works depends on the strength and duration of the immunity acquired.
For the measles vaccine, where lifelong immunity is induced, herd immunity is highly effective – however, this situation is relatively rare – as immunity for many other infectious diseases, such as coronavirus disease 2019, change over time.
This makes herd immunity less effective and over time, outbreaks can still occur.
Even if the herd immunity threshold is surpassed, pockets of susceptible individuals are still at risk from local outbreaks.
Population structure, variation in transmission dynamics between populations, and changes in immunity will lead to variation in the extent of indirect protection of people who are more susceptible to disease by herd immunity.
In order to approach widespread herd immunity for coronavirus disease 2019, two possible things could occur – a mass vaccination campaign, which would not happen any time soon, or allow enough people to get the virus naturally.
The consequences of the latter approach are catastrophic – a large fraction of the human population would need to become infected with the virus, causing millions of deaths. Therefore, establishing herd immunity should NOT be the ultimate goal.
Instead, emphasis should be placed on policies that protect the most vulnerable groups such as mask wearing and social distancing.
Pathogen: bacteria, virus or other organisms that can cause disease.
Acquired: to get/obtain.
Susceptible: someone who is likely to catch a disease.
Immunocompromised: individuals with an impaired immune system.
Randolph HE & Berreiro LB. Herd immunity: Understanding COVID-19. Immunity, 2020; 52(5): 737–741. https://pubmed.ncbi.nlm.nih.gov/32433946/