##### < Immunity

# What is herd immunity and can it protect me from COVID-19 ?

Text updated on 2020-07-20

**When a sufficiently large proportion of individuals are already immune to an infectious agent, it cannot spread in a population. This is called herd (or group) immunity. In the case of the COVID-19 epidemic, the majority of studies consider that the threshold of herd immunity will be reached when 60-70% of the population is immunized. Other studies that take into account the heterogeneity of SARS-CoV-2 coronavirus transmission indicate that the threshold for herd immunity may be lower, but its precise value is difficult to determine.**

A COVID-19 epidemic progresses in a population when an individual infected with the SARS-CoV-2 coronavirus infects, on average, at least one other. As the epidemic progresses, the likelihood of an infected individual transmitting the virus to an uninfected individual decreases. When the proportion of individuals who have already been infected is large enough, it becomes unlikely that an infected individual can transmit the virus to a new individual and the epidemic declines. From then on, individuals who have not yet been infected are protected indirectly against infection, known as herd immunity. This herd immunity can be achieved through natural infection (if it gives lasting protection to individuals who have been infected) or through vaccination (if a vaccine is successfully developed).

**Estimation of the threshold for herd immunity**

Mathematical models can be used to determine the proportion of the population that needs to be protected against the SARS-CoV-2 coronavirus (after natural infection or vaccination) to achieve herd immunity. This proportion depends on the average number of people infected by a contagious individual in an unprotected population (this is called the baseline reproduction rate or R0). In the simplest models it can be shown that the threshold for herd immunity is 1 - 1 / R0. As many studies have shown that the R0 of CoV-2-SARS is between 2 and 6, it is generally accepted that herd immunity will be achieved when at least 60-70% of the population has been immunized.

**Collective immunity in a heterogeneous population**

This estimate of the threshold for herd immunity is correct if the virus is transmitted homogeneously in the population. In reality, it is known that the transmission of the SARS-CoV-2 coronavirus depends on multiple factors such as age and exposure of individuals. For example, children and young adults have a low susceptibility to infection and are less likely to transmit SARS-CoV-2 than older individuals. As a result, COVID-19 spreads less efficiently in certain categories of the population. Models that take into account this heterogeneity of SARS-CoV-2 coronavirus transmission suggest that the threshold for herd immunity may be lower than expected, although the exact level is still uncertain.

**herd immunity has not yet been reached.**

In many countries, less than 10% of the population has been infected with the SARS-CoV-2 coronavirus. Even assuming that all these people are immune and protected against further infection on a long-term basis, this is insufficient to confer herd immunity on the rest of the population and to stop the current spread of the epidemic. The best way to establish herd immunity will be to conduct vaccination campaigns once a safe and effective vaccine has been developed.

__Sources__

Description of the concept of herd immunity and its implications for COVID-19.

Randolph, H. E., & Barreiro, L. B. (2020). Herd Immunity: Understanding COVID-19. Immunity, 52(5), 737-741.Study showing how heterogeneities in the population can impact herd immunity:

Britton, T., Ball, F., & Trapman, P. (2020). A mathematical model reveals the influence of population heterogeneity on herd immunity to SARS-CoV-2. Science.Study taking into account the heterogeneity of SARS-CoV-2 coronavirus transmission within a population to estimate the threshold of herd immunity.

Gomes, M. G. M., Aguas, R., Corder, R. M., King, J. G., Langwig, K. E., Souto-Maior, C., ... & Penha-Goncalves, C. (2020). Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold. medRxiv.Analyses on the state of the epidemic in Spain:

Instituto de Salud Carlos II, Second national round sero-epidemiology study of SARS-COV-2 infection in Spain [in Spanish] (2020)Analysis of the state of the epidemic in France in May 2020:

Salje, H., Kiem, C. T., Lefrancq, N., Courtejoie, N., Bosetti, P., Paireau, J., ... & Le Strat, Y. (2020). Estimating the burden of SARS-CoV-2 in France. Science.Analyses on the state of the epidemic in Europe:

Flaxman, S., Mishra, S., Gandy, A., Unwin, H. J. T., Mellan, T. A., Coupland, H., ... & Monod, M. (2020). Estimating the effects of non-pharmaceutical interventions on COVID-19 in Europe. Nature, 1-5.