Herd immunity: basic concepts, epidemiological consideration and its application during pandemic like COVID-19

Smita Sinha, Rishita Chandra


Herd immunity is referred as the indirect protection from infection caused to susceptible individuals when a sufficiently large population of immune individuals exist in a population. The transmission dynamics of a pathogen needs to be well characterized. The basic reproduction number R0 and the effective reproduction number Re are the centric concept in the epidemiology of an infectious disease. The point at which the proportion of susceptible individuals falls below the threshold needed for transmission is known as the herd immunity threshold, and is calculated by 1 – 1/R0. The vaccination programmes and herd immunity complement each other. Through vaccination, a randomly mixed population can be considered immune, once it reaches herd immunity threshold. The proportion of immune individuals should exceed (R0 − 1)/R0, or 11/R0, in order to bring decline in the infection incidence. The vaccine based eradication programmes have been successful in achieving herd immunity for various infectious diseases. As the transmission dynamics of SARS-CoV-2 is not well understood and there are no vaccines available, the objective of establishing herd immunity seems to be theoretical. The previous outbreak of SARS-CoV had shown genome instability and hence, to comment over the indirect protection was not found to be appropriate. In the current pandemic caused by a novel virus of the same family, when the availability of vaccines might take 12-18 months, exposing the population to encounter infection with the vision of achieving indirect protection can appear to be detrimental for the nations where health care infrastructure is already not robust.


Herd immunity, SARS-CoV-2, Coronavirus infections, Basic reproduction number

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