# Complexities of herd immunity

Note4Students

From UPSC perspective, the following things are important :

Prelims level: Herd immunity

Mains level: Paper 2- Herd immunity and issues related to it

#### What is herd immunity

• The herd immunity concept is based on lowering the number of susceptible individuals.
• If sufficient individuals in the population are immune either through vaccination or a prior exposure, then the number of susceptible individuals drops.
• For example, if the immune population is 70%, then the susceptible population is 30%.

#### Does herd immunity really protect from subsequent waves?

• The number of daily cases depends on three factors: The number of infectious people in the population, the number of susceptible individuals, and the rate of transmission of the virus.
• The rate of transmission is dependent on the nature of the virus and the extent of contact between individuals.
• So, if the rate of transmission increases due to change in social behaviour and increased contact then even with a large percentage of the immune population, a significant number of daily cases can result.
• The “herd immunity” number is not a static number but it changes depending on the rate of transmission of the virus and the extent of virus present.

#### Estimating exposures in metro cities

• Serosurveys indicated that Covid had touched 56% of population in Delhi by January; 75% in some slums Mumbai in November, and about 30% in Bengaluru in November.
• The population touched by Covid can also be estimated by the Infection Fatality Rate (IFR).
• This is the total number of deaths divided by the total people infected. In India, the estimate is 0.08%.
• So this number can be used to back-calculate the number of infections based on the number of deaths in the different cities.
• The table given below shows the number of people exposed to Covid in some metros until January 31 using the method above.

#### What are the reasons behind the recent surge

• The reasons behind the recent surge are not fully understood.
• The one factor that is not in doubt, however, is that interaction and contact with the population has increased since February.
• Such increased contact increased the virus in circulation and led to increased cases in the susceptible population.

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