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.