Understanding Herd Immunity and Rising Covid Cases in India

Synopsis: Herd Immunity can’t be attained solely by a rise in Covid-19 positive people. It is also dependent on the absolute number of susceptible individuals and the rate of transmission of the virus.

Background:

• The data from various serological surveys in metro cities shows high immunity levels in the metro cities. As per them, Covid-19 had touched: 
  • 56% of population in Delhi by January 2021.
  • 75% in some slums in Mumbai in November 2020.
  • About 30% in Bengaluru in November 2020.
• This indicates attainment of herd immunity but still, the Covid-19 cases are rising.

What is Herd immunity?

• It is a stage of an epidemic in which some members of a population group remain protected from infection. 
• This happens as the majority of those around them have already developed immunity either through vaccination or prior infection.

What is a Serological Survey?

• It is a blood test that seeks to assess the prevalence of disease in a population by detecting the presence of specific antibodies against the virus.
• The test only indicates past infections (which triggered an immune response) and is not used to detect active infections.

Understanding the current spread: –

• The number of daily cases depends on 3 factors: 
  • Number of infectious people in the population (those who have developed immunity)
  • Number of susceptible individuals (those who are not infected)
  • Rate of transmission of the virus – It depends on the nature of the virus and the extent of contact between individuals.
• As per experts, the cases are rising due to:
  • Increased interaction and contact among the population since February 2021. This increased the virus in circulation and led to increased cases in the susceptible population.
  • The high absolute number of Susceptible population among which the virus is circulating. For instance, in this second wave, the affluent class and youth are getting more infected who were less targeted in the first wave.
• Therefore, solely relying on the first factor to check the virus spread will not yield rational results. 

Future Trajectory:

• Firstly, a decline would be observed if it is proved that the virus (even the new strain) is infecting the susceptible population more.
• Secondly, a greater surge could occur if the new strain is again infecting the previously infected individuals. This scenario would also question the efficacy of our vaccines in fighting the virus. Source: Indian Express