herd-immunity

Context

Britain has announced a different strategy to tackle the Corona situation by containing the spread of the virus but not suppressing it completely. 

  • The aim in Britain would be to allow immunity to build up among members of the population who are least at risk of dying from COVID-19.

What is herd immunity ?

  1. As per the World Health Organisation (WHO), when a sufficient percentage of a population is vaccinated against a disease, it slows the spread of disease. It is also referred to as community immunity or herd protection.
    1. The premise is that if a certain percentage of the population is immune, members of that group can no longer infect another person. 
    2. This breaks the chain of infection through the community (“herd”), and prevents it from reaching those who are the most vulnerable.
  2. The decline of disease incidence is greater than the proportion of individuals immunized because vaccination reduces the spread of an infectious agent by reducing the amount and/or duration of pathogen shedding by vaccines, retarding transmission.
  3. The approach would require those exposed to the virus to build natural immunity and stop the human-to-human transmission. 

Source: Immunology.Org

How does it work?

  • The scientific principle goes by the fact that presence of a large number of immune persons in the community, who will interrupt the transmission, provides indirect protection to those who are not immune.
  • To estimate the extent of spread and immunity, a measure is used which is called the basic reproductive number (R0). 
    • This indicates how many persons will be infected when exposed to a single case.
    • An R0 of more than 1 indicates one person can spread the infection to multiple persons.
    • The RO for COVID-19 ranges between 2 and 3.

Immunisation level of Community

Introduction of infectious cases with RO value of 1

No immunisation

  • There is a possibility of the entire community being infected, with a few exceptions.

Some persons protected under immunisation

  • Only these immunised persons will not be infected when at least two infectious cases are introduced in the community.

When the majority of the community is immunised

  • The spread can take place only in exceptional cases, like in the elderly or other vulnerable persons. 
  • Even in such a situation, the immunised persons protect the non-immunised by acting as a barrier which is herd immunity.

When do we know that a population has achieved herd immunity?

  1. It is dependent upon multiple factors.
  1. Effectiveness of  the vaccine for a given disease 
  2. Long-lasting immunity from both vaccination and infection
  3. Populations who  form critical links in transmission of the disease.

 

  1. Mathematically, it is defined on the basis of a number called herd immunity threshold, which is the number of immune individuals above which a disease may no longer circulate. 
  1. The higher the R0, the higher the percentage of the population that has to be immunised to achieve herd immunity.
  2. Polio has a threshold of 80% to 85%, while measles has 95%. With the current data for COVID-19, experts have estimated a threshold of over 60%
  3. That means more than 60% of the population needs to develop immunity to reach the stage of herd immunity.