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 ?
- 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.
- The premise is that if a certain percentage of the population is immune, members of that group can no longer infect another person.
- This breaks the chain of infection through the community (“herd”), and prevents it from reaching those who are the most vulnerable.
- 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.
- The approach would require those exposed to the virus to build natural immunity and stop the human-to-human transmission.
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
- 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?
- It is dependent upon multiple factors.
- Effectiveness of the vaccine for a given disease
- Long-lasting immunity from both vaccination and infection
- Populations who form critical links in transmission of the disease.
- 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.
- The higher the R0, the higher the percentage of the population that has to be immunised to achieve herd immunity.
- 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%.
- That means more than 60% of the population needs to develop immunity to reach the stage of herd immunity.