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Last week, the UK'’s Government Chief Scientific Adviser Sir Patrick Vallance hinted at a strategy that would allow the novel coronavirus to infect 60% of the country'’s population so that a degree of “"herd immunity”" could be achieved.

Following widespread criticism, and with Imperial College London projecting a dire scenario if the pandemic remains uncontrolled, the UK has now retracted — and is looking at self-isolation for the elderly. Herd immunity refers to preventing an infectious disease from spreading by immunising a certain percentage of the population. While the concept is most commonly used in the context of vaccination, herd community can also be achieved after enough people have become immune after being infected.

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.

However, the discussion on herd immunity to fight COVID-19 in the UK has not been based on this conventional definition. The UK government had wanted the entire population to be exposed to the novel coronavirus infection, so that the majority could develop Immunity to COVID-19.

The scientific principle is that the 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, epidemiologists use a measure called the '‘basic reproductive number’' (RO). This indicates how many persons will be infected when exposed to an single case; an R0 of more than 1 indicates one person can spread the infection to multiple persons.

Scientific evidence shows that a person with measles can infect around 12-18 persons; and a person with influenza can infect around 1.2-4.5 persons, depending on the season. On the basis of the available evidence from China, and according to various experts, R0 COVID-19 ranges between 2 and 3.

There are three ways in which an infection can spread in a community. The first scenario looks at a community that is not immunised. When two infectious cases, both with an R0 value of 1, are introduced, there is a possibility of the entire community being infected, with a few exceptions.

In the second scenario, there may be some persons who have been immunised; and only these immunised persons will not be infected when at least two infectious cases are introduced in the community.

The third scenario is when the majority of the community is immunised. So, when two infectious cases are introduced, 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 depends on multiple factors: how effective the vaccine for a given disease is, how long-lasting immunity is from both vaccination and infection, and which populations 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.

According to the passage, which infection has the highest R0?

A) Measles

B) Corona Virus

C) Influenza

D) Not stated in the passage