Herd immunity is a form of indirect protection from infectious disease that occurs when a high percentage of a population has become immune to it, either through vaccination or previous infection.
To cause an outbreak, a virus must have a continuous supply of potential victims whose immune system do not know how to fend it off before it makes them ill.
When a virus or bacteria enters the body the immune system creates substances called antibodies, which are designed to destroy one specific type of bug.
People who have these antibodies normally enjoy long-term protection, or immunity, against an illness.
If nobody is immune to an illness – as was the case at the beginning of the coronavirus outbreak – it can spread like wildfire.
However, if, for example, half of people have developed immunity there are only half as many people the illness can spread to.
As more people become immune, the bug finds it harder to spread until its pool of victims becomes so small it can no longer spread at all.
The threshold for herd immunity is different for various illnesses, depending on how contagious they are – for measles, around 95 per cent of people must be vaccinated to it spreading.
For polio, which is less contagious, the threshold is about 80-85 per cent, according to the Oxford Vaccine Group.
Experts say herd immunity will only work for Covid-19 if about 60 to 70 per cent are immune.
WHY IS IT CONTROVERSIAL?
Herd immunity is considered a controversial route for getting out of the pandemic because it implies encouraging the spread of the virus, rather than containing it.
When UK Government scientists discussed it in the early days of the pandemic, it was met with criticism and therein swept under the carpet.
The Chief Scientific Adviser Sir Patrick Vallance said at a press conference on March 12, designed to inform the public on the impending Covid-19 crisis: ‘Our aim is not to stop everyone getting it, you can’t do that. And it’s not desirable, because you want to get some immunity in the population. We need to have immunity to protect ourselves from this in the future.’
Sir Patrick has since apologised for the comments and said he didn’t mean that was the government’s plan.
In a Channel 4 documentary aired in June, Italy’s deputy health minister claimed Boris Johnson had told Italy that he wanted to pursue it.
The Cabinet Office denied the claims made in the documentary and said: ‘The Government has been very clear that herd immunity has never been our policy or goal.’