The UK’s chief scientific adviser Sir Patrick Vallance has said building up “herd immunity” is part of the government’s response to the coronavirus pandemic.

“Communities will become immune to it and that’s going to be an important part of controlling this longer term,” he told the BBC, adding: “About 60 per cent is the sort of figure you need to get herd immunity.”

The prospect of 60 per cent of the UK population (about 40 million people) coming down with Covid-19 has prompted some scary headlines today.

Here’s what it all means, courtesy of experts contacted via the Science Media Centre.

What is herd immunity?

If enough people become immune to an illness – because they’ve already had it or been vaccinated – the disease can’t spread anymore, so even people who aren’t immune probably won’t catch it.

Let’s say you come down with a virus. You’re contagious for a few days and you are in contact with a certain number of people. The virus has a certain number of chances to infect others.

The more people you meet who are resistant to infection, the less likely the illness has to infect someone who is susceptible.

After a while the virus will not be able to infect enough people to sustain itself in a population.

It will stop spreading, and even people who are not resistant will be out of danger – although herd immunity does not give those individuals the same level of protection as a vaccine.

Why 60 per cent?

The more infectious an illness, the more people we need to achieve herd immunity.

With a highly contagious disease like measles – where the average carrier can expect to infect more than 12 other people – we need more than 90 per cent of the population to be immune to get herd immunity.

Influenza is much less transmissible than measles. The average infected person only passes it to 1.3 others. Only around a quarter of the population need to be immune to flu to achieve herd immunity.

The evidence so far suggests coronavirus is in the middle: more transmissible than flu but less than measles. We think the average person with Covid-19 infects two to three others, although we don’t know the exact number.

That’s why Sir Patrick Vallance thinks that around 60 per cent of people need to be immune to coronavirus to achieve herd immunity – although other experts put the percentage slightly higher.

Is that as terrifying as it sounds?

Since we don’t have a vaccine against Covid-19, the scary thought is that there is nothing to stop at least 60 per cent of the population catching the disease eventually.

At that point, we might expect to see the benefits of herd immunity kick in, but only after tens of millions of people have been infected.

And if 1 per cent of infected people died, we would be talking about hundreds of thousands of deaths.

But experts say an outcome as dramatic as this is not inevitable in the UK, for several reasons.

We can slow the rate of transmission

The government’s advice so far – for people to wash their hands thoroughly, and for certain groups of people to self-isolate – should have the effect of reducing the number of other people infected by the average coronavirus patient.

Professor Matthew Baylis from the Institute of Infection, Veterinary and Ecological Sciences at the University of Liverpool said: “By reducing the number of people that one person infects, on average, then we lower the point at which herd immunity kicks in.  If we reduce it to 1.3, Covid-19 becomes more like flu.

“Herd immunity kicks in when about one quarter of the population has had the disease and is now immune.

“So, from an epidemiological point of view, the trick is to reduce the number of people we are in contact with (by staying more at home), and reduce the chance of transmission to those we are in contact with (by frequent hand washing) so that we can drive down the number of contacts we infect, and herd immunity starts earlier.”

There’s a catch, though: “We will need to sustain this until we have a vaccine: only at that point can we return to normal behaviour patterns, with herd immunity now achieved by vaccination, not disease.”

We can try to protect the most vulnerable

Again, the government has signalled that it will announce new measures to try to protect older people, who may be at the most risk of becoming seriously ill if they catch the coronavirus.

It’s theoretically possible that younger and healthier people could provide herd immunity for those elderly people, meaning they would get some protection from the disease without having to catch it.

Professor Peter Openshaw, Professor of Experimental Medicine at Imperial College London, said: “Generating herd immunity in the population, and particularly in younger individuals who are less likely to experience serious disease, is one way to stop the disease spreading and provide indirect protection to older, more vulnerable groups.”

Professor Martin Hibberd, Professor of Emerging Infectious Disease, London School of Hygiene & Tropical Medicine, said: “In a good scenario, the 70 per cent infected, recovered and immune would be people who were expected to have mild disease and the 30 per cent who were vulnerable to severe disease would be protected by this herd immunity.”

What we don’t know

The experts stress that there is still a lot of uncertainty about how the coronavirus behaves, meaning there are no guarantees about herd immunity.

We don’t know if people who catch the coronavirus will become immune for life afterwards.

It’s not clear whether people can become infected again, or whether the disease will behave like flu in the future and mutate into new strains to which people are not immune.

See here for the latest NHS advice for the public on the coronavirus.