A risky strategy? Very. At complete odds with the rest of the globe and the World Health Organisation? Definitely. Politically unpopular? Absolutely. But sensible? Quite possibly. Britain's approach has three core elements: enact social distancing measures much more slowly than other countries; shield at-risk groups like the elderly and sick from contact with the general population; and then let COVID-19 slowly sweep through everybody else. The latter approach is called "herd immunity" – a phrase likely to enter the lexicon shortly in the same way as "flatten the curve". Herd immunity describes a scenario where so many people become resistant to a disease – either through vaccinations or exposure – that it becomes much harder for the virus to spread through the rest of the population. Mass immunity could effectively cause the virus to burn out over the course of one or two seasons, or buy time until a vaccine is developed and distributed. A woman wears a face mask as she visits Buckingham Palace as the outbreak of coronavirus intensifies. Credit:Getty Images Johnson's chief scientific adviser, Sir Patrick Vallance, who along with chief medical officer Chris Whitty has been entrusted by the government to manage the spread of virus, says between 60 to 70 per cent of the population would need to be infected to achieve herd immunity.

"We think this virus is likely to be one that comes year on year ... like a seasonal virus," Sir Patrick said on Saturday. "Communities will become immune to it and that's going to be an important part of controlling this longer term." Officially, the government won't say that herd immunity is actual strategy: "Herd immunity is not our goal or policy. It is a scientific concept," Health Secretary Matt Hancock said on Sunday. Ian Donald, an emeritus professor from the University of Liverpool, suspects the government wants younger, healthier people to get infected right up until the point hospitals begin to reach capacity. At that point, authorities will try to slow – but not stop – infection rates. This could be achieved by closing schools, restaurants and maybe even large offices. A man wearing a mask walks over London Bridge in central London with the Tower Bridge behind. Credit:Getty Still confused? Imagine a bucket of water and a plastic jug with a hole penetrating about half-way up its side. The bucket represents the general population, the jug represents the health system and the hole in the side is for people to leave hospital after they recover. The government wants to tip the bucket into the jug at a rate that brings the water near its capacity but never over it. As people leave hospital or recover at home, immunity builds.

While this is happening, the government would try to isolate the elderly and other vulnerable groups from the rest of the population. There are reports Downing Street is considering a ban on visitors to nursing homes and hospitals for many months. "After a while, most of the population is immune, the seriously ill have all received treatment and the country is resistant," Professor Donald outlined in a long Twitter thread. "The more vulnerable are then less at risk. This is the end state the government is aiming for and could achieve." However, Professor Donald and other experts warn there are big risks. For starters, the jury is still out on whether people infected by COVID-19 will never contract it again. Data on infection rates also needs to be highly accurate and the social distancing measures to slow the rate of infection need to be timed perfectly. The public needs to trust the strategy and be prepared to follow the advice over many months. And the "cocooning" of at-risk groups needs to be highly effective. In an open letter to the government, British Society for Immunology president Arne Akbar raised "significant concerns" about the strategy and its "severe" consequences if vulnerable people aren't properly protected.

Loading Professor Akbar also warned the virus was so new that there were many unanswered questions about how it interacts with the immune system. "For example, we don’t yet know if this novel virus will induce long-term immunity in those affected as other related viruses do not. Therefore, it would be prudent to prevent infection in the first place." Johnson has been critical of blunt measures designed to stop the virus from spreading, accusing some leaders of bowing to political pressure instead of following the science. Italy and Spain have virtually shut down, while Norway, Denmark and Poland have closed their borders. Unlike Britain and Australia, most countries have also closed schools and universities. The government says these measures may well be enacted in Britain but warns their impact on reducing spread will be limited. It argues asking people with symptoms to stay home for seven days could have a much bigger impact on reducing the size of the peak, possibly by as much as 25 per cent.

Professor Donald says the Italian lockdowns aimed at trying to stop as much infection as possible are appealing, "but then what?" He warns such restrictions are not sustainable for months and will have to be relaxed, leading to a new surge in infections. "So they will have to reintroduce the restrictions each time infection rates rise," he says. "That is not a sustainable model and takes much longer to achieve the goal of a largely immune population with low risk of infection of the vulnerable." Researchers from Imperial College and Oxford University seem to agree. They recently released a chart using red, blue and green lines to show three possible options for the next six months. The red line shows a huge spike in cases if the outbreak is not controlled or managed. The blue line shows a rise in infections early in the pandemic, but a major reduction as harsh social distancing measures are enacted. But that blue line rises sharply to form a 'second wave' as soon as the measures are lifted. The green line – which the UK has gambled on – shows more people infected in the short term than the blue line, but in a way that eventually peters out.