The UK could face indefinite lockdown without a vaccine against coronavirus, scientists have warned.

Their modelling of how the disease could sweep through the country, kill more than 250,000 Britons and overwhelm the NHS forced Boris Johnson’s change in tack today.

The scientists, from Imperial College London’s Centre for Global Infectious Disease, said the world was now in uncharted territory with the strict measures on limiting social contacts and quarantining households only able to “buy time” rather than stop the virus spreading.

They estimated the new measures may mean the NHS can cope but would still lead to up to 20,000 deaths.

Their modelling for the government, which has been shared with France and the US, was based on new information from Italy. The data showed around 30 per cent of Britons could end up needing intensive care – meaning the NHS would be overwhelmed by the start of April.

Ultimately around 260,000 people could have died without the government’s new measures today.

But despite the government’s steps, the scientists warned there was no end in sight for the world without a vaccine. They said countries would have to go through repeated cycles of restrictions being lifted and re-imposed.

Professor Neil Ferguson, professor of mathematical biology, said the government had wrestled with the idea of adopting strict measures and then going back to normal. He said: “We don’t think that’s now possible.

“It does leave the world in a quandary that what we would expect to happen is when we lift these measures that transmission would resume.”

He warned that the modelling showed that restricting people’s social lives would in effect only buy the world “a breather”, adding this was the “main downside of where the world sees itself now”.

He added: “We may well be ending up in a really quite different world for at least a year or more.”

Asked if this was unparalleled in human history, Professor Ferguson said it was, adding: “The idea of controlling a virus like this, while still having a vaguely normal society does put us in uncharted territory.”

However, he said there was “a lot of research and developments on how can we successfully suppress transmission without shutting down society” that may offer countries a way out of never-ending cycles.

His colleague Professor Azra Ghani, a professor of infectious disease epidemiology added: “We have explored a scenario where these measures stayed in place for five months, which is what is taking us through to the summer. We haven’t found any way, at least in our understanding of this so far, that we can ever release these methods until some other intervention can be put in place.

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“So really, we are essentially waiting for a vaccine. A vaccine is not five months away. We know it’s at least 12 to 18 months away. So we will have difficult choices to make.

“If the transmission characteristics of the virus stay as they are, then I think we are in this situation. But it’s a situation we face and the world as a whole faces.”

She said the estimate of 260,000 deaths was not a worst-case scenario but the “best estimate” of the situation before the measures announced by the government today.

Scientists around the world are now watching China as it emerges from lockdown, to see what its rate of transmission is – that is, how many people are being passed the virus per infected person.

Under the modelling research, it is thought around a third of infections come from within people’s own family and homes, while another third is via their workplace and the final third is other social contact.

The British government is trying to stop the virus spreading by cutting two-thirds of transmission scenarios.

On school closures, Professor Ferguson said this remained a likely option for the government and it does help to prevent spread. But he added there were significant unintended consequences of closing schools, with around 25 to 30 per cent of NHS nurses having a child in school who would need care.