Moments after the Prime Minister stood down from his lectern having announced the nation’s lockdown, the academic on whose data the decision was based rushed from Downing Street to the Wellcome Trust building in Euston to deliver a jaw-dropping press briefing. In a fifth floor meeting room Professor Neil Ferguson, the country’s foremost epidemiologist and adviser on the outbreak’s trajectory, explained to a small group of science journalists why 20,000 coronavirus deaths in the UK would be a win. The alternative would leave 260,000 dead, he explained. The change in strategy had been necessitated by new data, said Prof Ferguson. The true position of demand on the NHS had only now become evident. Having run the new numbers, he had advised the government to change course. While previously the new social-distancing measures would have been temporary, now they would need to be held in place until such time as a vaccine or treatment was found, perhaps a year or more. If they were lifted early, many more would likely die – and that would be a political choice. Those gathered were shocked. Are we then in a permanent state of outbreak?, one asked. “That's our current understanding,” replied Prof Azra Ghani, a member of the modelling team which is based at Imperial College London. “The only exit strategy from this long term is vaccination or some other kind of innovative technology,” added Prof Ferguson. The briefing was troubling but the data contained in the accompanying report was positively frightening. Academics and MPs had been demanding its release since Thursday, when the government had announced only very limited measures, fuelling suspicions it was pursuing a “herd immunity” strategy in which the bulk of the population would become infected. So what does the data reveal? And what is the new strategy the government is following? Is it effectively the same one China imposed in January and the rest of the world followed suit with like dominoes in the following weeks and months? The data reveals three things. First, it shows for the first time that the pandemic, were it to hit Britain uncontrolled, would top the “reasonable worst cases scenario” for 500,000 deaths that Public Health England has long used for contingency planning purposes. This is a serious matter in regards to contingency planning for future outbreaks. Second, the modelling shows the major pinch-point is a lack of NHS “surge capacity”, especially in critical care or ICU beds. To put it another way, if we had many more beds and ventilators we could afford for more people to catch the virus because we would be able to treat them. Only last week, the Department of Health and Social Care released a chart showing the planned flattening of the epidemic curve below a horizontal line representing NHS capacity. But the new data shows an entirely different picture. Rather than sitting above a flattened curve, the line representing NHS capacity hardly moves above the x axis.

Prof Ferguson explains this pivotal change in his report like this: “In the UK, this conclusion has only been reached in the last few days, with the refinement of estimates of likely ICU demand due to COVID-19 based on experience in Italy and the UK (previous planning estimates assumed half the demand now estimated) and with the NHS providing increasing certainty around the limits of hospital surge capacity.” Expect MPs to interrogate this. China has been throwing up new hospitals since January. Italy’s intensive care crisis has been building for weeks. Britain’s relatively low level of critical care beds (seven per 100,000, compared to Germany’s 30 and Italy 14) has been known about for years. And last week the Telegraph reported that we were 7.5 times short of ICU beds. Can it really be the case that the NHS has only just got round to accurately modelling its surge capacity accurately and if so why? Third, and not unrelated, the report details likely hospitalisation figures by age. Overall it predicts nearly 4.4 per cent of the population will be hospitalised, rising to 27.3 per cent for those over 80. “We assume that 30 per cent of those that are hospitalised will require critical care”, it adds.

These are the data driving the strategic change in direction. Once fed into the model, they predict 260,000 deaths on the government’s original “mitigation” strategy – a mixture of social-distancing measures to slow the spread of infection but not completely interrupt it. “[Mitigation] could reduce peak healthcare demand by two thirds and deaths by half. However, the resulting epidemic would still likely result in 260,000 deaths and therefore overwhelm the health system,” the report says. The new strategy is described as “suppression”. It entails near identical measures – those outlined by the Prime Minister on Monday – but maintains them to hold back the virus. “The major challenge of suppression is that [it]... will need to be maintained until a vaccine becomes available (potentially 18 months or more)”, the report says.

Prof Ghani said: "The new strategy is to try and reduce transmission as intensively as we can now that the epidemic is starting to take off. "If we can do that, we are likely to remain at a level where our health service can cope and we should not exceed in particular the ICU capacity that is available." She added that it would result in an estimated 20,000 deaths, rather than 260,000. Official statistics put the current UK death toll at 71, up from just six seven days ago.

The government is being careful not to say explicitly that it has switched course. It is able to maintain this because the only real difference between the two strategies is the length of time the measures are in place. The chief scientific officer, Sir Patrick Vallance, said yesterday the difference was only “semantic”. Speaking before the Health and Social Care Committee, he said: “It’s a semantic difference whether you’re calling it suppression, delay or mitigation – the aim is exactly the same of: how do you keep this thing down and how do you keep it below the level that you want to keep it, and how do you keep it down for long enough to ensure that you’ve actually managed to achieve that suppression.” It is difficult to read the modelling report and come to the same conclusion. The public reacted with fury last week when the government took much a lighter approach than other countries, allowing people to think that their older friends and relatives were being put at risk.