A week is a long time in a coronavirus pandemic. Within days of Boris Johnson and his advisers announcing that anyone with symptoms of a cold should stay at home for seven days but otherwise live life as normal, the prime minister came out with a series of bombshells that will effectively confine most of the population to their homes.

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What changed was new data on the impact of Italy’s out-of-control epidemic on its health service. Basically, it is catastrophic, with 30% of hospitalised patients having to be admitted to intensive care. The teams of modellers at Imperial College and the London School of Hygiene and Tropical Medicine who advise government crunched those numbers – and the death toll and pressure on the NHS that came out were unacceptable.

Prof Neil Ferguson at Imperial College’s MRC Centre for Global Infectious Disease Analysis and colleagues found that the mitigation strategy, as they called it – or scenario 1 – that the government had just announced would lead to 260,000 deaths. That would be not just deaths from the virus, but from other illnesses that the NHS would be too hard-pressed to treat.

Their modelling looked at all the interventions that might help drive down the infections and deaths. There were five, they said, that would have an impact:

• isolating people with a cough and temperature at home for seven days.

• quarantining families where somebody has symptoms for 14 days, to allow time for any symptoms in the others to show.

• social distancing, involving cutting the normal contacts people make at home, school or work by three-quarters.

• social distancing for everyone over 70, by asking them to stay at home.

• closure of schools and universities.

Last week’s mitigation strategy was about people staying home for seven days with symptoms. It was said that quarantine for families plus keeping the over-70s at home would probably follow. That package would reduce peak healthcare demand by two-thirds and cut deaths by half. But, the researchers said, “the resulting epidemic would still likely result in 260,000 deaths and therefore overwhelm the health system (most notably intensive care units)”.

So now we have scenario 2, which the modellers call suppression. It takes things much further, adopting all the measures except closing schools and universities. And school closures, said Ferguson, are probably also on the cards before too long.

Ferguson and his colleague Prof Azra Ghani liken the new measures to China’s actions, which succeeded in driving down the epidemic to very low numbers. But unlike in China, anything that happens in the UK will be voluntary. The government will need population buy-in for this to work.

The bad news is that although it will keep death rates down to 20,000 or possibly just a few thousand, said Ferguson, we are looking at these social curbs through to July or August – and even when the brakes are taken off, they may have to be slammed back on again. The virus will not have disappeared and could resurge. Only a small proportion of the population will have been infected, recovered and become immune.

The government’s earlier hopes that it could rely on large proportions – maybe 60% – of the population getting ill, getting better and becoming immune to build up some herd immunity in the UK population – something many experts thought was dangerous – are dashed by the new strategy.

Herd immunity is normally created by vaccinating large numbers of children, safeguarding those who cannot be inoculated. Nobody has ever tried to do that by allowing infection with a disease before – and now, it appears to have been recognised that it’s not safe to try.