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First we were told to work from home (16 March). Then came the school closures (18 March), followed by restaurants and pubs (20 March). And then, on 23 March - the full lockdown; no one allowed to leave his or her own home except for essential purposes.

This decision changed our lives for nearly six weeks. As we consider how long these restrictions will continue was that decision worth it?

New estimates suggest that the lockdown may have come at a crucial time in our struggle against the virus. According to our modelling work, published in , on day one of the lockdown, the COVID-19 epidemic in the United Kingdom was likely on the verge of exploding, with around 100,000 people being infected every day.

How did we get to this number? We used a database called the First Few Hundred (FF100) that kept track of reported COVID-19 cases during the first few weeks of the epidemic. Reported is key as this won’t be the case for everyone who becomes infected with COVID-19.. Some are asymptomatic, others are just never tested. So we only looked at those recorded as being admitted to critical care, since they must all be symptomatic, and a much higher proportion of these are tested.

We also used the database CHESS (COVID-19 Hospitalisation in England Surveillance System), which NHS Trusts use to record their number of COVID-19 critical care admissions.

All this information was matched against numbers coming out of China and the USA (and later Italy) telling us the percentage of infected cases who needed critical care. Put together, our model suggests that fewer than 1% of infected people in the UK were being reported as COVID-19 cases – and that the true number of new cases being added daily was creeping towards 100,000 in that crucial week of March 16-23.

The UK, as many other countries, is going through a very difficult time; thousands have died on ventilators or in their homes over the last few weeks. But our analysis suggests that it could have been much worse – 100,000 new cases can become half a million new cases after a week of unmitigated exponential growth.

New data we are getting from household surveys indicates that the daily number of contacts people made after the lockdown, so this intervention must have put a crimp in virus transmission.

The week that changed everything may have come in the nick of time.

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