Locking down a week earlier in spring 2020 could have saved 23,000 lives in England during the first wave, a new report has shown.
The peer-reviewed study, published in the journal Science Translational Medicine, found elderly people were nearly three times more likely to die from Covid in care homes than in the community.
In the absence of vaccines at the time, only lockdown from March 23, 2020, drove the R number below 1 consistently, the researchers' modelling showed.
Despite a devastating first wave, on December 2, 2020, England was still far from herd immunity; the reports show 7.9% in the south west had antibodies, rising to 22.5% in London.
One of the report's authors, Dr Marc Baguelin from Imperial College said: “Early intervention is really critical to reducing the number of cumulative deaths. There was also a high burden of Covid-19 deaths in care homes as once an outbreak has begun it is very difficult to reduce transmission.”
Last week, Health Secretary Matt Hancock said ordering an earlier lockdown would have meant going against scientific advice.
Mr Hancock insisted he had scrutinised the advice but "ultimately we didn't know how long people would put up with it and now it seems obvious that people will put up with lockdowns - it was not at all obvious.
"These are huge decisions; to take those decisions against the scientific advice is an even bigger decision to take."
He said: "Challenging the scientific advice is one thing, but overruling a scientific consensus is much harder, especially when the costs of the lockdown are immediate and are obvious."
Mr Hancock has been under pressure following claims by the prime minister's former aide Dominic Cummings that untested patients had been discharged to care homes in England, thereby allowing the virus to spread.
Care home deaths accounted for an estimated 28% of Covid-19 deaths as of August 1, 2020.
The researchers' model showed that while community transmission rates fell during lockdown, transmission within care homes continued to rise, with infection risk peaking in care home residents between March 31 in London and April 20 in the East of England.
The study says that London and the South East of England were already in the grip of a Covid epidemic when a national lockdown was announced on March 23.
Despite a younger population and fewer care homes, London saw a mortality rate of 96.3 per 100,000 during the first wave, compared with the estimated national average of 86.4.
The research builds on the work from Report 41 and highlights the challenges of containing transmission, particularly in care homes.