As the UK introduces fresh restrictions on social contact to curb the spread of coronavirus, controversy continues to rage about whether the government had initially considered trying a very different approach.
At the start of the pandemic, the government's chief scientific adviser, Sir Patrick Vallance, spoke about "herd immunity" - the idea that once enough of a population had been exposed to the virus, they would build up natural immunity to it.
Sir Patrick and the government have both insisted this was never official policy. The government also denies there was any delay in locking down the country, as some critics have suggested.
Emails obtained by the BBC reveal the alarm among the government's top scientific advisers at the reaction to Sir Patrick's words.
In one email from March, Sir Patrick asks for help to "calm down" academics who have expressed anger at his repeated references to herd immunity and the delays in announcing a lockdown.
The material, obtained by the BBC via a Freedom of Information Act request, consists of every email sent by Sir Patrick and chief medical officer for England, Professor Chris Whitty, from the start of February to the start of June, containing the words "herd immunity".
There is no reference in any email until after 13 March, when Sir Patrick discussed herd immunity in a number of media interviews.
"Our aim," he told BBC Radio 4's Today programme that morning, is to "try and reduce the peak - not suppress it completely, also because most people get a mild illness, to build up some degree of herd immunity whilst protecting the most vulnerable".
To many, his words appeared an unequivocal endorsement of herd immunity. They also appeared to explain the government's reluctance to order the kind of lockdowns and social distancing measures that were already in place in many other countries, despite cases increasing and worrying scenes in hospitals in Italy.
Speaking to Sky News on the same day, Sir Patrick talked about not suppressing the virus completely, to help avoid "a second peak," and also to "allow enough of us who are going to get mild illness to become immune to this".
When asked how much of the British population would need to contract the virus for herd immunity to become effective, he calmly replied "probably around 60%".
With an approximate 1% case fatality rate, the interviewer responded, that would mean "an awful lot of people dying".
At the time, there was no strong evidence that being infected by coronavirus would result in long-lasting immunity.
The following day, a group of more than 500 academics published a joint letter, criticising the lack of social distancing restrictions imposed by the government, adding that "going for 'herd immunity' at this point does not seem a viable option, as this will put the NHS at an even stronger level of stress, risking many more lives than necessary".
In an email to Sir Mark Walport, the UK's former chief scientific adviser, discussing the scientists' letter, Sir Patrick suggests the message in response should be "herd immunity is not the strategy. The strategy is to flatten the curve… and to shield the elderly… As we do this we will see immunity in the community grow".
Sir Patrick appears clearly rattled by the backlash to his use of the phrase.
In response to an email titled "Covid-19 and herd immunity", from an academic, he writes brusquely "No it is NOT the plan". He does not, however, explain his previous references to herd immunity.
On the same weekend, he writes to a colleague, "anything you can do to calm our academic friends down over herd immunity would be greatly appreciated".
Sir Mark Walport told the BBC he believed the interviews had been misunderstood.
He suggested what Sir Patrick had meant when saying it was not desirable to completely suppress the virus, was that it would be so "draconian and difficult to do that it would not be achievable".
Others, however, have suggested, despite the denials, that "herd immunity" was indeed the strategy for a period of time.
The first public use of the term by a UK official appears to be in a BBC interview on 11 March with Dr David Halpern, chief executive of the government-owned Behavioural Insights Team, known as the "nudge unit", and a member of the Scientific Advisory Group for Emergencies (Sage).
He told the BBC: "You'll want to protect those at-risk groups so that they basically don't catch the disease and by the time they come out of their cocooning, herd immunity's been achieved in the rest of the population."
However, the emails obtained by the BBC suggest herd immunity was under discussion as early as January.
In one email from April, Prof Whitty confers with colleagues about a report in the Times newspaper - in which an unnamed senior politician says he had conversations with Prof Whitty in January that "were absolutely focused on herd immunity".
In the email, Prof Whitty complains he has been misrepresented, stating he never thought herd immunity "was actually a sensible aim of policy", but suggesting the concept was talked about when answering "questions put to me by ministers".
In another email to the president of the Faculty of Public Health, which sets standards for health professionals - who had raised questions about the lack of testing - Prof Whitty insisted "the government had never pursued a 'herd immunity strategy'". He added it would not make "epidemiological or public health sense".
In a statement, a government spokesman said the emails "make clear… herd immunity has never been a policy aim".
However, that is unlikely to put an end to the controversy, particularly given the lack of references to herd immunity prior to the interviews given by Sir Patrick on 13 March.
Campaigners representing families of some of those who died in the pandemic are calling for a public inquiry into the government's response to the disease.