Coronavirus: Did 'herd immunity' change the course of the outbreak?
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Prof Whitty argued that beginning social distancing measures "too early" would risk people becoming tired of them and public compliance waning.
And Sir Patrick began to talk about the concept of herd immunity.
Speaking about the coronavirus he said: "It's not possible to stop everybody getting it and it's also actually not desirable because you want some immunity in the population.
We need immunity to protect ourselves from this in the future."
Mr Johnson reiterated that people should remember to wash their hands.
image copyright Getty Images Sir Patrick was on BBC Radio 4's Today programme.
He said the thinking behind the government's approach was to try to "reduce the peak", and because most people would only get a "mild illness", to "build up some degree of herd immunity… so that more people are immune to this disease and we reduce the transmission".
At the same time, he said, the vulnerable would need to be protected from the virus.
"So that's the flattening of the peak.
You can't stop it, so that you should end up with a broader peak, during which time you would anticipate that more people will get immunity to this and that in itself then becomes a protective part of this process."
He said previous epidemics had shown that measures to strongly suppress the virus risked it bouncing back when they were ended.
Herd immunity "Herd immunity" is a concept describing the point at which a population has developed protection against a disease.
But with any new virus it's impossible to say how long it will take to develop a vaccine, if ever.
The other way is for people to catch the disease and build up some form of immunity.
If exposed to the virus again, it is assumed they have protection.
And if most of the population catches the disease, many thousands might die.
On 13 March, Sir Patrick stated that about 60% of the population would need to become infected for society to have "herd immunity" - effectively some 40 million people in the UK.
"Communities will become immune to it and that's going to be an important part of controlling this longer term," he told Sky News.
A spokesman for the Department of Health and Social Care said Sir Patrick's comments had been misinterpreted.
"Herd immunity is not part of our action plan but is a natural by-product of an epidemic," he said.
That same evening, the Telegraph website published an article by the Health Secretary Matt Hancock.
"We have a plan, based on the expertise of world-leading scientists.
Herd immunity is not a part of it.
That is a scientific concept, not a goal or a strategy."
On Sunday 15 March, the health secretary appeared on the BBC's Andrew Marr and Sky's Sophy Ridge programmes, restating that herd immunity was not the government's policy.
image copyright Getty Images The Telegraph website said the government had appeared to "U-turn on the idea".
The Express said herd immunity had been abandoned after "a huge backlash".
But despite repeated government denials, the BBC has learned that on 13 March, when Sir Patrick Vallance was outlining the government's approach to tackling the virus, herd immunity was being discussed at the heart of the health service.
"We want people to be infected with Covid-19," the notes say.
"The best way of managing it is herd immunity and protect the vulnerable."
Mr Enright was clear where the idea had come from, according to the notes.
"What it shows is that at the heart of the government's response there was no clear command-and-control structure as to how we were going to manage this outbreak.
"It's very important to get these facts on the record about herd immunity, because history is being rewritten at the moment and herd immunity is being written out of the history."
A government spokesperson said: "This is a new virus and an unprecedented global pandemic, and our strategy to protect, delay, contain, research and mitigate was clear from the outset.
It is categorically wrong to suggest herd immunity was the government's aim."
Suppression versus mitigation Some critics believe it is problematic if herd immunity was part of the government's thinking at the time.
Sir David King, former chief scientific adviser to the government between 2000 and 2007, has been a vocal critic of the UK government's efforts to fight coronavirus.
"I can only give you one rational explanation for this tragedy and that is that they had decided to go for herd immunity.
We did hear the phrase 'herd immunity', although the government subsequently somehow denied that they were doing that."
"We made the decisions at the time on the guidance of Sage [the government's scientific advisory group], including Prof Ferguson, that we thought were right for this country," he said.
The BBC has spent months speaking to more than a dozen scientists advising the government on its response to the outbreak to try to find out just how important the concept of herd immunity was to the scientific thinking that drove government strategy at the beginning of the outbreak.
Some of Sage's papers model different ways of responding to the outbreak.
"If a tsunami is coming we don't try to stop the tsunami, we just try to ensure that the minimum number of people are harmed by it."
"Herd immunity was on the table," says Prof Ian Hall, an epidemiologist, head of the University of Manchester modelling team, and a member SPI-M who has also attended multiple Sage meetings.
He described it "as a concept", a technical term typically used when discussing the spread of disease.
Dr Hall says SPI-M discussed a range of options to be able to prevent the NHS being overwhelmed and buy it time to prepare, while the population achieved "some sort of herd immunity".
Herd immunity can be achieved by an uncontrolled spread of infection which can happen quickly, but that would overwhelm the NHS.
Hall says this was never realistic because the government would always act to reduce the number of deaths.
But it can also be achieved by measures to mitigate the disease which look to protect lives, in the absence of any vaccine, and build up immunity over a long time.
Jeremy Hunt, chairman of the health and social care select committee, has criticised the scientific advice at the start of the pandemic, calling it "wrong".
"Ministers were given the choice of extreme lockdown or mitigated herd immunity," he recently told the Times newspaper.
He said that Sage did not model the adoption of a testing regime used in previous coronavirus outbreaks in East Asia.
image copyright Getty Images The need to "protect the vulnerable" had been mentioned repeatedly by the government and its advisers.
Graham Medley, chairman of SPI-M, gave an illustration of the challenges of delivering this when explaining herd immunity on BBC's Newsnight on 12 March.
He argued that theoretically - although impractically - one way to achieve it would be to move all vulnerable people to the north of Scotland, and the rest of the population to Kent.
That way "a nice big epidemic" in Kent would allow herd immunity and the re-mixing of the entire country again, he said.
"We can't do that, so what we're going to have to try and do, ideally, is manage this acquisition of herd immunity and minimise this exposure of people who are vulnerable."
Health Secretary Matt Hancock has on many occasions defended the government's handling of the outbreak in care homes, insisting that from the beginning it had tried to throw a "protective ring" around them.
Questions Four months on from the introduction of the lockdown, scientists remain unsure how long any immunity from coronavirus might last.
No-one knows when an effective vaccine will come.
But Dr Hall says the UK's response will ultimately end with herd immunity.
"Without a vaccine we will go on with transmissions at a relatively low level hopefully and with contact tracing and all the other interventions to mitigate the disease, we will eventually achieve herd immunity one way or another."