Video report by ITV News Political Correspondent Shehab Khan
Decisions taken by the government in its response to coronavirus are "not led by science" the chief scientific adviser has said, "they are informed by science".
In an apparent rebuke to comments often used by ministers explaining decisions, Sir Patrick Vallance told the Health and Social Care Committee that a range of options are presented at meetings and ministers are responsible for Covid-19 policy decisions.
In March, announcing the start of daily coronavirus press conference, a Downing Street source said "at all times, we will be led by the science" - a comment which has often been repeated by ministers.
Responding to a question about decisions made on herd immunity and when the UK went into lockdown, Sir Patrick said: "We give science advice, and then ministers have to make their decisions."
"Clearly, what we don't give advice on is absolute precise policy decisions or absolute timings," he added.
He and Deputy Chief Medical Officer for England Dr Jenny Harries faced a grilling from MPs during the lengthy committee appearance which offered up some revealing details, but raised more questions than answers.
Sir Patrick said the evidence of the effectiveness of wearing face coverings in public was "not straightforward" but added they could have a "marginal but positive" impact on reducing the spread of the virus.
He also revealed how asymptomatic transmission - when people pass on infection without symptoms - "may be an important feature of this illness".
ITV News Political Editor Robert Peston said this begs the "hugely important" question of "why mask wearing has not yet been formally recommended by the government".
On testing, which the government has been working to increase, Dr Harries said community testing was stopped in the middle of March because of issues with capacity.
She said: "If we had unlimited capacity, and the ongoing support beyond that, then we perhaps would choose a slightly different approach."
Asked if there was anything he would have changed when he looks back, Sir Patrick told the committee: "I think if we’d managed to ramp testing capacity quicker it would have been beneficial.
"And, you know, for all sorts of reasons that didn’t happen."
It was also revealed in the committee that the main "seeding" of the epidemic in UK was as a result of people coming to the UK from Italy and Spain in UK in early March.
Peston says this raises the question of "why we didn't have a policy of quarantining those returning from countries where there were significant outbreaks, as did happen in other nations".
The committee also heard how the decision to impose lockdown on the UK was taken by Boris Johnson after he was told guidance needed to be toughened up and combined with “hard-hitting emotional messaging”.
At the Sage meeting on March 23, the day the PM announced the new policy, a paper drawn up by behavioural science experts said: “The perceived level of personal threat needs to be increased among those who are complacent, using hard-hitting emotional messaging.
“To be effective this must also empower people by making clear the actions they can take to reduce the threat.”
Also at that meeting Mr Johnson was told evidence suggested intensive care units across the country would be swamped if more stringent measures were not imposed.
Sir Patrick also clarified the idea of 'herd immunity', which was spoken about early in the crisis but has since been criticised as dangerous.
Asked about his comments back in March on herd immunity, Sir Patrick apologised if he was not clear in his explanation then.
He said: “I should be clear about what I was trying to say, and if I didn’t say this clearly enough then I apologise.
"What I was trying to say was that, in the absence of a therapeutic, the way in which you can stop a community becoming susceptible to this is through immunity and immunity can be obtained by vaccination, or it can be obtained by people who have the infection.”
He reiterated that, as yet, “exactly what degree of protection you get from natural infection” is not yet known.
He added: “I think expectation is that antibody responses will correlate with immunity to some degree, maybe very high, may not be so high, and that some degree of protection, whether it’s to reduce the severity of the disease, or to reduce the overall effect of the disease, I think, and the ability to catch the disease, I think we’ve still got some work to do to find out about that.”
On immunity, Sir Patrick said evidence from around the world suggests the vast majority of people who have had the infection have “some form of antibody response”, adding “so that looks quite promising”.
He said: “What we don’t know is the degree of immune protection that provides.”
He said “some degree of protection” would be expected, but that it “almost certainly” will not provide absolute immunity.
Whether someone can still carry the virus and be infectious after having developed antibodies also remains unknown, he said.
Giving a potential timeframe for immunity based on experience from other coronaviruses, he said: “It may last for one, two, three years, but not for many, many years.”
Coronavirus: Everything you need to know