On 12 March Boris Johnson warned the country that, as a result of Covid-19, “families are going to lose loved ones before their time”, yet took little immediate action to stop the virus spreading. Four days later, on 16 March, his tone was far more urgent, as the prime minister requested everyone “to stop non-essential contact with others and to stop all unnecessary travel”. The most vulnerable, especially those over 70, were told to prepare to stay home for up to 12 weeks. After that, the measures escalated; the economy was effectively shut down and all but the most essential activities prohibited.

What explains the sudden change of course? The established narrative suggests that the government’s strategy for managing the pandemic initially depended on “herd immunity” – a policy tantamount to doing nothing to contain the spread of the virus. But the combination of public revulsion at the government’s apparent readiness to let tens of thousands get sick and die, combined with a frightening report published by Imperial College London about what would happen if stronger measures were not adopted (with a death toll of up to 250,000), forced the government to embrace new containment strategies.

There is a kernel of truth to this narrative. There is a real concern among scientists and politicians that the country might succeed in seeing off the first wave of infections through extraordinary efforts, only to be faced with a more devastating second wave. Given the time taken to prepare and test a reliable vaccine, the more the first wave leaves a large proportion of the population with a natural immunity, the better placed we are to cope with a resurgence of Covid-19. There is a potential trade-off between protecting as many people as possible from the first wave and being best prepared for the second. What became apparent to government advisers in those few days between 12 and 16 March was that the sort of short-term measures required to suppress the spread of Covid-19 could no longer be delayed.

The government had always intended to launch measures to contain the spread of the infection. Herd immunity was always a secondary objective rather than a core strategy, but was rather an attempt to explain why ministers had not introduced an early ban on large social events, such as concerts and sporting fixtures. But as the world saw the unfolding tragedy caused by Covid-19 in Italy, the UK government’s response became more urgent and the public was prepared to accept tougher measures that might have been resisted a week earlier.

It is possible to understand the sequence of events in those few days between 12 and 16 March through government records. In response to claims that Downing Street was relying on flawed and callous advice, the Scientific Advisory Group for Emergencies (Sage) published key documents from its deliberations since the early days of the epidemic. These received some media attention, but made little impact on the dominant narrative around No 10’s erratic response to coronavirus. However, the documents are invaluable in deciphering the reasons behind the initial herd immunity response, and the government’s thinking in those critical days.

The aim of the policy was always to “flatten the curve” – slowing the virus’s spread so that fewer people need treatment at any given time – so that the NHS could cope when the rate of infections reached its peak. The strategy was to move in sequence; first attempting to contain the virus by testing known cases and tracing their contacts, while publishing advice on how to detect symptoms, as well as information on the benefits of self-isolation and hygiene. The next stage was to reduce human interactions by isolating the infected and the vulnerable, and then, if necessary, drastically reduce all social activities. Sage’s main concern was public compliance if these measures were introduced too early, and that self-isolating would be hard to maintain over a long period of time.

By 9 March, 319 cases of Covid-19 had been reported in the UK, with five deaths. With the growth in cases appearing relatively moderate, Sage advised the government in a report that there was time to introduce new measures. The paper advocated enacting measures for isolating those with symptomatic cases of Covid-19 over the coming fortnight, followed by isolation for entire households with any infected members, and social distancing for the over-70s in the weeks after that.

But, crucially, this depended on how the outbreak unfolded in the UK. The government advisers were divided about what measures should follow: the stringent quarantine measures, as introduced in Wuhan, or the less draconian social distancing seen in Hong Kong and Singapore? Tougher measures would undoubtedly reduce the spread of the virus, but would this mean a much higher peak in the second wave? Much would depend on public compliance. For the moment the focus was on treating those with symptoms – along with their households – and also the elderly. Together these measures promised to reduce peak hospital bed demand by 50 to 70 per cent and deaths by 35 to 50 per cent.

By contrast, the more drastic interventions – school closures and banning large events – would help to flatten the curve but with negligible impact on total deaths. The advice was counterintuitive: closing schools would mean that parents, including many in the health service, would have to take over childcare duties or else hand them over to grandparents (the age group most vulnerable to Covid-19). The most serious contagion took place in small groups. There would be clusters of groups at sporting events, but if these were cancelled, the same people were as likely to congregate in the confined space of a house or a pub.

This issue of large gatherings had been addressed as early as 11 February by the Scientific Pandemic Influenza group on Behaviour (SPI-B) which was set up during the 2009 “swine flu” pandemic. “On the one hand,” this group reported, “stopping some public gatherings could mean people replace this with other activities (ie, playing football behind closed doors could mean fans watch the match in the pub), potentially slightly accelerating epidemic spread.” Yet there were also possible advantages. “On the other hand, the message sent by stopping them would be expected to change people’s behaviour in other ways, potentially slowing epidemic spread. It is not possible to quantify either of these effects.”

A further consideration added to the uncertainty and the divisions among government advisers. The next critical step for the government was to get those most at risk – people over 70 and those with existing illnesses – to stay at home and avoid social contacts for an extended period. Was it possible to delay “widescale social isolation at the same time as recommending [protective] isolation to at-risk groups”?

The issue of isolating the elderly and vulnerable was on Cobra’s agenda for 12 March. In preparation for this emergency meeting, Sage asked all of its subgroups to review evidence “on public gatherings, including risk to individuals and the impact of restricting gatherings on UK epidemic evolution”. The point of contention was still the potential divisiveness of isolating only the most vulnerable. This is where the concept of herd immunity was raised.

It was a justification for letting mass gatherings continue, while isolating the elderly and the vulnerable at the same time. “One view,” noted an SPI-B paper for Sage, “is that explaining that members of the community are building some immunity will make this [approach] acceptable. Another view is that recommending isolation to only one section of society risks causing discontent.” Although opinion at Sage was divided, the argument for this herd immunity approach won out.

By the time of the Cobra meeting on 12 March the number of reported cases in the UK had risen to 590, with ten deaths. On its conclusion, Boris Johnson addressed the nation to explain that the government was moving from an attempt to contain the disease to one geared to delaying its spread. But the steps announced were modest and tentative. Those with symptoms were told to stay at home for seven days, and those over 70 advised not to go on cruises. The government communications campaign following this statement hardly helped, as the concept of herd immunity was introduced. The night before, on 11 March, David Halpern, chief executive of the government-owned Behavioural Insights Team and a member of Sage, explained to BBC News the importance of shielding vulnerable people until enough of the UK population had been infected with Covid-19 to acquire immunity.

On the morning of 12 March, ITV’s political editor Robert Peston outlined the concept in similar terms, noting that the acquisition of this immunity would still need to be at “a much-delayed speed so that those who suffer the most acute symptoms are able to receive the medical support they need”. The next day, Patrick Vallance, the government’s chief scientific officer, elaborated on the idea. But there was no suggestion from any of these people that herd immunity constituted the whole strategy; Vallance warned that people would soon need to be isolated for long periods of time.

The term "herd immunity" was striking and the implications alarming. It lent itself to accusations that the government was preparing to let the disease rip through the community as part of a cold-blooded experiment in social engineering. This was reinforced by the Imperial College analysis, published on 16 March, which contrasted a suppression strategy – accompanied by active measures to reduce the rate of infection – with a mitigation strategy, which allowed the virus to run its course while encouraging better personal hygiene, isolation of the most vulnerable and providing whatever treatment was possible. Suppression offered the prospect of keeping deaths in the tens of thousands, but a mitigation-only strategy pointed to a still more terrifying scenario in which hundreds of thousands of people would die. All this encouraged the view that the focus on herd immunity represented extraordinary recklessness on behalf of the government.

Johnson’s statement after the 12 March Cobra meeting did not rule out the gradual introduction of more draconian steps “at some point in the next few weeks”. Although he highlighted the seriousness of the situation he failed to capture the urgency. Other governments were being much more interventionist. Large public gatherings were banned in Berlin; schools were being shut in Spain and Greece. Ireland had cancelled St Patrick’s Day parades. By 12 March, French president Emmanuel Macron had announced school closures and banned gatherings of more than 1,000 people. All of this was being done against a frightening Italian backdrop, as the death toll there rose from 52 to 1,266 in only 12 days.

The Johnson government risked losing the initiative. A poll in the Observer on 15 March showed that just over a third of the population trusted the prime minister’s management of the crisis, and 40 per cent thought that the government had “underreacted”. Government-sponsored polling on attitudes to suppressive measures had been underway since February. At the start of March only about a third of those polled agreed that large sporting events should be cancelled. But a few weeks on and opinion had shifted – a majority was in favour of cancelling them. From assuming that it was pointless to implement measures that would face popular resistance, make scant difference and even be counterproductive, the government now confronted rising popular anxiety and accusations that it was exposing the population to unnecessary risk. As universities sent home students and the Football Association took the initiative to suspend all fixtures, it appeared to be following rather than leading.

On 13 March, an addendum was added to the original 12 March SPI-B paper for Cobra. The advisers now warned of the dangers of a slow response. They had “pointed out repeatedly that trust will be lost in sections of the public if measures witnessed in other countries are not adopted in the UK and that not pursuing such routes needs to be well explained”. They added that communications was not within their remit “but this point bears repeating again”. The Health Secretary, Matt Hancock, was soon on air denying that the government’s strategy was driven by herd immunity.

Now, promoted by the Imperial study, the advice from the operations subgroup of the Scientific Pandemic Influenza group on Modelling (SPI-M-O) on 16 March was notably different in tone from a week earlier. The measures at first envisaged – case-by-case isolation, household isolation and social distancing of vulnerable groups – was now “unlikely to prevent critical care facilities being overwhelmed”. Everything now had to be tried, including “general social distancing and school closures”, which offered the best chance of disease control. “There would be a two-three week delay between measures being put into place and their impact being felt in ICU [intensive care units].”

The overall picture of this extraordinary moment changes with new information about the virus itself. Policies based on the idea that by far the most vulnerable group are elderly people become increasingly suspect as the fit and young are regularly struck down. There are already some indications from China and Hong Kong that the risk of a second wave of infections is real. It is still too early to say whether tough measures make a difference, or how well they can be sustained over long periods. Words such as “quarantine” and “lockdown” suggest a completeness that is impossible to achieve, other than in communities that lend themselves to isolation.

The pandemic illustrates the complex interaction between expert advice and political judgment. Language matters at a time of national emergency, especially when it is migrating from a specialist scientific discourse into public messaging. The right words have to be found if confidence is to be maintained among a population whose normal routines are being upended and with people fearful for both their lives and their livelihoods.