In his seminal work, The Structure of Scientific Revolutions, the philosopher of science Thomas Kuhn developed the notion of the “paradigm” as a way of understanding how a community of researchers makes its judgments. A shared paradigm, observed Kuhn, is a mode of seeing a problem that makes certain presumptions and privileges particular perspectives. It sees things. But it also misses things.

As the coronavirus pandemic spread across the world, the British government relied on the wrong paradigm. The health secretary, Matt Hancock, has insisted that herd immunity – the idea that allowing a virus to spread will eventually build up sufficient resistance in a population – was never a “part of the plan” in the battle against Covid-19. But today’s Guardian’s investigation into the government’s early handling of the crisis leaves little room for doubt: the concept was fundamental to the government’s decision-making in the crucial months of February and March, as most of the rest of the world took a different path. It was abandoned only when it became clear that the NHS would be utterly overwhelmed and hundreds of thousands of Britons could conceivably die.

There were other reasons why Britain remained a relatively relaxed outlier in early March, as Italy, France and Spain moved towards lockdowns and Germany followed South Korea in pursuing a rigorous test, trace and contact strategy. The distraction of Brexit played a part. A reluctance on the part of Boris Johnson to overly impinge on the rights of “free-born Englishmen” also influenced government strategy. It may have been assumed that because outbreaks of deadly diseases such as Sars-CoV had been confined to Asia, the new coronavirus would follow a similar pattern. Our investigation also suggests that ministers could have exaggerated the likely resistance from the public to more stringent measures.

Above all, though, the framework was wrong. The laissez-faire blueprint for the government’s response to the crisis was overwhelmingly derived from previous experience of pandemic influenza. In the absence of a vaccine, flu spreads fast, becomes milder as it mutates, and populations can eventually acquire resistance once a majority of people have become infected. Herd immunity, distasteful as it sounds, can work. But as we now know, Covid-19 is horribly, devastatingly different. In the words of David Nabarro, the World Health Organization’s envoy to Britain: “Coronaviruses are horrible. You can’t just let this thing … wash over your society, because it will kill lots of old people, and a few younger people, it will make hospitals into a big mess and it will endanger health workers.”

Britain finally entered into lockdown on 23 March. By then, the despair of overwhelmed Italian doctors had been broadcast around the world. That footage, and a now famous Imperial College advice paper, which predicted up to 250,000 deaths if the government’s strategy was continued, smashed the herd immunity framework. The same figure had appeared in a 2 March paper. Only now did it concentrate ministerial minds.

Well over 26,000 Britons have died from Covid-19 and there are fears that Britain will emerge from the crisis with the worst fatality rate in Europe. The ministerial mantra has been that “the right decisions were taken at the right time”. But lives would have been saved if rigorous social distancing had been imposed earlier and the transmission of the virus reduced. This is a truth that should be acknowledged by the prime minister and his government. But it should also inform policymaking now that the debate has turned to easing the lockdown amid fears of economic collapse. There is still no vaccine – and nothing approaching a herd immunity strategy can be contemplated again. Until a testing regime can be established that matches the efficiency of those in Germany and South Korea, stringent restrictions must remain. No more unnecessary risks should be taken with people’s lives.