Our immune system is the pandemic’s biggest mystery

What have we been witnessing these past few months? A worldwide crisis caused by the arrival of a new virus of exceptional virulence — or a crisis of awareness, in which incomplete information led to a wildly disproportionate reaction? Have lockdowns, face coverings and the rest saved millions of lives worldwide? Or have they had relatively little effect on the course of the pandemic, and ended up causing more harm than good? And why, so far, is Britain not seeing the surge of Covid-19 infections reported in Spain and France? What are we missing?

We still know a lot less about...