Covid-19 is in retreat across the US. Cases have plunged by 77 per cent in six weeks and hospitalisations have dropped by almost 50 per cent in the past month.

The US vaccination programme is going well – 15 per cent of Americans have now received their first dose. But vaccines alone cannot explain this dramatic shift. Build-up of natural immunity may provide a more plausible explanation.

Covid cases have been rapidly and consistently declining since 8 January, at a time when almost nobody outside the healthcare industry had been vaccinated. Moreover, vaccines take weeks to kick in.