Has this Brazilian city reached herd immunity without lockdown?

Throughout the Covid crisis, the international response to the disease has rested on a simple assumption: that none of us have any resistance to it, being caused by a novel virus. Therefore, if allowed to let rip through the population, the virus would exponentially spread until around 60 – 70 per cent of us had been infected and herd immunity was reached. This was the assumption behind Neil Ferguson’s paper in March, claiming that Covid-19 would kill 500,000 Britons if nothing was done and 250,000 of us if the government carried on with the limited mitigation polices it was then following.

Yet real world data has challenged this assumption. First came the accidental human experiment of the Diamond Princess cruise ship, where in January the disease spread unnoticed among 3,711 passengers and crew in the closed environment of a ship. But when all those people were tested for the disease, only 19 per cent turned out to have been infected.