Is coronavirus over in some cities?

More than half a million people have died from COVID-19 globally. It is a major tragedy, but perhaps not on the scale some initially feared. And there are finally signs that the pandemic is shuddering in places as if its engine is running out of fuel. This has encouraged many governments to relinquish lockdowns and allow everyday life to restart, albeit gingerly.

The spread of SARS-CoV-2 has been difficult to predict and understand. On the Diamond Princess cruise ship, for example, where the virus is likely to have spread relatively freely through the air-conditioning system linking cabins, only 20 percent of passengers and crew were infected. Data from military ships and cities such as Stockholm, New York, and London also suggest that infections have been around 20 percent – much lower than earlier mathematical models suggested.

This has led to speculation about whether a population can achieve some sort of immunity to the virus with as little as 20 percent infected – a proportion well below the widely accepted herd immunity threshold (60-70 percent).