The complications of herd immunity for Covid-19

Herd immunity is like a forest fire, once enough trees have burned down, it is hard for a new fire to start. — AFP

One year into a changed world, the numbers defy comprehension.

More than 21 million confirmed cases of Covid-19 in the United States – a rate approaching one in 15 people – plus untold millions who had mild or no symptoms and were never identified.