Carl Bergstrom co-authored this OpEd piece in the New York Times, on what is not said about herd immunity.

In the absence of a vaccine, developing immunity to a disease like Covid-19 requires actually being infected with the coronavirus. For this to work, prior infection has to confer immunity against future infection. While hopeful, scientists are not yet certain that this is the case, nor do they know how long this immunity might last. The virus was discovered only a few months ago.

But even assuming that immunity is long-lasting, a very large number of people must be infected to reach the herd immunity threshold required. Given that current estimates suggest roughly 0.5 percent to 1 percent of all infections are fatal, that means a lot of deaths.

Perhaps most important to understand, the virus doesn’t magically disappear when the herd immunity threshold is reached. That’s not when things stop — it’s only when they start to slow down.