[https://www.nytimes.com/2020/08/17/health/coronavirus-herd-immunity.html?action=click&module=Top%20Stories&pgtype=Homepage](https://www.nytimes.com/2020/08/17/health/coronavirus-herd-immunity.html?action=click&module=Top%20Stories&pgtype=Homepage) In what may be the world’s most important math puzzle, researchers are trying to figure out how many people in a community must be immune before the coronavirus fades.