“A man has got to know his limitations.” That is one of the memorable phrases uttered by “Dirty Harry” Callahan, the fictional police inspector played by Clint Eastwood. Today, the world is learning its limitations in the ongoing COVID-19 pandemic, and the most important lesson is that a key strategy we banked on to defeat the virus — herd immunity — appears unobtainable. Herd immunity is reached when enough people in a community acquire immunity to a disease, either through infection or vaccination or both, that it makes sustained transmission impossible and protects even those who are not im…