Will it be a shot in the arm — or the foot? Americans have invested high hopes in a Covid-19 vaccine. Its development is racing ahead, with researchers deploying novel approaches and governments spending billions of dollars to try to get billions of human bodies to self-generate coronavirus-fighting defenses.

But how safe and effective will a Covid-19 shot be? Can the U.S. government, with an already checkered record in responding to the pandemic, deal with all the p's and q's required to safeguard many millions of children, women, and men? Will the vaccine-hesitant or -resistant undermine the "herd immunity” that will be fundamental to the mass protection provided by a coronavirus shot?