There has been much criticism of the decision by the Centers for Disease Control and Prevention (CDC) to let Covid-infected people who are feeling better to stop quarantining after five days and simply wear a mask. But this sort of pragmatic decision has a long history in public health.

The C.D.C.'s latest policy guidance that people who have been fully vaccinated against the coronavirus virus no longer need to wear face masks indoors gets the science right, but policymaking wrong.