When the next dead­ly pan­dem­ic flu hits, the first chal­lenge will be to de­vel­op a vac­cine. But loom­ing behind that ob­sta­cle is another: How to get an in­oc­u­la­tion to millions of people with­out in­ad­vert­ent­ly ex­ac­er­bat­ing the cri­sis.