A previous vaccine shortage helped reveal the crucial importance of distribution strategies. In 2009 the flu strain H1N1, known as the swine flu, emerged unexpectedly, resulting in production delays. So the C.D.C. had to decide who would be first in line for the limited doses. Because H1N1 seemed to sicken young people far more often than older adults, the agency advised that health care workers inoculate as many people as possible between the ages of 6 months and 24 years (as well as those in other high-risk groups) before treating older adults. For seasonal influenza, its policy at the time was to vaccinate those most vulnerable to severe illness or death, including children from 6 months to 18 years and adults over 50.

But in a paper that year in Science, Alison Galvani, of Yale University, and Jan Medlock, now at Oregon State University, argued that we could actually prevent more people from getting sick and dying from H1N1 and other flus if instead of simply trying to protect those facing the greatest dangers, we vaccinated those most likely to transmit the viruses. “Children 5 to 19 are responsible for most transmission and for the spread of infection to their parents’ age groups,” they wrote. Therefore, they showed, first vaccinating people ages 5 to 19, as well as those roughly in their parents’ age group (30 to 39), would protect “the remainder of the population” most effectively. Tens of thousands of infections and deaths would be prevented, along with billions in economic costs. Those insights led the C.D.C. to advise that everyone 6 months and older get a seasonal flu vaccine yearly.