In the United States, the standard approach to distributing a routine vaccine is based on the assumption that there will be plenty of doses available. Ideally, everyone will then follow their doctors’ recommendations when it comes to being immunized against the germ in question. As a practical matter, though, rarely are both of those conditions fully met. Some people are unable to get a vaccine; others refuse it. (For the 2019-20 season, the influenza vaccine was distributed to about half the U.S. population.) The next-best vaccination outcome is the inoculation of enough people so that, together with those who are immune after being infected, there are too few available hosts for the pathogen to circulate widely in the population — a situation known as herd immunity.
But herd immunity is especially difficult to achieve for a brand-new disease like Covid-19. It takes time to make enough vaccine to give it to everyone who would benefit from it, and then it takes even more time to distribute it. And no vaccine completely protects everyone who receives it. Still, even with these limitations, a vaccine can help. “We don’t need a perfect vaccine” to achieve herd immunity, says Eric Toner, the lead author of guidelines for vaccine allocation and distribution created by the Johns Hopkins Bloomberg School of Public Health. But how many people need to be vaccinated to get there, he says, “depends on how good the vaccine is.” The more effective it is, the fewer people will need it. What that number of people turns out to be, though, and how soon we reach it, will also depend on how we decide to deploy those initial doses.
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.