The vaccine rollout is not going as planned. Since mid-December, the U.S. has distributed 21.4 million doses of COVID-19 vaccines; less than one-third have actually made it into people’s arms. The problems have been many and varied: holiday delaysscheduling scams, long lines in some places, and not enough demand in others. These initial kinks are getting worked out, but that alone will not get us back to normal anytime soon. The next phase of the vaccine campaign—reaching tens of millions of elderly people and essential workers, along with the rest of the community—will be even harder.

The initial vaccine rollout is simpler than the phases to come because it targets hospitals and long-term-care facilities, where the relatively small number of eligible people are already concentrated. Finding and scheduling them should be straightforward. “This is the easy part,” says Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security. Finding and scheduling people in the next priority groups will bring a new tangle of logistics, for which the country is still not prepared.

In the coming months, state and local health departments will have to stand up mass-vaccination clinics that can handle hundreds or even thousands of people a day in the middle of a pandemic, when crowds are dangerous. Thousands of doctors’ offices and pharmacies across the country will also need to learn how to handle and administer unusually fragile vaccines. And eventually, as the pool of people eager to get a vaccine is exhausted, public-health officials will need to persuade the uninterested, the reluctant, and even the skeptical to get vaccinated, so that communities can reach the 70 to 90 percent necessary for herd immunity. The hardest work still lies ahead.

That work has largely fallen to states, which have traditionally organized vaccination campaigns, such as during the 2009 swine-flu pandemic. But this year, they are already strained from months of fighting COVID-19, and they haven’t had the money or resources to fully plan the biggest and most complicated pieces ahead of time. Congress waited until December to pass $8 billion for vaccine distribution, even though local and state officials had been pleading for funding for months. “States can’t make a contract or plan for anything or hire someone until they have that money on hand,” says Kelly Moore, the deputy director of the Immunization Action Coalition. That work is happening only now, as vaccines are already being distributed.

Florida might serve as a cautionary tale about trying to vaccinate large numbers of people without advance planning. The state is currently offering vaccines to seniors over 65, in addition to the federally recommended priority groups of health-care workers and long-term-care residents. But it delegated the logistics to counties, which came up with a host of different plans that ran into a host of different problems: long lines of seniors waiting overnight in some counties, confusing registration sites and outright scams in others. Without better coordination, these scenes could soon play out across the country.

On the most basic level, scale creates new challenges. And the first coronavirus vaccines require very particular storage conditions and skill to administer, adding to the difficulty. Exactly how community vaccination centers solve these problems will vary state by state, city by city. Ballad Health, a hospital system in eastern Tennessee and southwestern Virginia, is working with local health departments to open five community vaccination centers for the elderly, three in Tennessee and two in Virginia. The centers are in buildings near a hospital with ultracold storage, so doses can be quickly thawed and transported. Jamie Swift, the chief infection prevention officer at Ballad Health, says her team has had to plan for everything: staffing phone lines, traffic flow, even just having enough chairs available for seniors waiting for their vaccine.

The hospital drew on experience in vaccinating their own staff and local doctors, but planning for the community is more complicated. In hospitals, “it was a very controlled population. These are our employees. This is who’s eligible. You have to get your badge to get in … end of story,” Swift says. With the community vaccination clinics, the hospital is expecting more questions and more time for each person.

As more vaccines become available, doctors’ offices and pharmacies are expected to offer the COVID-19 vaccines as well. The sheer number of people getting vaccines will create more errors in handling and scheduling and other logistics. “The bigger this gets, the more sites that get involved, the less control,” Toner says.

Public-health departments will also have to balance reaching priority groups and making sure vaccines are going out as fast as possible. It doesn’t make sense to track down every person over 65 in a state before you let a single 64-year-old get a vaccine. But how soon do you decide to open up vaccination to the next priority group? “When the pressure builds up to get as much vaccine out as possible, some places may just start ignoring the priority groups,” says Saad Omer, a vaccinologist at Yale. At times, these decisions might be obviously appropriate. A hospital with doses expiring in hours should clearly vaccinate anyone it can. But other cases will be more ambiguous. If demand is simply low in a certain priority group, should states spend resources trying to persuade people to get vaccinated, or just move on?

Fights over vaccine access and confusion about different guidelines from state to state can erode people’s willingness to go out of their way for a vaccine. “When you’re confused and unsure, your default is to stay as you are and not step out and get vaccinated,” Moore says. “When someone is feeling confident and hopeful, then they’re more likely to take action.” To vaccinate enough people so that the U.S. achieves herd immunity, the shots need to reach people who are currently unsure—at a time when the danger of the virus itself is deeply politically polarized. This is the challenge that lies ahead, and the end of the pandemic depends on it.