Vaccines against COVID-19 have arrived with unprecedented speed. At least three candidates appear to be extremely effective and are likely to be approved in the U.S. in coming weeks. By the end of December, the U.S. Centers for Disease Control and Prevention (CDC) estimates the U.S. will have enough vaccines to treat 20 million people. Britain this week approved the vaccine made by Pfizer and BioNTech.

But for the first time in modern U.S. history, not everyone who wants a vaccine will be able to get one immediately. Figuring out who gets the first shots in that country and who has to wait is the job of an obscure CDC panel known as the Advisory Committee on Immunization Practices (ACIP). This week they announced their first official COVID vaccine distribution guidelines.

The committee makes such plans for all newly approved vaccines, but never before has ACIP had to work so fast. In its December 1 meeting, the committee voted 13 to one that the approximately 21 million health care workers in the country should be the first to receive any new COVID vaccine, along with residents of long-term care facilities like nursing homes. But deciding who comes second, and third, and even further back in the priority line, is trickier, as health experts weigh factors such as vulnerabilities of specific groups and whether immunizing others can have an outsize effect in stopping the harm to larger society caused by the disease.

Public health specialists who have worked on vaccine plans say the immense effects that COVID has had on the country make these decisions challenging. “We don't have everything in place, because we've just never faced this before,” says physician Helene Gayle, president and CEO of the Chicago Community Trust, who co-chaired a committee evaluating COVID vaccine priorities for the U.S. National Academies of Sciences, Engineering, and Medicine. “In my lifetime, there's never been such a huge challenge to major swathes of the population that also has such societal impact.”

Lawrence Gostin, a health law expert at George Washington University, says there is virtually universal agreement that health care workers should get the vaccine first. This group is one of the most likely to acquire the virus because of their exposure to infected patients. Health care workers have put themselves at the front lines and their well-being ensures that other patients are treated. This follows the recommendations of an October report from Gayle’s committee.

Still, when Britain approved the Pfizer/BioNTech vaccine, the country’s Joint Committee on Vaccination and Immunisation did not give health care workers the highest priority. Instead, it recommended that long-term care residents receive the very first vaccines. The committee cited this population’s very high risk of infection and death; in one study of four nursing homes, more than a quarter of residents died over the course of two months.

In the U.S., nursing homes account for nearly half of all COVID deaths. Still, including long-term care facility residents in the first group, known as phase 1a, was a more contentious point of discussion at the recent ACIP meeting. Committee members questioned whether this population, which tends to have weaker immune systems, might not respond as well to a vaccine. The lone dissenting vote on the initial distribution plan, infectious disease expert Helen Keipp Talbot of Vanderbilt University, said she felt there was not enough data on the vaccine’s safety and efficacy in this group, and was concerned that U.S. federal agencies’ surveillance systems weren’t sufficient to track these people’s outcomes.

Choosing the second group—people who could begin receiving vaccines in about a month—will present even more difficulties. Current plans put forward by the CDC suggest essential workers—a population of about 87 million—will be in this category, known as phase 1b. The following phase, 1c, which could start in about three months, would include the approximately 153 million people who have high-risk medical conditions and those over age 65.

But those plans are not final, and ACIP will continue to meet and hammer out details after the FDA approves each new vaccine and as more doses become available. For now, the committee has to consider several factors, says Jeffrey Duchin, a former ACIP member who now directs public health efforts in Seattle and King County in Washington State. Foremost is the scientific evidence about which populations are most likely to spread the disease and who is most likely to have severe effects. Epidemiological models suggest there is little difference in spread whether a vaccine is first given to high-risk adults, essential workers or people over the age of 65.

Deciding among these groups, therefore, may come down to factors such as logistics and ethical principles like access to health care. People from racial minority groups, for instance, comprise a high percentage of essential workers with a high chance of exposure. Because these groups have historically had less access to health care and because COVID is far more likely to be fatal in nonwhite Americans, ethical principles might suggest such underserved populations be prioritized, Duchin says.

Others have argued that vaccines be specifically targeted to minority ethnic groups, regardless of whether they are essential workers, in order to make up for historical inequities. “I think there is a very strong ethical justification for giving considerable advantage to people who are socially vulnerable, and I would go further and give explicit priority to racial minorities,” Gostin says. “COVID has really amplified public concerns about racial and social injustice.”

But Gostin concedes that such a strategy would face political and legal risks. Surveys have shown that Black Americans, for instance, are already skeptical about receiving a new COVID vaccine, reflecting a broader distrust of experimental treatments among minority populations. And a conservative-leaning U.S. Supreme Court would be likely to strike down any plan based explicitly on race, if that plan were challenged in a lawsuit. Targeting the vaccine to essential workers and people in underserved communities, Gostin says, might be a more viable strategy.

One question that should be relatively easy to resolve is determining which health conditions put people in a high-risk group. The CDC has amassed a large amount of data on conditions like diabetes and morbid obesity that increase the risk of severe COVID complications, and conditions like asthma that only slightly increase the risk. This list, Gayle says, will allow public health officials to prioritize the people whose preexisting conditions put them at most risk.

Logistical issues may dictate some of the allocation as well. The Pfizer vaccine, for instance need to be stored at –70 degrees Celsius, a temperature generally only achievable at health care centers. So getting this vaccine to rural populations could prove challenging.

Ultimately, the implementation of allocation decisions is not done by ACIP. It comes down to state and local jurisdictions. The federal advisory committee can recommend a vaccination triage scheme, but the CDC can’t force states to follow it. Still, Sara Rosenbaum, a legal expert at George Washington University, expects that states will accept the plans. “I’d be surprised if they fought prioritization,” she says, noting that, historically, local public health agencies have always followed federal vaccination guidelines. 

But the CDC plans will need to allow local public health officials some flexibility in determining who is part of a particular group, for instance who qualifies as an essential worker or member of a disadvantaged population. Gostin agrees that flexibility is necessary, but is concerned that “it’s a recipe for hundred different responses across America.” He notes that during this year many state and local governments have failed to follow federal guidelines on COVID testing and mask requirements: “We’ve seen American federalism fail spectacularly.”

For that reason, Duchin says that ACIP will need to carefully word its recommendations in order to ensure consistency. “If vaccination strategies differ in big ways, it will lead to confusion and potentially undermine confidence in the process,” he says.

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