The world is soon likely to confront a serious new challenge to the fight against Covid-19: vaccine hesitancy. 
In the U.S. and U.K., large numbers of people — at least 30 percent — have said in recent surveys that they would hesitate to take or refuse a vaccine that could protect them from the coronavirus and slow its spread. These numbers probably understate the problem. People might tell a researcher that they will get vaccinated even if they won't. And the problem might be even worse if a vaccine is made available under a speeded up "emergency use" exception to the usually lengthy approval process, amplifying public concerns about rushing it out.
What can be done? To answer that question, we need to understand why some people are reluctant to take vaccines. Research explores the influence of three factors, often known as the three Cs.
The first is convenience. Human beings suffer from inertia, and they also procrastinate. If it's not so easy to get vaccinated, many people won't do it.
Physical proximity to vaccination sites helps; so do short waiting times. Long lines hurt. So do paperwork requirements and administrative obstacles. If widespread immunity is the goal, officials must not underestimate the importance of eliminating inconveniences, both small and large. The good news is that when vaccines are easily available, the rate of vaccination increases greatly, even among people who have doubts.
The second factor is complacency. With respect to diseases, a lot of people tend to think that their personal risk is low. "Optimism bias," as it is called, makes vaccination seem unnecessary.
The third factor is confidence: public trust in the efficacy and safety of the vaccine, and also in the motivations and competence of those who are behind it. Many people distrust vaccines in general. Many others distrust particular vaccines, or new ones, which they might perceive to be inadequately tested. In extreme form, distrust turns people into anti-vax activists.
In the context of Covid-19, distrust might well be a particular problem, potentially leading many people to think, "I am not going to be a human guinea pig," or at least to wait for some period before getting vaccinated. During that waiting period, a lot of people might get sick.
For those seeking to promote vaccination, the first step is to specify the particular source of hesitancy, and to learn which is most important, and exactly where.
For poor people, a lack of convenience might loom large. Among young people, the main problem might be complacency. For people who distrust authority and science, and who think that experts have no idea what they are doing, the problem is a lack of confidence.
Convenience might turn out to be the easiest problem to solve. Once a safe and effective vaccine is available, it should be a high priority to ensure that it is easy for everyone to get (starting with people who are at particular risk). An economic incentive, such as a small gift certificate, can help.
Complacency is likely to be a special problem for those who believe, reasonably or not, that their personal mortality risk is low. A good response would be to offer vivid warnings, including truthful narratives about deaths and serious illness among those who are young, healthy and tough.
It can also help to emphasize that vaccination protects not only those who get vaccinated, but also others whom they might otherwise infect. If young people think that their own risks are low, they might nonetheless choose to get vaccinated if they are convinced that doing so will protect somebody's mother or grandmother.
Confidence can be the toughest nut to crack. Misinformation strengthens public distrust, and with respect to a Covid-19 vaccine, there is bound to be a lot of that. And for vaccination, some people show "reactance," which means that they have a negative attitude about official requests or requirements — and rebel against them.
The good news is that it's possible to debunk misinformation, above all by emphasizing facts, and not drawing a lot of attention to falsehoods (mentioning them, even in the course of a debunking effort, can backfire).
For people who distrust the authorities, an effective response is to meet them where they are, and to appeal directly to their sense of social identity or culture. Such responses, sometimes described as "community-based," inform people that their peers, and those with their own values, are getting vaccinated.
Credible or surprising "validators" — high-profile people who are respected and admired by those who lack confidence in vaccines — could be a big help. Suppose that people in a community are known to fear that a vaccine won't work, or that it is part of some kind of plot, or that it will have terrible side-effects. Trusted politicians, athletes or actors — thought to be "one of us" rather than "one of them" — might explicitly endorse vaccination (and report that they themselves have gotten the vaccine).
It's obvious that scientific questions have scientific answers. It's less obvious that questions about human behavior also have scientific answers. With respect to how to promote vaccination, our intuitions often go badly wrong.
But there's a lot of evidence out there, and in trying to protect people from Covid-19, nations can draw on it. The challenge, and the opportunity, is that everyone will be in uncharted waters, which means that we have to be flexible and willing to learn what works and what doesn't.
1. I am chair of a Technical Advisory Group for the World Health Organization on behavioral insights and sciences for health. This column represents only my personal views.
Cass R Sunstein is a Bloomberg Opinion columnist. He is the author of "The Cost-Benefit Revolution" and a co-author of "Nudge: Improving Decisions About Health, Wealth and Happiness."
Disclaimer: This article first appeared on bloomberg.com, and is published by special syndication arrangement