The White House now says there will be enough supply for all American adults to get a COVID-19 vaccination by the end of May. Convincing a sizable portion of the U.S. public to be immunized is another matter. 

Only about 54% of American adults who haven't been vaccinated say they definitely will, according to a Census Bureau survey conducted in February. Meanwhile, about 23% say they will probably be vaccinated, and another 23% will either probably or definitely not get vaccinated.

Reaching that highly coveted "herd immunity" level is central to the U.S. government's plans to fight covid and reopen businesses. Vaccine supply is one thing, but overcoming hesitancy to get the shot is another. To do that, officials must make the process of getting shots easier, and fight misinformation about the vaccines, concerns about the speed of development and distrust of government and health-care institutions.

"A lot of it comes down to things like what they perceive costs and benefits to be," said Dean Eckles, a social scientist and statistician who is a professor of marketing at MIT Sloan. People might think: "If I don't get this then I'm not helping other people, and they'll be disappointed or unhappy with me. Of course, the reverse can also happen in communities where the norm is not accepting certain vaccines."

The World Health Organization has called vaccine acceptance the "next hurdle" in fighting the pandemic, and said that to win over people, they must have information, be motivated and have barriers eased. Willingness to be vaccinated is lower in the U.S. than in many countries including the U.K., Spain and India, according to February data from market research firm YouGov.

Hesitancy is especially prevalent in minority communities, even as the Biden administration has sought to make equity a hallmark of its approach. A smaller percentage of Hispanic and Black adults say they will "definitely" get vaccinated, compared with White and Asian adults, according to a Census Bureau survey conducted Feb 3-15. A history of racial discrimination and inequities in the health-care system have left many people of color in the U.S. distrustful of medical providers.

"We are engaged with stakeholder groups across racial and ethic minorities, rural Americans, to reach young Americans -- all of those are important," White House COVID-19 adviser Andy Slavitt said Friday in a briefing. "Our whole perspective is leave nobody behind."

The choice of whether to get vaccinated is highly influenced by the perspectives and behaviors of someone's social and professional circles. Simply put, the more people you know who have been vaccinated or want to be, the more likely you are to do so.

"What your friends and family members and co-workers do is very powerful," said Noel Brewer, a professor of health behavior at the University of North Carolina. "Watching other people get the vaccine can set a lot of people's minds at ease."

About half of those who want to get vaccinated as soon as possible have a close friend or family member who has gotten a dose, compared with 36% who don't know anyone vaccinated, according to a Kaiser Family Foundation survey published in late February.

About 17.7% of the U.S. population has received at least one shot to date. As that increases, it could give the vaccine push more momentum.

The U.S. has administered 2 million doses or more over the past four days, according to Bloomberg's Vaccine Tracker. On March 6, the Senate passed a $1.9 trillion pandemic relief bill, including $160 billion for coronavirus vaccine and testing programs that could increase the availability of shots in coming months.

Taking a hyperlocal approach to vaccine messaging can also harness a sense of community and collective action. A campaign from the city lf New Orleans, called "Sleeves Up, NOLA," has gained the attention of public health researchers because it appeals to close-to-home traditions like Mardi Gras.

"If you're from New Orleans, a lot of that resonates for people who are local," said Alison Buttenheim, associate professor of nursing and health policy at the University of Pennsylvania. "We need inspiration, and local campaigns in every city. I think that was very successful and needs to be replicated."

Because fewer people of color have received coronavirus vaccines than White Americans, there are fewer representatives to promote the shot in minority communities, further exacerbating racial disparities, said Saad Omer, director of the Yale Institute for Global Health. Diverse leadership is an effective tool in reassuring people that the vaccine is safe, he said.

About 22% of white adults report having received at least one dose of the coronavirus vaccine, compared with 11% of Hispanic adults and 13% of Black adults, according to Kaiser Family Foundation data from late February.

Even among health-care workers, racial discrepancies have emerged. For many such workers, "your employment doesn't overcome all of those other aspects of your identity," such as race, gender and other factors, said Joshua Barocas, an infectious-disease physician at Boston Medical Center.

About three in 10 health-care workers expressed hesitancy about getting the vaccine in a December Kaiser Family Foundation survey. Another report by the foundation found that clearly communicating that personal information collected for vaccination cannot be used for immigration-related purposes may help reduce fears among noncitizens.

Coronavirus vaccines were developed at record speed. Former President Donald Trump's administration dubbed its vaccine development program "Operation Warp Speed." Moderna Inc.'s vaccine, for instance, went from design to emergency-use authorization in about 11 months, compared with the decade vaccines can typically take to develop. That has sown doubts.

"No one wants a fast vaccine. They want a safe vaccine developed with the best science available," Brewer said.

What many Americans may not realize is that vaccines still went through three phases of testing for safety and efficacy, during which tens of thousands of people received the immunizations. While "this timeline was shortened," said Kizzmekia Corbett, a National Institutes of Health researcher who helped develop the Moderna vaccine, speaking at a forum on Feb. 22, "no integrity was lost."

In Harrisburg, Ill., a rural community of about 8,500 in the state's southern tip, widespread vaccine hesitancy led pharmacist J. Cody Sandusky to start soliciting and answering people's questions about the shots.

Sandusky has done so both internally for employees at Harrisburg Medical Center, where he is director of pharmacy, and the general public through the hospital's social media account. He has also heard concerns about vaccines being used to track people, and spotted misinformation on social media linking the vaccines with Bill Gates.

"Some people aren't going to change their mind," he said. But with most people, "once you paint the picture for them, you can almost see the 'Aha' moment, the light going off for them, because it all connects finally."

A third of those who aren't vaccinated said in a Kaiser Family Foundation survey that they thought or weren't sure about common coronavirus vaccine misinformation: that the vaccines caused infertility, that they contain live coronavirus -- and thus could make recipients sick -- and that patients have to pay out-of-pocket to get them. None is true.

In that survey, the most common concern cited by respondents was that they might experience serious side effects from the vaccine. Severe reactions to COVID-19 shots are rare, according to the U.S. Centers for Disease Control and Prevention.

The Biden administration has said that political leanings shouldn't drive vaccination decisions.

"We are targeting specific groups with conversations and answering questions that people have, and those questions may differ based on where you live or how you identify," Slavitt said Friday. "But this is not, nor should it ever become, something that is a question of politics."

Early recipients of vaccines had to be highly motivated to get an appointment, navigating confusing websites and often waiting in line. Many people are simply not willing to expend that kind of time and energy.

"One of the more strong ways of ensuring that there's high acceptance is not from persuasion, it's from making the clinic experience, the vaccination experience appealing," said Yale's Omer.

Humans are inherently somewhat lazy, so reducing the so-called hassle factor increases the likelihood of us getting something done, said Katy Milkman, a professor at University of Pennsylvania's Wharton School and co-director of the university's Behavior Change for Good Initiative.

While the supply of shots today remains a severe constraint, it should expand rapidly in the coming months, because of accelerated production timelines and the advent of a third coronavirus vaccine produced by Johnson & Johnson.

"This is not a March problem, this is not a January problem, this is not a February problem. It's an April problem," she said. "But you know, April is coming."

The Johnson & Johnson vaccine was authorized by U.S. regulators on Feb. 27, and consists of just one shot that can be stored in a refrigerator, smoothing the logistics of vaccine distribution. Yet experts are already worried that Americans will unfavorably compare its efficacy with that of the two vaccines already on the market.

Detroit Mayor Mike Duggan for instance, said at a March 2 news conference that J&J's shot isn't as good as Pfizer and Moderna's vaccines. He later reversed himself after getting a call from the White House concerned he was off-message. Duggan's initial remarks could be problematic in Detroit, a city whose population is almost 80% African American, and who already were less likely than other groups to accept vaccination.

The J&J shot was found to be 72% effective in preventing moderate-to-severe COVID-19, while Pfizer Inc. and Moderna have well-publicized rates of 95% and 94% effectiveness in preventing COVID symptoms. Yet the vaccines were studied in separate clinical trials, so the figures are not a direct comparison. Across all regions, J&J's vaccine was 85% effective in preventing severe disease after 28 days and it demonstrated complete protection against COVID-related hospitalizations and deaths.

Vaccine hesitancy also comes from distrust of scientific research, the health care system and government entities, especially in minority communities that have been mistreated by them in the past.

"You can't move forward until you acknowledge why people don't trust the system," like the forced sterilization of Puerto Rican women and experimentation on Black men in the notorious Tuskegee syphilis study, said Debbie Salas-Lopez, senior vice president of community and population health at the hospital system Northwell Health.

One of the most effective ways of alleviating vaccine fears is by addressing concerns, but instead, the public health community has pushed people to take the vaccine regardless of their concerns, said Barocas.

Barocas has been holding hour-long "listening sessions" at the Boston Medical Center where he explains the science behind the vaccine and then answers questions. Afterward, there are usually about five people who sign up to be vaccinated, he said.

"It's much harder and much more time consuming to listen than it is to tell, but that's much less effective," he said.