CHICAGO — When Tori Saylor, 27, stepped out of her apartment in Kalamazoo, Mich., last week, she knew that the Centers for Disease Control and Prevention had already given fully vaccinated Americans the go-ahead to shed masks in most situations.

Ms. Saylor, who is vaccinated, wore one anyway. And when she summoned an elevator in her apartment building, she confronted her first real test of the new era: Twice, the doors opened to reveal people who were not wearing masks, and twice, she let the elevator go.

“Am I to trust these people, having never met them?” said Ms. Saylor, who has multiple sclerosis and gets an infusion therapy that compromises her immune system. Despite her vaccination status, it is unclear whether her body will be able to effectively produce antibodies to fight off Covid-19. “How can I judge whether someone is vaccinated by making momentary eye contact with them?”

For many Americans, trust is in short supply after a year of a long pandemic and the conflicts that have come with it.

Our capacity to trust other people’s honesty has already been tested, and fibs — or omissions — may have happened along the way. Did every person who drove across a state line follow 14-day quarantine rules? Did everyone who got an early vaccine fit the eligibility rules at the time?

So it is no surprise that the latest honor code — the federal government’s guidance encouraging vaccinated Americans to take off their masks — was greeted with skepticism in parts of the country that have not already done so. Fewer than half of Americans over the age of 18 are fully vaccinated.

“It’s a very complicated symphony right now,” said Dr. Howard Markel, a medical historian at the University of Michigan who is an expert on pandemics. “There’s been such an erosion of trust, distrust for government, distrust for the virus, distrust for this party or that party. So when you tell the public what to do, there are people who say, ‘How can I trust the guy without the mask?’”

Health experts say that vaccinated people should be protected from severe disease, even if people around them are not vaccinated and not masked. But the unusual sight of bare faces has arrived at a time when Americans’ trust in institutions and one another is particularly fragile.

After all, evidence of pandemic-era wrongdoing has been rampant: Prosecutors have charged dozens of people who are accused of fraudulently obtaining loans and other funds from the federal government related to the CARES Act. High schools and colleges, including the prestigious Geisel School of Medicine at Dartmouth, have investigated students for cheating on remote exams while school buildings were closed because of the coronavirus.

Even before the pandemic, trust in the federal government was near record lows, and 7 in 10 people thought that Americans’ trust in one another had declined over the past 20 years, according to the Pew Research Center. Still, a majority of Americans had confidence that people could work together in a crisis. About 75 percent of Americans believed that people would cooperate with one another in a crisis, even if they did not trust one another.

Teamwork became a motif of the pandemic’s early days. Holed up inside their homes last spring, crafty Americans sewed homemade masks, neighbors planted yard signs supporting health care workers and essential workers, and politicians spoke in lofty language about working together to “flatten the curve.”

Then came a partisan division over masks, screaming crowds outside state capitols, death threats against local and state health officials. On the other side of the debate, some people who supported Covid-19 restrictions embraced the job of mask policing.

It quickly became apparent that, even in a crisis, Americans struggled to come together.

“We couldn’t even trust people to do the right thing and wear masks when it was rampant, when it was the highest it’s ever been,” said Deborah Burger, a president of National Nurses United, who described nurses rushing to the grocery store in their uniforms only to be berated by fellow shoppers. “People were accosting them, accusing them of lying about the pandemic.”

National Nurses United, the nation’s largest union of registered nurses, has come out publicly against the new C.D.C. guidelines on masks, which were announced last week and left state and local government officials, business owners and ordinary people scrambling. The guidelines allowing vaccinated people to go without masks do not apply to hospitals, but Ms. Burger said the changes create confusion and put the burden on health care workers to enforce face coverings.

“It feels like somebody has pulled the rug out from under us and taken away our protections,” she said.

Other frontline workers, including business owners, are also grappling with new pressures.

“We used to have a sign on our door that said you can’t come in unless you’re masked, and now I don’t know what my sign should say: ‘If you are not vaccinated, please wear a mask?’” said Louise Orlando, 55, who owns the Bakery on Mason in Cape Charles, Va.

Until recently, the business fell under a statewide mask mandate. Now, Ms. Orlando feels the onus falls to her. “I don’t know who is going to be honest anymore.”

Rebecca Johnson, of Winfield, Ill., is among those who will not be taking off her mask indoors in public any time soon.

She wants to protect her 11-year-old son, Mitchell, who is too young to qualify for a vaccine. And while she is thrilled to shed the mask when outside or around other vaccinated people indoors, there are plenty of situations where it feels like the old rules still apply.

“I still think when you walk indoors you have to look at every person that you’re around, thinking they could have it,” she said.

Not everyone is fretting over the new guidance or pondering trust.

“I’m not that troubled by the honor system,” said Tim Lovoy, 62, a retired accountant in the San Pedro neighborhood of Los Angeles who said he felt assured by the data.

Mr. Lovoy is fully vaccinated, he said, which offers him strong protection. In his home area of Los Angeles County, new virus cases have dropped to about 3 per 100,000, the lowest since the beginning of the pandemic last year.

These days, Mr. Lovoy has assessed his risk of getting Covid-19 to be lower than getting in an accident on the freeway. He is getting ready to return to normalcy, including practicing karate indoors again, and said he is wasting little time worrying about whether other maskless people he encounters are vaccinated.

“If people are vaccinated, their risk in taking off their mask is very, very low, and that’s their own decision to make,” he said. “And if people are not vaccinated and don’t wear a mask, they’re putting themselves at risk.”

Throughout the pandemic, views on the coronavirus have often split along partisan lines, and the new questions about masking — and about other people’s vaccination status — may be no different.

The changes by the C.D.C. are likely to be most jarring to Democrats, who have been more likely than Republicans to see the coronavirus as a major threat, more likely to overstate risks from the virus, and more likely to get vaccinated.

Many Republicans, by contrast, have emphasized their individual liberties on virus decisions from the beginning and may welcome the freedom that comes with the new guidance.

At the same time, people who identify as conservative are less likely to be vaccinated, whether because of skepticism about the safety of a fast-tracked vaccine, or a belief that the coronavirus itself is not very dangerous. In recent polls by Monmouth University and Quinnipiac University, almost half of Republicans surveyed said they did not plan to pursue vaccinations.

“We don’t even agree on what ground truth is,” said David DeSteno, a professor of psychology at Northeastern University in Boston, who said that the lack of shared information sources in partisan politics makes building trust particularly difficult. Without a common understanding of the risk of getting seriously sick from the virus, or any side effects from getting a vaccine, “we can’t agree on what is an acceptable sacrifice, or what is an acceptable trustworthy behavior.”

Eli Finkel, a psychology professor at Northwestern University in Evanston, Ill., who has studied romantic relationships and American politics, said that trusting one another inherently involves a gamble — whether it is letting your guard down in a marriage, or trusting the behavior of fellow citizens during a pandemic. Still, he said, trust is often essential for society to move forward.

“It’s a willingness to allow yourself to be vulnerable with the hope that life will be better for having done so,” Dr. Finkel said.

In the case of the coronavirus, the benefit of trust — and widespread honesty — would be collective freedom from pandemic restrictions that have disrupted the country for more than a year.

It is a well-established principle in social psychology that a common enemy is supposed to bring people together, Dr. Finkel said. So when the pandemic first erupted last spring, he was intrigued.

As months passed, though, he saw the opposite.

“It’s almost like American society has crossed the Rubicon of distrust, where even things that should bring us together — like a big external threat that we need to come together to make better — even those things that should bring us together don’t, and even push us further apart,” he said.

Julie Bosman reported from Chicago, and Sarah Mervosh from New York.