Several hospital systems that previously held off making vaccines mandatory for health care workers are now willing to do so. Google employees in California who have returned to the office on a voluntary basis are again wearing masks indoors. Goldman Sachs is considering whether to reinstitute testing for fully vaccinated employees in the company's New York City offices, according to a person familiar with the situation who spoke on condition of anonymity because nothing had been decided. And Monday, Apple told its workforce that it would push back its return-to-office date from September to October.

When companies began announcing tentative return-to-office plans this spring, there was a sense of optimism behind the messages. COVID cases were dwindling in the United States as the vaccine rollout picked up pace. Employers largely hoped their workforces would get shots on their own, motivated by raffle tickets, paid time off and other perks, if not by the consensus of the medical community.

In recent days, that tone has suddenly shifted. The delta variant, a more contagious version of the coronavirus, is sweeping through the country. Less than half of Americans are fully vaccinated, exacerbating the situation.

Nationally, the average of new coronavirus infections has surged 180% in 14 days, to more than 45,343 a day Thursday; and deaths — a lagging number — are up 30% from two weeks ago, to nearly 252, according to New York Times case counts. Vaccines are still unavailable for children younger than 12, many of whom are preparing for an in-person return to school this fall.

It all adds up to a difficult calculation for America's business leaders, who hoped the country would already be fully on a path to normalcy, with employees getting back to offices. Instead, individual companies are now being forced to make tough decisions that they had hoped could be avoided, such as whether to reverse reopening plans or institute vaccine mandates for employees. All the while, they continue to grapple with the unpredictable nature of the pandemic.

"It's emotionally draining on all of us, and it drives the top management teams crazy," said Bob Sutton, a psychology professor at Stanford University who studies leadership and organizations. He said some executives he had advised were "pulling their hair out" over what to do.

For employers wary of the legal ramifications and political backlash of mandating a vaccine, the tide has begun to turn, if ever so slightly.

"At the beginning, there were a lot of employers that were concerned about jumping in too soon and being the one out front; it is a divisive issue," said David Barron, a labor and employment lawyer at the law firm Cozen O'Connor. "The calculus starts to shift a little bit when you see another spike."

New York Mayor Bill de Blasio on Friday encouraged private employers to require workers to get vaccinated. He also said the city might broaden the number of city workers required to get vaccinated or tested weekly.

Recent court decisions have upheld employers' rights to require vaccinations, including a ruling that said Houston Methodist Hospital could require health care workers to get shots. On Monday, a federal judge ruled that Indiana University could require students to get vaccinated as well.

"The legal authority continues to line up on the side of employers being allowed to mandate vaccines if they choose to," said Douglas Brayley, an employment lawyer at the global law firm Ropes & Gray.

When Twitter reopened its San Francisco office this month at 50% capacity for those who wanted to come back, only vaccinated workers were allowed inside. In June, a civilian group that oversees the Los Angeles Police Department was examining the possibility of requiring police officers to get shots. And numerous colleges around the country have required students and staff to be fully inoculated before they step foot on campus in the fall.

"The recent news of delta surging in some places is just adding to that determination to be as safe as we possibly can," said Tim Killeen, president of the University of Illinois public school system, which instituted a vaccine requirement Wednesday.

Novant Heath, a North Carolina-based health care company with more than 35,000 employees, said Thursday it would make vaccinations mandatory for its workers by Sept. 15. The move came after efforts to overcome vaccine hesitancy through education and making shots easily accessible stagnated.

"Now that almost 4 billion doses of vaccine have been given around the world, and we see that it's safe and effective, we see that the delta variant is obviously here, and we have it in our communities, and that almost all the patients being added to our hospitals are unvaccinated, the time was right to say, 'We've got to move forward with requiring vaccines of our team members,'" said Dr. David Priest, the company's chief safety officer.

For others, high voluntary vaccination rates among employees have made requiring the shot simpler. Morgan Stanley, an investment banking firm, is requiring employees and guests at its New York offices to be fully vaccinated, according to a person familiar with the situation who spoke on condition of anonymity to discuss company protocols. By the time it imposed the mandate in June, 90% of its employees were vaccinated.

Vaccine mandates are still not the approach that most companies are taking. And the risk that the coronavirus poses to much of the population is far from what it was at the worst of the pandemic. New cases, hospitalizations and deaths remain at a fraction of their previous peaks, largely localized to areas with low vaccination rates. Vaccines remain effective against the worst outcomes of COVID-19, including from the delta variant.

"The big question is not so much, 'Can we keep workers safe in our buildings?' but, 'Will workers feel comfortable enough coming back, even if good controls are in place?'" said Joseph Allen, an associate professor at the Harvard T.H. Chan School of Public Health who advises companies on COVID-19 strategies. "There's a renewed anxiety that maybe started to dissipate in the spring — but it's back."

That tension may make it more difficult to convince workers to return to the office. In California's Silicon Valley, tech companies largely embraced the new era of remote work during the pandemic. But not all have been eager to let their employees stay home for good.

In June, Apple CEO Tim Cook told employees that they would be required to return to the office at least three days a week, starting in September. About 1,800 employees sent Cook a letter calling for a more flexible approach.

He did not respond, but days later, Apple posted an internal video in which company executives doubled down on bringing workers back to the office. In the video, Dr. Sumbul Desai, who helps run Apple's digital health division, encouraged workers to get vaccinated but stopped short of saying they would be required, according to a transcript viewed by the Times.

The video did not sit well with some employees.

"OK, you want me to put my life on the line to come back to the office, which will also decrease my productivity, and you're not giving me any logic on why I actually need to do that?" said Ashley Gjovik, a senior engineering program manager.

When the company on Monday delayed its return-to-office date, a group of employees drafted a new letter, proposing a one-year pilot program in which people could work from home full time if they chose to. The letter said that an informal survey of more than 1,000 Apple employees found that roughly two-thirds would question their future at the company if they were required to return to the office.

In Los Angeles, Endeavor, parent company of the William Morris Endeavor talent agency, reopened its Beverly Hills headquarters this month. But it decided to shut down again last week when the county reimposed its indoor mask mandate in the face of surging case counts. An Endeavor spokesperson said that the company decided enforcement would be too difficult and would hinder group meetings.

The employment website Indeed had been targeting Sept. 7 as the date when it would start bringing workers back on a hybrid basis. Now it has begun to reconsider those plans, the company's senior vice president of human resources, Paul Wolfe, said, "because of the delta variant."

Some companies said the recent spike in cases had not yet impacted their return-to-office planning. Facebook still intends to reopen at 50% capacity by early September. IBM plans to open its U.S. offices in early September, with fully vaccinated employees free to go without a mask; and Royal Dutch Shell, a gas company, has been gradually lifting restrictions in its Houston offices, prompting more of its workers to return.

Hewlett-Packard Enterprise began allowing employees to return to its offices Monday, bolstered by a survey of its California employees that found 94% were fully vaccinated.

"That gives us an added layer of comfort," said Adam Bauer, a company spokesperson.

Wells Fargo told its employees July 16 it would begin to bring employees currently working remotely back to the office Sept. 7. But unlike banks that earlier called workers back with declarative language ringing in a new stage of the pandemic, the memo, sent by the bank's chief operating officer, Scott Powell, had a notable degree of caution.

"The timing communicated in this message is dependent on our assumption that the pandemic continues to remain stable or further improves," Powell wrote. "We continue to actively monitor the situation and any developments, including new variants."