Up until a few weeks ago, corporate leaders felt confident about what to expect this fall.

Offices would reopen after Labor Day. Business travel would resume more broadly. Long-delayed work gatherings, conventions and off-site meetings would finally take place.

The pandemic has, once again, upended many of those plans.

The swift, startling resurgence of Covid-19 cases and hospitalizations across the U.S. is causing corporate leaders to rip up playbooks for the next few months.

No longer is a September return a target for many companies. Some employers, such as banking giant Wells Fargo & Co. and managed-care company Centene Corp. , have in recent days shifted return-to-office dates to October. Meanwhile, a range of other prominent companies now predict it will be 2022 until most workers return.

No longer is a September return a target for many companies, like Amazon.com.

Photo: David Ryder/Getty Images

Amazon.com Inc. on Thursday delayed its return for corporate employees to at least Jan. 3, from September. Lyft Inc. pushed back its planned office reopening to February 2022. Other companies, such as Dell Technologies Inc., have postponed fall returns in the U.S. without giving new reopening dates. Business travel at the company also remains largely restricted.

In a memo to employees Tuesday, Dell Chief Operating Officer Jeff Clarke said that, because of the Delta variant, many cities where Dell operates in the U.S. have moved from “green” to “red” on an internal company risk dashboard.

“We expect this trend to continue for the next several months,” Mr. Clarke said, adding, “when we have line of sight to new reopening dates we’ll communicate to you with plenty of time to plan.”

The delays come at a precarious time for many companies and workers. White-collar employees, while often not eager to give up remote arrangements entirely, have looked forward to some measure of normalcy in the coming months, with more in-person interactions with colleagues. Bosses remain concerned about their workers’ mental health and how to keep them motivated as the pandemic drags on.

“I think it is fair to say that we are all feeling a mix of anticipation, anxiety, frustration, confusion, and exasperation,” CNN President Jeff Zucker told staff Thursday in announcing a month-long delay of the network’s reopening to October. “But we will get there. Continue to take care of yourselves and each other.”

CNN President Jeff Zucker said the network’s official return-to-office date in September will now be postponed.

Photo: sergio perez/Reuters

Much is changing, and quickly. Industry events that had been planned for later this month or this fall have been scrapped or gone virtual. Organizers of the New York Auto Show, scheduled to run from Aug. 20 to Aug. 29, canceled the event, citing challenges related to the Delta variant. In Nashville, Tenn., the 3686 Festival, which draws startup founders, investors and others, canceled its event in September. “We look forward to connecting again in person in 2022,” organizers said in a note to attendees.

UKG Inc., an HR and workforce management software company, plans to hold an internal sales kickoff with thousands of employees in October. That is in addition to two user conferences, slated for November and December in Las Vegas, that typically draw roughly 5,000 customers each. So far, the events are still on, though CEO Aron Ain says the company will evaluate as the dates get closer. “We’ll do what we have to do,” Mr. Ain says.

In a video message from his home office on Friday, Mr. Ain told UKG’s more than 13,000 employees that the company would delay the next phase of its voluntary office reopening, slated for mid-August for vaccinated employees.

“We had plans all set a week ago, and we’ve adjusted those just in the past week again, but what are we supposed to do? It is what it is; it’s out of my control,” Mr. Ain said in an interview.

The company, which has corporate offices in Lowell, Mass., and Weston, Fla., plans to bring more employees back beginning Oct. 4, but that timeline could change, too. One of the challenges of managing in the current moment is that there is little parallel, Mr. Ain said. “I have 42 years of experience to draw on, but I don’t have experience on what we’re navigating right now,” he said.

Some office workers are back, but a range of companies now predict it will be 2022 until most people return.

Photo: Gabby Jones for The Wall Street Journal

At Bank of America Corp. , executives have sent memos in recent days requiring employees on site to wear face coverings while away from their desks and in spaces like conference rooms. JPMorgan Chase & Co., in a note to employees Friday, said that the rise in Covid-19 cases “is a cause of increasing anxiety for many.” The company said it would continue its return-to-office schedule but reinstate a mask mandate for its U.S. employees.

Many companies have stopped making reopening estimates altogether. The Houston corporate office of retailer Mattress Firm Inc. is still largely closed, with no reopening date set. “I don’t want to make any predictions,” CEO John Eck said. “Where we’re at now would indicate it’s going to be later than sooner.”

Corporate executives have flooded medical consultants with calls in recent days, looking for guidance on how to proceed. Roslyn Stone, CEO of Zero Hour Health, a health advisory firm, has told companies to restart Covid-19 task forces that have been disbanded. She has asked restaurants what they did with the plexiglass dividers some removed in recent weeks and encouraged them to resume daily wellness checks and take stock of the personal protective equipment, like masks, on hand.

As recently as July, a number of large-company executives were calling her for advice on how to safely resume large, in-person company meetings planned this fall. “The hot question two weeks ago was to help us develop a plan to reduce our risk,” she said. The conversation now: “When do you think we might be able to reschedule it?”

Ms. Stone added: “I wish I knew the answer to that question.”

SHARE YOUR THOUGHTS

How has your workplace responded to the recent rise in Covid cases and hospitalizations? Join the conversation below.

The situation now is different from the beginning of the pandemic in that employers better understand the virus and can act to safeguard their workforce, like mandating vaccines for all employees, said former Medtronic PLC CEO Bill George, a senior fellow at Harvard Business School.

Even so, part of the frustration, corporate leaders say, is that much remains fluid. Eyeglasses seller Warby Parker, which employs about 3,000 people, made the decision in recent days to delay bringing more corporate employees back to its offices in New York and Nashville.

Instead of a Sept. 14 reopening, the company is planning on Oct. 19 at the earliest. But the new timeline will depend, in part, on whether schools reopen as planned, said Neil Blumenthal, Warby Parker’s co-founder and co-chief executive.

“We’re back at the same level of uncertainty that we were a year ago,” Mr. Blumenthal said. “This is frustrating, and it’s maddening.”

Neil Blumenthal, Warby Parker’s co-founder and co-chief executive.

Photo: Patrick T. Fallon/Bloomberg News

—Benjamin Mullin, David Benoit and Ben Eisen contributed to this article.

Write to Chip Cutter at chip.cutter@wsj.com