Amazon pushes its return to office to January.

More large corporations, including Amazon, BlackRock and Wells Fargo, announced on Thursday that they would delay their return to office plans as the spread of the Delta variant causes the number of coronavirus cases to climb.

Amazon took the biggest step, telling its corporate employees that they did not need to return to their offices until Jan. 3, pushing back a deadline that had been set for early September.

The company said in the spring that it wanted to retain its “office-centric” culture, though later embraced more of a hybrid approach preferred by many employees.

The notification to employees did not indicate any changes to its vaccination policy, which encourages but does not mandate vaccines, or its mask policy, which allows workers at both its corporate offices and warehouses to be unmasked if they provide proof of vaccination.

BlackRock, the world’s largest money manager, and Wells Fargo, one of the nation’s largest banks, said in internal memos to U.S. employees that they would postpone their mandatory return plans until early October, from September. Both attributed the changes to worrisome new developments in the spread of Covid-19 in the United States.

“We are following the Delta variant in different parts of the country and closely monitoring the latest guidance from public health officials and local government authorities that encourages people to wear masks in indoor public spaces in areas of substantial or high transmission,” BlackRock’s chief operating officer, Rob Goldstein, and other officials wrote in their memo to employees. “We know this raises concerns about returning to the office.”

To better accommodate workers, BlackRock said it would allow only vaccinated employees in its offices until Oct. 1, rather than the original date of Sept. 8. The firm has also reestablished mask mandates for workers currently in the office in cities including Atlanta and Washington, where local authorities now require indoor masking.

When BlackRock employees return to their offices in larger numbers, the money manager will introduce a hybrid policy whereby employees can work three days per week in the office and two from a remote location.

At Wells Fargo, employees who are working from home will be called back starting Oct. 4, instead of Sept. 7, according to a memo to staff. Call-center and operations workers will be expected to come in first, followed by corporate and support staff in November. The delay comes just weeks after the company said it would allow more flexibility for employees to work in the office and remotely, in contrast with other banking giants that have summoned staff back earlier, and in greater numbers.

“We will continue to monitor the situation and make further adjustments if required to prioritize the health and safety of our employees and customers,” Wells Fargo’s chief operating officer, Scott Powell, wrote in a memo to staff.

Both BlackRock and Wells Fargo are encouraging employees to get vaccinated.