The delta variant of the coronavirus is undoing major employers' plans to invite workers back to the office.

Twitter closed its offices in New York and San Francisco on Wednesday, just two weeks after reopening them, the San Francisco Chronicle reports. On the same day, Google pushed back its official return-to-office date to at least mid-October from its initial plan to come back after Labor Day, and Lyft won't be calling its office workers back until February at the earliest, The New York Times reports.

Twitter and Google made their decisions just over a week after Apple also pushed its return date back to October, with all three companies citing the rise in cases driven by the delta variant as cause for the change. Google is also among a raft of companies mandating vaccinations for employees when they do return to the office, including Facebook, Bloomberg reports.

The Equal Employment Opportunity Commission has confirmed that employers are within their rights to require vaccines of their workers, with The Durst Organization going so far as to threaten workers who refuse vaccines with termination, Business Insider reports.

Vaccinations alone won't be enough to allow an office to operate as it did before the pandemic. The Centers for Disease Control and Prevention issued new guidance recommending wearing masks indoors in areas with high infection rates regardless of vaccination status. Citigroup and Disney have reimposed mask mandates on their employees, a decision that is rapidly becoming more common, especially in retail businesses, the Times reports.

Sixty percent of the U.S. population over 18 is fully vaccinated, but portions of the remaining 40% are more determined than ever to refuse. Unless and until cases start to decline again, more policies and behaviors that had been discarded as the weather warmed are likely to return.