Facebook parent company Meta said on Monday that it will require Covid-19 booster shots for employees, and it is delaying a full employee return to U.S. offices until March 28 as the Omicron variant rewrites office reopening plans in Silicon Valley.

Meta said on Monday that it made the choice to push back its return-to-office date to give employees more flexibility during the pandemic. The company previously planned to fully reopen offices for vaccinated employees on Jan. 31. Its headquarters are in Menlo Park, Calif.

“We’re focused on making sure our employees continue to have choices about where they work given the current COVID-19 landscape. We understand that the continued uncertainty makes this a difficult time to make decisions about where to work, so we’re giving more time to choose what works best for them,” Janelle Gale, Meta VP of human resources, said in a statement.

If employees want to work remotely after March 28, they will need to request a deferral from Meta by mid-March, the company said. Those deferrals will last between three and five months.

The Omicron variant of Covid-19 has forced several of Meta’s neighbors in Silicon Valley to push back their return-to-work plans, but Meta is one of the first big companies to tell its employees that proof of a booster shot will be required to work in the office.

Apple told employees last month that it did not have a firm date to be back in the office. In December, Google parent Alphabet pushed back its Jan 10. return date indefinitely until the workers can return to a “stable, long-term working environment.”