The federal agency that President Biden has tasked with implementing a national vaccine mandate has struggled to respond to the coronavirus pandemic, drawing fire for failing to issue broad safety rules specifically aimed at keeping workers safe from workplace transmission.

But with the vaccine mandate announced Thursday, Biden has finally given the Occupational Safety and Health Administration (OSHA) the green light to take aggressive action.

The new rule compels private companies with more than 100 workers to require employees to get vaccinated or submit to regular testing for the virus. Agency officials, who only learned about the mandate last week, must now move quickly to draft a plan for implementing and enforcing the mandate, a complex undertaking that represents one of the biggest challenges in the agency’s 50-year history.