General Electric Co. GE 0.59% , Union Pacific Corp. UNP -0.08% and other large U.S. employers are imposing Covid-19 vaccine mandates for their workers to comply with a Dec. 8 deadline set by the Biden administration for companies that are federal contractors.

Boeing Co. , International Business Machines Corp. and Raytheon Technologies Corp. are other federal contractors that have already announced Covid-19 vaccine mandates for their U.S. staff. Together with GE and Union Pacific, these companies collectively employ more than 300,000 U.S. workers.

Employees of government contractors are required to get vaccinated against Covid-19 under an executive order signed in September by President Biden. Contractors can request an accommodation for a religious belief or disability, but cannot opt out of the shot through Covid-19 testing. The White House has also said it plans to require companies that employ 100 or more workers to require their employees be vaccinated or undergo regular Covid-19 testing, but that policy is awaiting a formal rule from the Occupational Safety and Health Administration.

While some large companies, such as Walt Disney Co. and meatpacker Tyson FoodsInc., have imposed vaccine mandates, most businesses are awaiting more details from OSHA. Some companies have pushed back return-to-office plans and some business groups have raised concerns about the burden of complying with the standard.

Union Pacific said it is abiding by the executive order pertaining to federal contractors because the railroad operator ships items like military equipment around the country for the armed services.

“Along with being our best defense against severe Covid-19 illness, vaccines are our best path forward out of the pandemic,” Beth Whited, Union Pacific’s human resources chief, said in a note to staff last week.

Workers must report their vaccination status or have an approved medical or religious accommodation to exempt them. Union Pacific is offering incentives like extra vacation time to nonunion workers and cash payments to unionized workers. Failure to do so will result in discipline including termination for nonunion workers, while unionized workers can be medically disqualified from duty.