EXPLAINER: Who must follow Biden's vaccine mandates?

Millions of health care workers across the U.S. must be vaccinated as a result of a mandate from President Joe Biden's administration that is taking effect

Millions of health care workers across the U.S. now must be vaccinated against COVID-19 as a result of a mandate from President Joe Biden's administration.

The vaccine requirement for Medicare and Medicaid providers was one of several mandates Biden's administration imposed upon private-sector employers to try to drive up vaccination rates and slow the spread of the coronavirus. Health workers in about half the states must have had their first dose of the vaccine by Thursday, while the rest will have to meet deadlines in February.

Two of Biden's other high-profile orders are on hold. The Supreme Court blocked a rule requiring employers with more than 100 workers to be vaccinated or tested weekly for COVID-19. A lower court also blocked a requirement for employees of federal contractors to be vaccinated.

Biden's various vaccine orders were challenged in court by Republican-led states, conservative groups and some businesses. The lawsuits argued in part that the mandates exceeded federal executive powers and infringed on states’ rights to regulate public health matters.