Tens of millions of Americans who work at companies with 100 or more employees will need to be fully vaccinated against COVID-19 by Jan. 4 or get tested for the virus weekly under government rules issued Thursday.

"We know that vaccines absolutely save lives and we know that mandates work," says Dr. Anthony Fauci, Director, National Institute of Allergy and Infectious Diseases.

The federal government’s mandates have a deadline.

People who work with companies that have 100 or more employees, certain health care workers, and federal contractors will need to be fully vaccinated by Jan. 4.

People with big companies will have the option of wearing masks and getting tested every week.

However, pushback against the mandate is building.

Louisiana, Indiana, and Mississippi filed a lawsuit against the Biden administration on Thursday. The states say the mandates constitute federal overreach and risk negatively impacting the workforce. The attorney general in Florida says she will legally challenge the mandate for large companies. In South Carolina, Gov. Henry McMaster issued an executive order prohibiting state agencies from requiring employees to get vaccinated. The state is one of a long list of states that have already filed lawsuit against the administration over the mandates.

At least 40 senate Republicans say they plan to challenge the mandate for large employers using the Congressional Review Act.

However, the White House is standing its ground.