Watch Biden Live Stream: Covid Vaccine Mandate Speech

President Biden on Thursday used the full force of his presidency to to get two-thirds of the American work force vaccinated against the coronavirus, including private sector employees, health care workers and federal contractors — as well as the vast majority of federal workers, who could face disciplinary measures if they refuse.

The sweeping moves, which the president announced in a White House speech, are the most expansive actions he has taken to control the pandemic since he assumed the presidency in January, and will affect almost every aspect of American society.

Initially reluctant to enact mandates, Mr. Biden is now waging an aggressive effort that will also put pressure on private businesses, states and schools to enact stricter vaccination and testing policies as the Delta variant continues its spread across the United States.

Mr. Biden is acting through a combination of executive orders and new federal rules. Under his plan, federal workers, contractors and health care workers employed by institutions that accept Medicare and Medicaid reimbursement will not be allowed to opt out of testing by undergoing mandatory vaccination. Private sector businesses that have 100 or more employees will have to require vaccination, or mandatory weekly testing, for their workers after Mr. Biden instructs the Occupational Safety and Health Administration to draft a rule.

OSHA, which is part of the Department of Labor, oversees workplace safety, which experts say extends to vaccine mandates. The agency has issued other guidelines for pandemic precautions, such as a rule in June requiring health care employers to provide protective equipment, provide adequate ventilation and ensure social distancing, among other measures.

Slightly more than half of Americans, 53 percent, are now fully vaccinated. While the number of people seeking shots ticked up considerably in August, as Delta caused virus cases to soar, the vaccination rate has yet to help the nation achieve “herd immunity” — the tipping point that occurs when widespread vaccination, coupled with natural immunity, slows the spread of a virus. If it continues to spread, officials fear that it will mutate into another, even more dangerous variant that evades vaccines.

“When you have 75 to 80 million people who are eligible to be vaccinated, who don’t get vaccinated, you’re going to have a dynamic of continual smoldering spread of the infection,” Mr. Biden’s top medical adviser for the coronavirus pandemic, Dr. Anthony S. Fauci, warned in an interview, adding, “It’s very frustrating, because we have the wherewithal within our power to be able to actually suppress it.”

The mandate for federal workers is an especially assertive move by the president. Jen Psaki, the White House press secretary, told reporters on Thursday that, aside from some religious and disability exemptions, the vast majority of federal workers would be subject to a 75-day grace period for receiving a vaccine.

If workers decline to receive shots in that…