The U.S. government on Saturday authorized Johnson & Johnson's single-dose COVID-19 vaccine, enabling millions more Americans to be vaccinated in the coming weeks and setting the vaccine up for additional approvals around the world.

The J&J vaccine is the third authorized in the United States, following ones from Pfizer-BioNTech and Moderna, both of which require two doses.

The U.S. Food and Drug Administration announced the emergency use authorization of the J&J vaccine for adults aged 18 and older following Friday's unanimous endorsement by the agency's panel of outside experts. Shipments to vaccination sites are expected to begin Sunday or Monday.