NEW YORK — Johnson & Johnson's coronavirus vaccine was cleared for use in the U.S., making a third shot available that could plug gaps in the nation's immunization campaign as concern grows over an influx of virus variants.

The Food and Drug Administration said in a statement Saturday that it had granted an emergency-use authorization for the single-dose vaccine for people 18 and older.

The decision comes after the U.S. hit a sad milestone with 500,000 Americans dead from the virus, and at a time when health officials are raising alarms that recent declines in the number of new cases may be stalling.

The J&J shot is highly effective at preventing severe COVID-19, with no serious side effects, agency staff said in a report Wednesday. On Friday, a committee of outside advisers to the FDA voted unanimously that the vaccine's benefits outweigh any risks. In the week ahead, J&J is prepared to ship 3 million to 4 million doses, Biden administration officials have said.

“We're in a race between the virus mutating — new variants coming out that could cause further disease — and stopping it,” said Jay Portnoy, director of the division of allergy, asthma and immunology at Children's Mercy Hospital, who supported an emergency authorization. “We need to get this vaccine out.”

The clearance marks another milestone in an unprecedented global scientific sprint. Since the World Health Organization declared the coronavirus a pandemic nearly a year ago, numerous vaccines have been deployed around the world, including shots developed in China and Russia. Worldwide, more than 231 million doses have been administered in 100 countries, according to data collected by Bloomberg.

J&J's shot is expected to be easier to distribute and administer than the Pfizer Inc.-BioNTech vaccine and the Moderna shot that were cleared by the FDA late last year. To reach full effectiveness, the earlier shots require two doses several weeks apart. They must also must be kept at very cold temperatures, while J&J's doses can be stored in a refrigerator for long periods.

Its simplicity could make J&J's shot especially beneficial for hard-to-reach rural areas that have seen higher rates of virus cases and deaths.