WASHINGTON — At least 400,000 people in the United States have received COVID-19 booster shots since the extra injections were authorized last week, the Biden administration said Tuesday.

“Our planning and preparation on boosters have propelled a strong start,” Jeff Zients, a White House COVID-19 response coordinator, told reporters during a livestreamed news conference.

Zients said most of the 400,000 injections were administered over the weekend, and nearly 1 million people have scheduled appointments to get their third shot of the Pfizer-BioNTech vaccine. The administration’s partnerships with states, long-term care facilities, doctors and pharmacies enabled it to “immediately” roll out boosters following last week’s approval of the shots by the federal government, Zients said.

“We are off to a very strong start with the booster campaign,” Zients said. “We will continue working with our partners to execute so we can efficiently and equitably provide the added protection of booster shots to eligible Americans.”

The Centers for Disease Control and Prevention on Thursday greenlighted boosters of Pfizer’s vaccine for people over age 65, those living in long-term care facilities and other adults with certain underlying health conditions. Officials also cleared the way for adult workers facing a high risk of contracting the virus to get boosters. Eligible Americans can get the third shot at least six months after they receive their second.

A majority of vaccinated Americans have taken the Pfizer shot, according to the CDC. Regulators are expected to eventually approve some form of booster shots for those who received the Moderna or Johnson & Johnson vaccines.

Despite the recent focus on booster shots, Zients said increasing overall vaccination rates remains the government’s “top priority.”

About 77% of eligible adult Americans have had at least one vaccination shot, according to federal statistics, with about 65% considered fully vaccinated. About 23% haven’t gotten any shots, President Biden said Monday just before getting his booster shot on national television.

Earlier this month, a panel of experts that advises the Food and Drug Administration rejected a plan that would have made booster shots available to most Americans because it is unclear how it would substantially slow the spread of the deadly virus, they said.

Experts have urged the White House to focus on increasing vaccination globally to limit the chances that new and potentially more deadly variants emerge.

Zients said the Biden administration is “laser-focused on getting more Americans vaccinated” but is also determined to increase manufacturing and shipping capacity to help other countries get vaccines.

In the last 10 days, the U.S. has shipped millions of doses to Mexico, Vietnam, Honduras, South Africa and the Philippines, Zients said. Biden pledged to ship more than 1 billion doses to other countries by next fall, a fraction of what experts say is needed to safeguard the world.

“This virus knows no borders,” Zients said, “and stamping out the virus everywhere protects our health and our economy here at home.”