Demand for coronavirus vaccines has spiked in the United States in recent weeks, as more Americans are eligible for booster shots and concerns grow over the omicron variant.

Health-care providers administered 2.18 million doses of coronavirus vaccines on Thursday, according to the Centers for Disease Control and Prevention — the “highest single-day total since May,” the White House said. According to the latest CDC report, over the week ending on Thursday, the average number of daily administered vaccine doses reported to the agency was 22 percent higher than the previous week.

Since omicron was first confirmed in southern Africa on Nov. 25, and soon after listed as a “variant of concern” by the World Health Organization, the Biden administration has been urging as many Americans as possible to get booster shots as the best means to protect themselves against it. All U.S. adults became eligible for boosters on Nov. 19.

For most of October, fewer than or slightly over 1 million doses of coronavirus vaccines were reported to the CDC as being administered every day in the United States. By mid-November, those numbers hovered around 1.5 million on average. In the past three reporting days, they neared or exceeded 2 million.

Much is still unknown about omicron, against which some scientists and vaccine makers predict existing vaccines may not be as effective, but the new variant and “the idea that we have a virus that may evade two shots but that three shots are likely to be pretty protective, I think, has also gotten a lot of people much more focused on getting a booster,” says Ashish K. Jha, dean of the Brown University School of Public Health.

The increased demand for coronavirus vaccines is largely driven by demand for booster doses, CDC data shows. White House covid-19 data director Cyrus Shahpar, calling out the booster numbers, cited the spread of omicron in a tweet, saying the country needs “to keep this up.”