First on CNN: More Americans are getting their booster shots, but ‘it’s not enough,’ Biden administration warns

(CNN) - More Americans are getting their Covid-19 updated booster shots, new data show, but Biden administration officials caution the recent uptake is still not enough as the winter months and the threat of new outbreaks approach.

Data from the US Centers for Disease Control and Prevention expected to be posted later Thursday and shared first with CNN show that 4.5 million Americans received the bivalent booster, which targets both the original strain of the coronavirus as well as the Omicron subvariants BA.4 and BA.5, over the past week.

"In the first three weeks, we saw about 7.5 million shots; in the last three weeks, we've seen almost 12 (million). That's a near 60% increase -- that's important progress," an administration official said, for a total of approximately 20 million Americans, which is still less than 10% of the eligible population.

"That's the largest weekly increase since our fall vaccination program launched -- up 40% from the prior week," the administration official noted, adding that 3.27 million Americans received the shot the previous week.

Uptake of the bivalent booster vaccine, authorized by the US Food and Drug Administration in late August, got off to a slow start but has accelerated in recent weeks.

The official touted "good progress" in reaching high-risk Americans, including seniors, noting that, "Half of shots have gone to seniors. That means almost 10 (million) seniors -- nearly 1 in 5 seniors -- has already gotten their updated vaccine."

But as more Americans head indoors and the flu season begins, the official cautioned there is still much more work to do.

"This is really important progress, but it's not enough. We need everyone to step up and get their updated vaccine as soon as possible," the official said.

White House Covid-19 coordinator Dr. Ashish Jha echoed that warning.

"The work we're doing to reach Americans through on the ground work with trusted organizations in communities across the country and paid media is helping drive the urgency for all Americans to get the protection they need ahead of the winter," Jha said in a statement to CNN. He added, "But to be very clear: it's going to take everyone talking to their family and friends to ensure the country is as protected as possible. Our message is simple: do not wait to get your updated vaccine."

The Omicron subvariant BA. 5 still dominates cases in the United States, but a flurry of new Covid-19 variants appear to be gaining traction globally, raising fears of a winter surge.

In the United States, these are BQ.1, BQ.1.1, BF.7, BA.4.6, BA.2.75 and BA.2.75.2. In other countries, the recombinant variant XBB has been rising quickly and appears to be fueling a new wave of cases in Singapore. Cases are also rising in Europe and the United Kingdom, where these variants have taken hold.

While Covid-19 cases are dropping in the US, virus experts fear that the downward trend may soon reverse itself.