There's no need for variant-specific vaccines, they said, while hedging on the question of redefining what it means to be fully vaccinated, saying instead that Americans should get booster shots because it's the sensible thing to do.
And they noted that the Delta variant is wreaking plenty of havoc on its own. The US Centers for Disease Control and Prevention predicted an increase in the rate of deaths from Covid-19 over the coming weeks. It projected between 837,000 and 845,000 people will have died by January 8.
An average of close to 120,000 new Covid-19 cases are being diagnosed each day, according to data from Johns Hopkins University -- 50% more than a month ago. And more than 1,200 people are dying every day, on average.
"We are already in a Delta surge," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN's Wolf Blitzer.
"The cases are going up. We have an average of about 117,000 cases. We have an increase in the percentage of hospitalizations. Deaths are still over a thousand," Fauci said. "Then you have, looking over your shoulder, the Omicron variant, which we know, from what's going on in South Africa and in the UK, is a highly transmissible virus.
"That's the reason why we are encouraging people, if they haven't been vaccinated, to get vaccinated but, as importantly, for those who've been fully vaccinated to get a booster."
There is no doubt Omicron will infect people who have been vaccinated, Fauci said.
"There will be breakthrough infections, no doubt about that," he said.
Asked whether that means the federal government should redefine what it means to be fully vaccinated, Fauci said that does not really matter.
"Well, it's almost a semantic thing for the regulation, as it were," Fauci said. It just makes sense for people to get booster shots, he said.
"We know what optimal is. Optimal is getting a boost. So instead of worrying about what the definition of fully versus not fully is, I'm telling people, if you want to be optimally protected and you're vaccinated, get boosted. That's the message and not worry about a definition," he added.
For now, the federal government considers people to be fully vaccinated if they have received two doses of Moderna's or Pfizer's vaccine, or a single dose of the Johnson & Johnson vaccine -- even with the strong recommendations to add a dose to any of those regimens.
"The definition right now is two doses of an mRNA vaccine or a single dose of the J&J vaccine," US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky told a White House Covid-19 briefing.
More important is getting more people vaccinated, Fauci said.
"The vulnerable people are the people who have not been vaccinated, and I hope that the possibility that we're seeing -- that we're going to be getting a surge of Omicron, which is almost inevitable given its characteristic of high degree of transmissibility," Fauci said. "We have the tools to be able to blunt this. We just need to implement them."
Vaccine makers have said they are working ahead to ready formulations of their vaccines that would specifically protect against the Omicron variant. But Fauci said there's no need for them yet.
"At this point, there is no need for a variant-specific booster," Fauci told the White House briefing Wednesday morning.
It's probably too late, anyway, Dr. Peter Hotez, a vaccine specialist and dean of tropical medicine at the Baylor College of Medicine, told CNN's Alisyn Camerota and Victor Blackwell.
"It takes time to make that. They are talking maybe three or four months. This freight train is here already," he said. "It looks like it's going to be here around Christmas so that there's not going to be enough time to really have that booster in place to make a meaningful difference."
Americans are taking the advice to get booster doses to heart, with more than half of the nearly 1.8 million vaccines going into arms each day being booster shots, according to the CDC. Just over 61% of the population is now fully vaccinated and the pace of vaccinations is up 23% over a month ago.
"Across the first two weeks of December, we've gotten 14 million booster shots. That's our highest ever two-week total. So our booster program is accelerating," White House Covid-19 response coordinator Jeff Zients told the White House briefing.
"In looking at early data on transmissibility of Omicron from other countries, we expect to see the proportion of Omicron cases here in the United States continue to grow in the coming weeks," Walensky told the White House briefing. "Early data suggest that Omicron is more transmissible than Delta, with a doubling time of about two days."
But US health officials were keeping a wary eye on Britain, which reported its highest daily number of new coronavirus infections ever Wednesday -- 78,610.
Dr. Jenny Harries, Chief Executive of the UK Health Security Agency, said case numbers will be "quite staggering compared to the rate of growth that we've seen in cases of previous variants."
More studies indicated that current vaccines will not fully protect people against infection, and one worrying study from researchers in Hong Kong indicated that one of the most widely distributed vaccines, China's Sinovac Covid-19 vaccine doesn't provide sufficient antibodies to protect against the Omicron variant.
The researchers tested samples of Omicron variant virus in blood taken from 25 people who had received both doses of either Sinovac's Coronavac or Pfizer/BioNTech vaccines.
"None of the Coronavac recipients had detectable neutralizing antibody to the Omicron variants," they wrote in a report posted online on the pre-print server MedRxiv. They say it has been accepted for publication in the journal Clinical Infectious Diseases.
Blood taken from people who got the Pfizer/BioNTech vaccine also showed reduced immune response to the Omicron samples, with just 20% to 24% of samples producing sufficient neutralizing antibodies, the study by Kelvin Kai-Wang To and colleagues at the University of Hong Kong found.
And a study from US researchers showed the protection from two doses of Moderna's vaccine wears off, but a booster dose restores much of it -- even against the Omicron variant.