(CNN) The new Omicron variant of coronavirus, with its many mutations and seemingly quick spread in South Africa, is worrying scientists and government officials.
But doctors want to remind Americans that they're already facing a pretty formidable coronavirus variant, and that's Delta.
Delta managed to take over the entire United States in a matter of weeks in early summer, changing the outlook for a country that was rolling out vaccines and hope with equal speed.
"In late June, the seven-day moving average of reported cases was around 12,000. On July 27, the seven-day moving average of cases reached over 60,000. This case rate looked more like the rate of cases we had seen before the vaccine was widely available," the US Centers for Disease Control and Prevention says on its website. "The Delta variant is highly contagious, more than 2x as contagious as previous variants."
Delta currently accounts for more than 99% of cases of coronavirus that are genetically sequenced in the US, according to CDC.
It remains to be seen whether Omicron will outcompete Delta, but it will be tough.
"We still have, of course, in the US, a serious surge of the Delta variant. We should be thinking about that," National Institutes of Health Director Dr. Francis Collins told CNN on Monday.
The US is now averaging 70,094 new Covid-19 cases and 730 deaths each day, according to Johns Hopkins University. And JHU says 75% of intensive care unit beds in the US are occupied, 15% of them by Covid-19 patients.
Much is being made of the 50 mutations that mark the Omicron variant -- 32 of them on the spike protein, which is the club-shaped structure that covers the surface of the virus and is used to attach to human cells so the virus may infect them.
But Delta has its own constellation of scary mutations, and they've made it the worst version of the virus yet seen. It races through populations, replacing more worrying variants that have mutations that should allow them to evade the effects of vaccines, like the Beta variant, for instance.
Robert Garry, a virologist at Tulane University, has done a head-to-head comparison of the mutations seen in Delta and Omicron.
Omicron does have "a chunk of them at once," Garry told CNN. "But we've kind of seen that kind of leap in evolution before," he added.
"There are definitely hotspots where this virus likes to mutate now," he said. But just because there are many mutations doesn't necessarily mean they'll add up to a meaner virus.
"What all those changes in the aggregate are going to do for the things that matter for this virus, we don't really know yet," Garry said.
But he does not see many important mutations that might make the Omicron version more contagious than Delta.
"The ones that might affect transmissibility, I mean, I'm just not seeing a whole lot that would give it a real strong advantage over Delta," he said.
"That's really the big question. You know, when it gets into a population that has Delta, is it going to out-compete or not out-compete?"
Other genetics experts also note Omicron does not carry some of the changes that helped make Delta so very contagious.
"Given that Omicron lacks so many of the non-spike mutations that have seemed to contribute to Delta's increased fitness I wouldn't be surprised if its intrinsic transmissibility is similar to Gamma," Trevor Bedford, a genome scientist and epidemiologist at the University of Washington and Fred Hutchinson Cancer Center in Seattle, said on Twitter. He referenced a study in September from researchers at the Broad Institute who found at least three mutations on the Delta variant that they said appeared to help make it more transmissible.
Some of the mutations that increase transmissibility are also seen in variants that died out, such as one called Kappa.
Garry does see mutations that might help Omicron to evade the body's immune response -- especially from previous infections.
"It's likely immune evasive. So is Delta, and before that Alpha and Beta. Should we be working on a specific vaccine? Yes," he said.
The immune response prompted by vaccination is broader than the response produced by a natural infection, so vaccinated people may still be protected from severe disease, doctors said.
"Your best protection against Delta is to get vaccinated, and if you've already been vaccinated and six months have passed since you got Pfizer or Moderna, get your booster, two months since J&J, get your booster," Collins told CNN.
"That was a reason already, but now add Omicron to the mix," he said. "And we do believe that this new variant, which will probably come to our shores, will also be something vaccines and boosters can help you with."
The CDC strengthened its guidance on boosters Monday, saying all adults should get booster shots six months after finishing the first two doses of Moderna's or Pfizer/BioNTech's vaccine and two months after getting Johnson & Johnson's single-dose vaccine.
And there's no known mutation that can make a virus evade precautions such as face masks, handwashing and physical distancing. Even if a mutation helps a virus become more viable as an airborne pathogen, better ventilation can help prevent transmission.