LOS ANGELES — With a handful of cases now confirmed across the country, it’s clear that the worrisome, mutated omicron variant has secured a toehold in the United States.

But whether these initial infections ultimately fade out or prove to be the beachhead for a new viral assault depends largely on how the strain stacks up against a now-familiar foe: the delta variant.

Since it officially burst onto the scene last week, much of the discussion surrounding the omicron variant of the coronavirus has centered around what scientists admit they don’t know — whether it might spread more easily than other strains, change disease severity or more readily evade vaccine protection.

Yet just as important a question, suggests John Moore, a professor of microbiology and immunology at Weill Cornell Medical College, is this: What if it’s no match for the devil we know?

“Can delta outcompete omicron, or will omicron thrive in the face of delta?” Moore said Thursday at a forum sponsored by UC San Francisco. “That’s just a complete unknown at the moment.”

Omicron may not become dominant in the U.S.

The delta variant has long been the dominant strain circulating in the U.S. — and it was the culprit behind a renewed wave of cases, hospitalizations and deaths that swept across the country during the summer.

But even the term “dominant” undersells just how widespread delta is. In the United States, it’s nearly omnipresent.

“I know that the news is focused on omicron, but we should remember that 99.9% of cases in the country right now are from the delta variant,” said Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention. “Delta continues to drive cases across the country, especially in those who are unvaccinated.”

The reason for the strain’s supremacy is simple. It’s the most transmissible variant of the coronavirus yet.

And because of its infectivity, delta has been able to elbow out any other variants that otherwise might have spread more widely.

Look no further than the beta variant, which scientists also considered to be a potential threat because it exhibited characteristics some worried could imperil the efficacy of COVID-19 vaccines. Scientists worried it could become a newly dominant variant.

“Never happened,” Moore said. “Beta was squelched out by delta. Well, that could happen to omicron.”

Uncertainty swirls around new strain

It remains unknown whether omicron can spread as readily as delta.

However, it’s clear that omicron dwarfs its coronavirus compatriot in the number of mutations it possesses. Scientists are still trying to wrap their heads around the full ramifications of that.

Omicron hasn’t really been tested against delta. Its rise in South Africa, for example, came at a time when there were a relatively low number of cases in the region, and delta was not being transmitted as much as it is now in the United States.

“Omicron arose in South Africa in an environment where it didn’t have a lot of delta going around to compete with,” Moore said. “We have a lot of delta infections still, and we’re going to get more in the coming weeks.”

Still cause for concern

That’s not exactly good news. While the scope of the potential danger posed by omicron is still being assessed, delta has proved more than potent enough to cut a devastating swath through the U.S.

Even without factoring in omicron, “we already are facing a delta-driven winter surge that is going to kill another 100,000 to 150,000 Americans,” Moore said.

“Most of us think that’s a pretty bad winter. So how much worse could it get? Well, it could get very much worse, but I think my money is on delta dominating the winter pandemic.”

Some 6,000 Americans are dying of COVID-19 every week, a trend that the CDC predicts will continue or even worsen through Christmas Day. California is averaging more than 500 COVID-19 deaths a week, and there are some scenarios that forecast an uptick in weekly deaths later this winter, especially if too few vaccinated adults get booster shots as their immunity weakens.

Nationwide, 70.6% of Americans have received at least one vaccine dose, and nearly 60% are considered fully vaccinated, CDC figures show. In California, those shares are 79.1% and 63.4%, respectively.

“We have a highly transmissible and dangerous variant in delta already circulating,” Moore said. “So, for sure, we’re going to have as a nation a very bad winter surge — in January, February — just as we saw last year with Alpha. But it will be concentrated in the one-third of the country that’s refused to have given themselves protection.”

All available data show those who are unvaccinated remain especially at risk for COVID-19.

State health officials estimate that unvaccinated Californians are seven times likelier to get COVID-19, more than 12 times likelier to require hospitalization and 15 times more likely to die from the disease.

“Given the evidence to date that these vaccines have been very effective against all of the variants we’ve seen, the one step I would urge everybody to take is get vaccinated if you’re not vaccinated and go ahead and get that booster shot as soon as possible,” Los Angeles County Public Health Director Barbara Ferrer said.

Best- and worst-case scenarios

If Omicron is highly transmissible but produces less severe disease than delta, this development would be a good thing, Moore said.

But it’s not clear that such a situation would let omicron outcompete delta.

“That’s, again, the unknown,” he said.

In Moore’s mind, a worst-case scenario would be “if omicron spreads rapidly and causes severe disease and breaks through vaccine protection. That’s really bad.”

“But if delta just squelches out omicron in the way it squelched out Alpha, then we should be rooting for delta: Better the devil you know than the devil you don’t — because we know that the vaccines can deal with delta. Very few people who are vaccinated are dying of delta,” he said.

He continued: “If omicron is lethal, it would be better for delta to squelch it out. It’s a perverse way of looking at things ... but if you game out possibilities ... what you’re not rooting for is the highly transmissible, lethal variant that blows through vaccines, because that’s what we feared all along.”

Though anecdotal reports have trickled in since the variant emerged, officials say it’s still too early to know how an omicron infection will affect sickness levels.

“Usually, it takes time for a severe illness to really require hospitalization,” said Dr. Regina Chinsio-Kwong, an Orange County deputy health officer. “So we will find out, probably in another two weeks, how severe omicron can be on the system. But it will take time.”

South African doctors don’t seem to be panicking

Obviously things could change, but Moore said it’s noteworthy that the chairman of the South African Medical Assn. said in a newspaper op-ed published earlier this week that “no one here in South Africa is known to have been hospitalized with the Omicron variant.”

“That’s an interesting perspective. It may or may not turn out to be right. But it’s a viewpoint from someone in authority in South Africa, who is in touch with the data sets,” Moore said.

“So there is a possibility that all omicron is is a highly adapted variant that will spread but will not cause serious disease. It may be a move this virus is making toward a non-pathogenic variant that is akin to the common cold viruses — coronaviruses. I don’t know if that’s going to happen.”

It’s also entirely plausible that omicron could cause more vulnerable people to get severely ill and be hospitalized. Many of those recently infected have been younger, otherwise healthier people, who aren’t particularly likely to get severely ill with COVID-19.

Moore said he has read news accounts that the omicron surges in South Africa have related to college students meeting up at parties to commemorate the end of their academic year.

“Perhaps in South Africa, omicron hasn’t burst its way into a nursing home full of elderly and vulnerable patients,” Moore said.

Could reinfection rates be worse with omicron?

There’s some initial suggestion that omicron is more capable of reinfecting people who have recovered from earlier coronavirus infections, Moore said.

“Not a massive effect. But it’s statistically significant,” he said, based on preliminary analyses he’s reviewed. “And that would be consistent with the highly mutated nature of omicron.”