Delta vs. Omicron: Which COVID-19 variant will become dominant in the US?

A sign advertised COVID-19 testing in Times Square. There is evidence Omicron can infect vaccinated people.Yuki Iwamura/Associated Press

Now that the Omicron variant of the coronavirus has made landfall in the United States, with at least 10 cases reported in six states, scientists are racing to discover the virus’s capabilities, and public health officials are waiting anxiously to see what happens next.

Such as: Will Omicron overtake Delta and become the dominant variant?

“That’s kind of the million dollar question right now. We don’t know,” said Jacob Lemieux, an infectious disease specialist at Massachusetts General Hospital and co-leader of the viral variants program at the Massachusetts Consortium on Pathogen Readiness.

Two key issues will determine the fate of Omicron: how transmissible it is and how well it evades the body’s immune system. The new variant has mutations known to make it more infectious and better able to overcome vaccine protection. Scientists in South Africa reported Friday that Omicron appeared to spread more than twice as quickly as Delta.

“What we’ve learned over the last eight days is alarming, because in South Africa, the outbreak has accelerated extremely rapidly,” Lemieux said. “Omicron has clearly outcompeted Delta in South Africa. The question is, is it going to do that elsewhere?”

The situation in Africa differs from other places, though.

In South Africa, Omicron arrived after Delta cases had subsided. There wasn’t a lot of Delta virus there for Omicron to compete with.

In contrast, Delta has been driving a surge of cases in the United States, including in Massachusetts. Delta also spreads extremely fast and can infect vaccinated people.

“The question is, will Omicron win in a context where Delta is surging?” said Daniele Lantagne, a Tufts University professor who advises the World Health Organization on the coronavirus. “And we literally do not know that answer.”

Lemieux said he thinks that, because Omicron spreads so fast, “it’s more likely than not that Omicron will outcompete Delta everywhere. There’s a lot of uncertainty, but that’s the possibility that we need to be prepared for.”

In South Africa, few people are vaccinated, but a significant proportion have some immunity from having already been sick with COVID-19 — and Omicron has shown it can infect them. “There’s no question we never saw reinfections like this before in people who had COVID,” said Eric Topol, director and founder of the Scripps Research Translational Institute.

How will Omicron fare among vaccinated people? Most experts expect the vaccines to be somewhat less effective against Omicron — but no one knows how much.

Still, no one expects Omicron or any variant to render vaccines useless. Rather, the new variant may be able to overcome the vaccine’s protection to some degree.

Lemieux believes “there’s a real possibility” that Omicron will do a better job of infecting vaccinated people than previous variants.

“The biggest issue is going to be,” said Angela L. Rasmussen, a virologist specializing in emerging viruses at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, “will the vaccines continue to be protective against severe disease even in breakthrough infections?”

Topol said there’s already plenty of evidence from the cases so far that Omicron can infect vaccinated people. The vaccines will likely continue to protect against severe illness and hospitalization. But even people with mild cases can end up with the syndrome of lingering, often disabling, symptoms known as “long COVID.”

And whenever COVID-19 spreads, vulnerable people — the elderly, cancer patients, people with conditions that weaken their immune system — get sick and die. “There’s a lot of vulnerable people out there,” Rasmussen said.

“There’s good and bad,” Topol added. “There’s bad because it clearly has immune escape — from what we know, the worst immune escape yet. The good is, we have incredibly potent vaccines.”

That’s why experts are emphasizing that people need to fortify themselves now, to build as strong an antibody response as possible, by making sure they are fully vaccinated and also get a booster shot.

Rasmussen sees two possible scenarios: That Omicron proves to be “a nothingburger, and causes regional outbreaks but Delta reigns supreme. The other alternative, Omicron kicks Delta’s ass.”

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and co-director of the Texas Children’s Hospital Center for Vaccine Development, has posited a different scenario, in which Omicron and Delta coexist as “separate, twin outbreaks.” Delta would attack unvaccinated people while Omicron goes after those with partial immunity, either from a previous infection or incomplete vaccination, he wrote in the Daily Beast.

In an interview with the Globe, Hotez said it’s certainly possible Omicron will outcompete Delta in the United States. But Delta, which emerged from an unvaccinated population in India, has a firm grip on the world.

“Delta’s so dominant right now it’s going to take time,” Hotez said. “Just because [Omicron] is taking over in South Africa doesn’t yet mean it’s going to take over the rest of the world.”

Lantagne, the Tufts professor, sees three possible scenarios: The US wave of Delta cases could fade, and Omicron would not become the dominant variant. Or the new variant could overtake prior strains and partially escape the vaccine, causing increased hospitalization and death.

The third option “could be good — even great — for us,” Lantagne said. Omicron could be highly transmissible but turn out to be much less virulent than Delta. In that case, the new variant could overtake Delta but reduce infections to something closer to a common cold.

But, Rasmussen calls that “wishful thinking.” There is no evidence yet on whether Omicron is more or less likely than other variants to cause severe illness, she and others emphasized.

And Rasmussen makes another point: “Even without Omicron,” she said, “we were already staring down the barrel of a huge winter Delta surge. Delta is still here, still a big problem. Whether Omicron outcompetes it — it doesn’t get rid of the fact that we already have a problem.”