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As the Omicron variant snowballs in South Africa and widens its inroads in Europe, evidence is mounting that it can outcompete the highly transmissible Delta variant — a potential warning signal for the United States.

Viruses perform differently in different places, depending on what other variants are circulating and on what the landscape of immunity looks like. But Omicron is picking up speed in Europe, which has often served as a preview of what was headed the U.S.’s way. It’s an early sign that the already bleak situation here may get worse.

“It is more likely than not, at this point, that Omicron is going to dominate Delta systematically across regions and countries,” said Jacob Lemieux, an infectious diseases physician at Massachusetts General Hospital, who is helping lead a state program studying coronavirus variants. He cautioned researchers are still scrambling to better understand the new variant, but added, “We need to be prepared for the possibility that Omicron is going to cause a significant outbreak, not just in isolated locations, but elsewhere, including here.”

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All the recent attention on Omicron has distracted from the Delta trends in the U.S. Spikes in the Midwest and Northeast have driven average daily infections above 115,000; more than 1,200 people die each day in the U.S. Those ongoing outbreaks indicate that the conditions here are ripe for transmission, particularly with the holidays approaching. A better spreading variant could compound that and worsen the strain on short-staffed hospitals.

“We’re already seeing a dramatic increase in cases attributable to Delta,” said Yonatan Grad, an associate professor of immunology and infectious diseases at Harvard’s Chan School of Public Health. The impact of Omicron could be felt in a few weeks, he said, and added that flu activity — essentially dormant since March 2020 — is also reawakening (though it’s at much lower levels than during a typical year).

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Experts stress they’re still trying to figure out just how transmissible Omicron is and, if it can take over from Delta, how quickly that replacement will occur. They also note that the world has strategies that can combat the virus, from vaccines and boosters to testing to therapeutics to masks. Preliminary data so far indicate that vaccinated people should largely be spared from the worst outcomes of an Omicron infection, even if they’re now more vulnerable to infection, and that boosters can restore the protection that the variant erodes. There are some clues, too, that Omicron might be causing milder disease.

Still, more than one in four adults in the U.S. is not fully vaccinated, and only one in four adults has received a booster. Many people have recovered from Covid-19, giving them some protection, but it’s not clear how well infection-derived immunity holds up against Omicron. It’s hoped that people generally would be protected from severe disease.

“The extent of immune protection is not substantial enough to really, it seems, make a huge dent” in transmission, Grad said.

Omicron is certainly spreading in this country, with some two dozen states and counting identifying cases. It can take several weeks, even a few months, for a better spreading variant to gather enough steam to then seemingly explode in prevalence, and some evidence indicates that Omicron is catching up to Delta in Europe at a record clip. As the U.K.’s Health Security Agency forecasted this week, Omicron could account for the majority of the country’s Covid-19 cases in two to four weeks.

“Omicron is displaying a growth advantage over Delta,” the agency said Wednesday — an assessment made with “high confidence.”

When South African scientists alerted the world about Omicron late last month, experts cautioned that the apparent transmission advantage over Delta could be a mirage. Cases skyrocketed in South Africa’s Gauteng province at a time of really low Delta circulation, so perhaps Omicron was the beneficiary of a few superspreading events that made it look like it was racing ahead of Delta, one hypothesis went.

Even as Omicron expanded around South Africa, experts noted that its activity there might not model its behavior elsewhere. There wasn’t much other virus with which Omicron had to compete. The country had high levels of immunity from infection, but only about one fourth of the population was fully vaccinated. And as Anthony Fauci noted at a briefing this week, South Africa has a comparatively high rate of people who have HIV, which could dampen their immune responses.

But Omicron is also growing in prevalence in parts of Europe, where countries have vaccination rates on par with or exceeding that of the U.S., and where Delta was driving surges when Omicron arrived. Countries like Denmark and the United Kingdom with very strong surveillance networks have been tracking how Omicron has eaten into Delta’s dominance, raising concerns about how quickly Omicron is spreading in well-protected populations. All that has added to the evidence that Omicron can broadly outpace Delta.

There are two possible explanations for how that is occurring, and many experts say the answer could be a combination of both.

Earlier in the pandemic, with so many unprotected people out there, the SARS-CoV-2 variant that was inherently the best spreader was able to outrun other viral iterations. Omicron might get some of its apparent edge this way.

But the new variant has another evolutionary trick up its sleeve, one that is conferring an advantage at this particular moment.

There is such a high level of population immunity in most places that even a top-notch sprinter of a variant would have trouble finding many susceptible people to infect. Instead, the world could be at the point when the competitive advantage is transitioning to what’s known as immune escape — meaning a variant that can circulate at higher levels among protected people.

Delta could do this to some degree, which is why breakthrough infections increased, but Omicron seems to do this much more powerfully. Data from South Africa show a much higher rate of reinfections with Omicron, and lab experiments indicate that even fully vaccinated people could be widely vulnerable to infection from Omicron. If Omicron is infecting people who are vaccinated or have had Covid-19, it means those people can likely pass it along to others.

“Immune escape can really provide a fitness advantage over existing strains when there is a lot of immunity,” said evolutionary biologist Katia Koelle of Emory University. By allowing a variant to infect people that other variants can’t, “immune escape increases the number of susceptible individuals.”

The fear is not so much about the outcomes in most vaccinated people, but that they can contribute to the spread to unprotected people, or people who may not have mounted a strong defense even when vaccinated, including elderly people or people with certain underlying health conditions.

Immune escape certainly seems to be a factor in Omicron’s apparent advantage over Delta, but experts aren’t sure how much also may be coming from it being a better innate spreader. “Separating these two — separating inherent transmissibility from escape from antibody inhibition — I think that specifically may take a little time to clarify,” said virologist Jeremy Luban of the University of Massachusetts Medical School.

Despite the global dominance of certain variants at different points in the pandemic, there doesn’t have to be one variant to rule them all. Multiple variants could co-circulate — say, Omicron and Delta — and the composition could vary around the world.

A new variant can also become dominant locally without necessarily igniting another surge, but experts say that given the conditions, they can imagine Omicron taking off in the U.S. and worsening the ongoing wave.

“Hopefully it’ll just be in terms of cases, not in terms of deaths,” Koelle said.

That’s where population immunity remains important. Many experts believe that even if two doses of vaccines or an earlier Covid-19 case can’t block infection, they should still protect most people from serious outcomes even if they contract Omicron.

Indeed, some early evidence indicates that Omicron might be causing milder disease on average than other forms of the virus. Experts caution that it’s too early to conclude that — we need more cases and to follow them for longer — but say that one possible explanation isn’t a change in Omicron’s virulence, but that vaccinations and past infections are guarding people from severe outcomes. (It’s not clear what this could mean for rates of long Covid.)

The notion of a virus becoming “attenuated” — meaning it causes lower rates of disease — is framed as a positive development. But with Omicron, it might be more complicated than it seems. Given Omicron’s apparent transmission advantage, it could infect more people if it takes off, leading to a more tragic impact overall even if the individual risk is lower.

“If they’re allowed to spread unchecked, even though they’re not individually more virulent or lethal, they just generate more cases, they put pressure on the health system, and more people die,” Mike Ryan, the executive director of the World Health Organization’s emergencies program, said about more transmissible variants.

On the evidence that Omicron is more transmissible, Ryan said: “It doesn’t mean that the virus has become invincible. It’s just become better fit, better adapted to exploiting the contacts and the connections between us. And therefore, we have to double down … and use the measures at our disposal that we already have.”

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