Omicron is spreading with lightning speed. Scientists are trying to figure out why.

At the end of November, more than 110 people gathered for a crowded Christmas party at a restaurant in Oslo. Most of the guests were fully vaccinated. One had returned from South Africa a few days earlier and unknowingly carried the omicron variant of SARS-CoV-2.

Ultimately, around 70% of partygoers were infected.

Scientists who trace this super spread event concluded that this was evidence that omicron was “highly transmissible” among fully vaccinated adults.

A little over a month later, the rapid global rise of omicron now makes it clear that the holiday was not an isolated example. Country after country, the new variant has overtaken its predecessor, the delta variant – with one case of omicron causing an average of at least three other new infections. Cases have reached record highs in parts of Europe and now in the United States, where around half a million new infections have been recorded in a single day.

“It’s a game-changing virus, especially in the vaccinated population where people have had a level of invincibility,” explains Chanda Summit, Professor in the Department of Immunology and Microbiology at Scripps Research.

Indeed, in a world where vaccinations and infections built immunity, other variants were struggling to take hold. Still, omicron is booming.

“It changes the math for everyone,” says Chanda.

And so scientists are trying to figure out: what explains the rapid spread of the omicron lightning?

While it’s still early days, they’re starting to understand why the new variant is so contagious – and if that means old assumptions about how to stay safe need to be revamped.

A big question: how is it in the air?

Omicron’s best trick so far – which explains its success more than anything else – is to dodge our immunity: the antibodies and other immune defenses put in place by the body after vaccination and / or infection. earlier.

The variant’s many mutations on the spike protein allow it to infect human cells more efficiently than previous variants, again leaving many more people vulnerable. For this reason, “immune breakout” alone could be the main reason the variant seems so contagious compared to the delta, which was already highly transmissible.

In fact, omicron has spread at a rate comparable to the rate at which the original strain of the coronavirus spread at the very start of the pandemic despite new levels of immunity around the world.

“The playing field for the virus right now is quite different from what it was at the start,” says Dr Joshua Schiffer, infectious disease researcher at the Fred Hutchinson Cancer Research Center. “The majority of the variants that we have seen so far could not survive in this immune environment.” Even delta was essentially “on par,” he said, where it persisted, but “wasn’t increasing very quickly or declining very quickly.”

A new to study from Denmark suggests that much of the variant’s dominance comes down to its ability to evade the body’s immune defenses.

Researchers compared the spread of omicron and delta among members of the same household and concluded that omicron is approximately 2.7 to 3.7 times more infectious than the delta variant in vaccinated and boosted individuals. .

But here’s an interesting additional point: for unvaccinated people, there was no significant difference in infection rates between delta and omicron. This would indicate that the two variants are at roughly the same level of transmissibility among the unvaccinated – in other words, under these circumstances, omicron is not necessarily more transmissible than delta.

If confirmed, the results would support the idea that the increased transmissibility of omicron can be attributed to its “immune evasiveness” – and not to other characteristics that make the variant inherently more transmissible, the authors conclude.

It is also what a small study from the University of Maryland may hint, although here, too, the results are preliminary and have yet to be peer reviewed.

Researchers measured the amount of vaccinated virus that people infected with omicron released into the air after shouting and singing. Four in five exhaled large amounts of the virus into the air – comparable to the amount released by unvaccinated people earlier in the pandemic.

“But what is striking is that I expected to see the amounts much higher, and they are not,” says Dr Don Milton, an aerobiologist specializing in infectious diseases at the University of Maryland School of Public Health, who led the study.

The results suggest that the spread of omicron could, in part, depend on whether more people vaccinated are contagious and shed the virus, not necessarily whether each infected individual releases significantly more virus into the air. And if the results hold true, Milton says long-haul transmission is unlikely to become a new concern with omicron, beyond what has already been seen with other contagious variants like the Delta.

“With measles, for example, the force of the source is so intense that even in the next room, people are still at risk of getting infected,” he says. “And you don’t see much of that with this virus,” because it’s so diluted by the time it hits the next room.

But there is a downside. Milton says, “The bad news is that the vaccine doesn’t mean you aren’t going to pass it on to someone else.

And he adds that the conclusions are ultimately limited to those who are vaccinated: “Maybe you get an unvaccinated person there. [the amount of virus] is much more intense.

Omicron could have other advantages that would give it a head start

With so many mutations, it’s still entirely possible that omicron has some additional benefits that make it more contagious than other variants – benefits that rely on more than shattering our previous immunity.

Maybe omicron can make more copies of itself in a cell? Or maybe it adheres to cells more effectively? Or maybe it’s better to stay in the air and stay contagious?

“Any of these things would make the virus more contagious,” says Fred Hutchinson’s Schiffer.

A key difference emerging with omicron is how quickly an infected person becomes contagious.

Omicron appears to have a shorter incubation period and this can dramatically accelerate infections in the population. A to study The Oslo Christmas Party outbreak revealed that the incubation period could be around three days, compared to 4.3 days for the Delta and five days for the other variants. A small study of CDC also puts the incubation period at about three days.

“It’s actually a pretty big difference,” says Schiffer. This would mean that there are many more cycles of infections and less time for those exposed to take precautions not to expose others.

Mid-December to study of Hong Kong has also led scientists to consider that omicron may indeed replicate better in certain cells and therefore have a head start on delta, at least in the unvaccinated.

Researchers found that omicron multiplied about 70 times faster than delta in tissue samples from the bronchi, the large airways that lead from the trachea to the lungs. Meanwhile, omicron has had a much harder time infecting cells in lung tissue than the original version of the coronavirus which was first identified in Wuhan, China.

“You could potentially shed more virus in your upper respiratory tract than if most of the replication was happening deep in your lungs,” says Angie rasmussen, virologist at the University of Saskatchewan.

Like other variants, omicron is spread through the nose and mouth by short-range respiratory droplets and viral particles that float in the air and can stay suspended for some time, especially in places poorly ventilated.

Rasmussen says these data on faster replication in bronchial tissue “would suggest that you might have more virus in these respiratory secretions, which can come out as mucus if you have a runny nose or can certainly be exhaled as a runny nose. ‘aerosols and droplets’.

Airborne concerns as omicron spreads

If the omicron spreads more easily in the air, this faster replication in the bronchus would be one of the two most likely explanations, according to Linsey Marr, an engineering professor at Virginia Tech who studies how viruses are transmitted through the air. “Infected people release a lot more virus particles into the air or you can breathe less and still get infected – or a combination of these,” she says.

As this Hong Kong study focused on what goes on in the lab, a cautionary tale of omicron spreading through the air also emerged from an isolation facility there.

In one report published in early December, scientists in Hong Kong describe how a traveler quarantined at a hotel infected another person who was staying across the hall but never had face-to-face contact. “Airborne transmission through the corridor” is the most likely explanation, the authors conclude.

“This suggests that a very small amount of the virus may have caused an infection,” says Dr Michael Klompas, infectious disease physician and hospital epidemiologist at Brigham and Women’s Hospital. This could mean that omicron requires a lower dose than previous variants to infect people, although there is no data yet to establish whether this is true, he says.

Although worrying, these early anecdotes should be interpreted with caution. There are similar cases of airborne transmission with delta, and hotels and other locations reassigned to isolate infected travelers are difficult places to turn into quarantine facilities.

“Can we catch omicron more easily in the air than other variants?” I don’t think it’s known, ”says Rasmussen. “What is very clear is that you can catch it more easily, period.”

A bit reassuring

Despite the many unanswered questions about why omicron is so contagious, scientists say it’s important to realize that the coronavirus has not turned into an entirely new virus.

“The rules haven’t changed, it’s just that the margin for error has decreased a lot,” said Klompas.

When it comes to minimizing your personal risk, the same principles apply: wear a high quality mask like an N95, choose outdoors rather than indoors if possible, and avoid large gatherings with unmasked people , especially if they are not vaccinated.

“These harm reduction measures are additive and you should try to apply as many as possible”, explains Rasmussen.

Even rapid face-to-face interactions seem to be riskier with omicron, in part because people have relied on vaccines as their only layer of defense, Chanda explains. “If you walk into a room full of people and someone is infected, the chances of catching the virus have increased dramatically” – whether you are vaccinated or boosted or not.

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Robert M. Larson