The science behind the rapid spread of Omicron


As Omicron has quickly become the dominant variant of the coronavirus in South Africa and the UK, scientists are starting to piece together what gives it its evolutionary advantage.

Researchers are further refining and expanding their findings, but Omicron’s increased transmissibility appears to be a combination of several properties: it appears to be able to bind more easily to and break down in human respiratory cells; it seems to replicate faster once in our body; and it can substantially evade the immunity acquired from a previous infection or vaccination.

These advantages mean that Omicron is spreading across the world at a tremendous rate. Since scientists in South Africa first reported its presence last month, it has been detected in 77 countries and is likely present in most others, according to the World Health Organization.

Cases of Omicron have been detected in the United States, across Europe and Africa, and in China, which maintains strict border controls and a strict regime to handle even tiny epidemics.

“Omicron is spreading at a rate that we haven’t seen with any previous variant,” WHO Secretary-General Tedros Adhanom Ghebreyesus recently said.

In New York and New Jersey, Omicron is estimated to account for around 13% of Covid-19 cases in the week to December 11.


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There are signs that the variant is associated with less severe disease than earlier versions of the virus. Scientists warn, however, that the severity can vary from place to place depending on factors such as immunity levels and characteristics of the population, and that while it is not as dangerous, its ease of transmission means that Omicron could still cause waves of illness and death.

There are more and more examples of Omicron’s surprising transmissibility. After a Christmas party on November 26 for a renewable energy company at an Oslo restaurant, 80 of 111 guests tested positive for the coronavirus, with genetic sampling suggesting all were likely Omicron, according to the Norwegian Institute of public health.

In Hong Kong, a 36-year-old man was identified as one of the first known cases of Omicron on November 13 at one of the city’s quarantine hotels. Five days later, a 62-year-old man remaining in the room across the hall also tested positive for the variant, although he had no contact with his neighbor. The researchers concluded that the variant must have crossed the hallway.

In the UK, Omicron replaced Delta as the dominant variant of the coronavirus in England and Scotland in less than a month and it is only a few days behind Denmark, according to Danish health authorities. It is already the dominant variant in Ontario, Canada, accounting for 51% of new cases, according to a panel of scientists advising the provincial government. British health data suggests Omicron cases are doubling across much of the country in less than two days.

In the United States, the Delta variant accounted for about 96% of cases across the country in the week to December 11, while the Omicron variant accounted for about 3%. Some regions have higher figures: in New York and New Jersey, Omicron is estimated to account for around 13% of Covid-19 cases.

Behind Omicron’s rapid spread is a host of mutations that give it an advantage over older variants.

A number of studies indicate that Omicron is more resistant to current vaccines than previous variants of Covid, although boosters seem to be helping. WSJ’s Daniela Hernandez gets exclusive insight inside a lab testing how antibodies interact with Omicron. Photographic illustration: Tom Grillo

Of the approximately 50 mutations in Omicron, at least 30 are on the spike protein, the structure that helps the virus infiltrate cells and is also the primary target of vaccines.

Laboratory tests in South Africa, Sweden, the United Kingdom and the United States have shown that the ability of antibodies to prevent the variant from entering cells is lower than with other strains, which which gives Omicron a much better chance of infecting people who have already been vaccinated or infected. This means that it can spread quickly even in highly vaccinated populations.

Modeling released Friday by researchers at Imperial College London estimates the risk of re-infection with Omicron to be 5.4 times greater than with Delta, suggesting limited protection against a previous infection. The study, which was not peer-reviewed, also reported a reduction in vaccine efficacy against Omicron infection after two doses, compared to Delta.

Almost 70% of the UK population is fully vaccinated, but Omicron has propelled a record number of cases, prompting the government to ask people to work from home where possible, to limit mixing with other households in the days leading up to Christmas and receiving booster shots to build additional immunity. Denmark and Norway have also tightened public health restrictions to fend off a wave of cases.

A Covid-19 research lab in South Africa, where scientists first reported the presence of Omicron.


Waldo Swiegers / Bloomberg News

“If the virus is able to partially evade the immune response induced either by a previous infection with another variant or by vaccines, then it can become infected and spread more efficiently.” This is what we see with the Omicron variant, ”said Lawrence Young, virologist and professor of molecular oncology at the University of Warwick.

A growing body of evidence suggests that Omicron’s mutations may also give it inherent transmission benefits in addition to its ability to evade the body’s immune responses.

A study, not yet peer-reviewed, conducted by researchers in Hong Kong found that Omicron infects and multiplies 70 times faster than the Delta variant in the airways, which could explain why the new variant appears to be transmitted. faster.


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Things were different in the lungs, however, researchers found that Omicron replicates less efficiently in the lungs than the original version of the virus, which may help explain why Omicron has been linked to less severe disease in many. people it infects.

Another study, also not yet peer-reviewed, appears to corroborate the findings of the Hong Kong team: researchers showed that an Omicron pseudovirus – a synthetic version of the Omicron variant – was better at infecting cells than a Delta pseudovirus and the original version of the virus.

If the virus multiplies faster, “it can start spreading to others sooner,” said Julian Tang, clinical virologist and professor of respiratory medicine at the University of Leicester.

British public health officials say Omicron’s benefits mean the interval between one newly infected person becoming infectious to others appears to be shorter with Omicron, perhaps three days compared to six or more with Delta. They also estimated that the risk of someone passing the virus to another member of their household is three times higher with Omicron than with Delta.

One metric epidemiologists use to measure a virus’s transmissibility is its effective reproduction number, or R, the average number of people an infected person will infect. This will vary from place to place depending on the levels of immunity and the public health policies in place to control transmission. The higher the value, the more transmissible the virus is.

Nearly 70% of Britain’s population is fully vaccinated, but Omicron has propelled a record number of cases, prompting the government to ask people to get vaccinated.


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Susan Hopkins, who heads the UK Health Security Agency, told lawmakers in the UK Parliament on Thursday that her agency estimates Omicron’s current R-value to be between 3 and 5. The overall R-value of the UK outbreak is about 1 to 1.2, she says.

This high level of transmission is problematic, say scientists, even though Omicron is found to cause milder disease, especially in people who have been vaccinated or with a degree of immunity from a previous infection. Booster shots have been shown to increase protection against infection. Scientists are generally optimistic that vaccines will help ward off serious illness and death with Omicron.

An article awaiting peer review by disease modellers at the London School of Hygiene and Tropical Medicine, described several possible scenarios for the coming months in England as Omicron continues to spread, which varied in the how far the variant could evade immunity and how effective the boosters were. slowing down of infection. All of the scenarios indicate many more cases, hospital admissions and deaths.

“We better be careful not to predict it’s less serious, so it’s going to be better,” said Anthony Fauci, chief medical adviser to President Biden, in an interview with The Journal podcast of the Wall Street Journal. , published Thursday.

“Because if you infect a lot more people, the lack of severity could be overcome by having a lot more people infected. “

Write to Jason Douglas at [email protected] and Sarah Toy at [email protected]

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