Why is Omicron so contagious?

Omicron variants are spreading rapidly around the world. New travel bans and heightened anxiety follow this highly mutated version of the coronavirus currently reported in more than 60 countries. In South Africa, where about 50 mutations in Omicron were first identified, this variant was found in people who had already captured and survived previous versions of the SARS-CoV-2 virus, and those who were vaccinated against it. Indicates that it can be re-infected.

Scientists are now trying to model Omicron’s global orbit, which depends on two factors. The first is its innate infectivity, or transmissibility. The second is the ability to evade the human immune system. Mark Lipsich, an epidemiologist at Harvard TH Chan School of Public Health in Boston, said: “How much can each predict the number and rate of potential infections of Omicron in the spread of the variant? It’s about figuring out if infectivity and antigenicity contribute. ” ..

Infectivity reflects the ability of the virus to replicate within human cells and move from person to person. “It depends on all kinds of biological processes,” explains Jeffrey Shaman, an infectious disease modeler at Columbia University’s Mailman School of Public Health. “Does it bind more easily to the receptors in people’s lungs? Does it shed more efficiently and spit out more so that it can infect more people?” On the other hand, the avoidance of the immune system is that the virus The ability to avoid antibodies that are a sign of destruction by the body and the ability to dodge various immune system cells.

An important step in measuring the spread of a virus is to start with one infected person and estimate the number of others who will get the virus from that individual.In an ongoing pandemic, scientists try to capture that estimate with an effective reproduction number, or a value called R._t.. The variable “t” represents the number of secondary infections and depends on the effects of other people’s immunity, seasonal weather patterns, public health interventions, and other restrictions on viral infections. R_t “It can change from minute to minute, depending on the actual situation,” says Lipsitch. “Use this to determine how fast the outbreak is expanding or contracting.” R value₂For example, the value of R is_Five That is, the person spreads the virus to five people and increases the number of infected people much faster.

R_t Omicron estimates are currently emerging. December 9, South Africa National Institute of Infectious Diseases (NICD) report By early November, R_t In that country, stable at values ​​below 1, Delta is the predominant subspecies, meaning that the number of cases actually decreased during the period when the population was countering widespread immunity. But then R_t It fired suddenly in mid-November. Currently, it is over 2 in most of the countries and over 2.5 in the densely populated states of Gauteng, KwaZulu-Natal and Mpumalanga. NICD scientists calculated the values ​​using laboratory-identified cases and hospitalization data. R_t In this case, in addition to Omicron, other variants are included, but according to Carl Pearson, a math modeler at the London School of Hygiene and Tropical Medicine, the sudden rise is a mixture of new variants and many new ones. It indicates that it is causing an infection. A person who works closely with South African investigators.

Since then, British Health and Security Agency scientists have reported R_t Of 3.7 of Omicron itself. Its disturbingly high number is Technical briefing The data released on December 10 are based in part on data showing that Omicron infections in the United Kingdom are doubling every three days. At that pace, Omicron poses a far greater threat in terms of number of cases than Delta, wrote Trevor Bedford, an infectious disease modeler at the Fred Hutchinson Cancer Center in Seattle, in a detailed series of comments on Twitter. rice field.

As Bedford emphasized, what is not yet known is how much of the rapid rise of Omicron is due to its inherent transmissibility to the ability to evade immune defenses. He made the theory if one population was widely immune to other mutants. Omicrons spread rapidly, even without the inherently superior ability to transmit, because the human immune system suppresses competing mutants.

Evidence that Omicron evades the human immune response has accumulated from a variety of sources. One symptom is reinfection of people who are already infected with the virus. A team led by epidemiologist Juliet Priam, who directs the DST-NRF Center of Excellence in epidemiological modeling and analysis at Stellenbosch University in Cape Town, South Africa, reported on December 2nd: Over 35,000 SARS-COV-2 reinfections It occurred in that country among the 2.8 million people who tested positive for SARS-COV-2 within the last three months.Different study According to a team led by Alex Sigal, a virologist at the Africa Health Research Institute in Durban, South Africa, neutralizing antibodies in blood samples of people vaccinated with the Pfizer-BioNTech vaccine are about 1 / 40th as effective as neutralizing antibodies against Omicron. It turned out to be. Other variants.

But it’s still unclear whether experiments in the Cigal lab foretold a decline in vaccine protection in real life, said Sarah Covey, an epidemiologist and evolutionary biologist at the University of Chicago. “Immune avoidance is more than neutralizing antibodies.” Vaccines also activate specialized immune cells that destroy infected cells, so Omicron must also avoid them. “Immune avoidance cannot be clearly measured in the laboratory,” Cobey says.

In the coming weeks, scientists will evaluate Omicron’s R_t Values ​​are compared where infection history and vaccination rates differ. Sherman says it is not clear how much of what was observed in South Africa could be applied elsewhere. For example, the United States has never experienced a wave of infections caused by beta variants, but South Africa has. Exposure to various variants can alter the immune response of the US population. “We went straight to Delta from our ancestral alpha variants,” he says. “And these kinds of things may change how vulnerable the population is to this new Omicron variant. We need to see how this resolves over time. I have.”