Omicron: What we know about the dominant coronavirus variant

What is the omicron variant?

First identified in Botswana and South Africa in November, the omicron variant has surged around the world over the past few months, faster than any previously known form of the coronavirus. The variant has caused a rapid rise in new cases that pushed some hospital systems to the breaking point.

Scientists first recognized omicron thanks to its distinctive combination of more than 50 mutations. Some of them were carried by earlier variants such as alpha and beta, and previous experiments had demonstrated that they could enable a coronavirus to spread quickly. Other mutations were known to help coronaviruses evade antibodies produced by vaccines.

Based on those mutations, along with a worrying rise in omicron cases in South Africa, the World Health Organization designated omicron a "variant of concern" on Nov. 26, warning that the global risks posed by it were "very high." Since then, the variant has been identified in at least 175 countries. Omicron quickly surged to dominance in many parts of the world, living up to the potential that scientists recognized when it was first discovered.

At the beginning of December, a California resident who returned home from South Africa was identified as the first American infected with omicron. By Dec. 25, it made up three-quarters of all new infections in the United States, according to the Centers for Disease Control and Prevention. Today, the variant accounts for essentially all infections.

What is BA.2 and is it worrisome?

There are several genetically distinct versions of omicron. Initially, the subvariant known as BA.1 was the most common. In the United States this winter, BA.1 and the highly similar BA.1.1 drove an enormous surge in new cases, which peaked at an average of more than 800,000 a day in mid-January, more than three times as high as the nation's previous peak. Since then, cases have steadily declined, as have hospitalizations and deaths.

In the late winter and early spring, a different subvariant, known as BA.2, has gained steam, becoming increasingly prevalent worldwide. It is now causing slightly more than one-third of infections in the United States, the CDC estimates. BA.2, which is even more transmissible than BA.1, may now be fueling new surges in China, Hong Kong and South Korea, where cases have spiked in recent weeks.

Does omicron spread faster than other variants?

Yes. It is two to three times as likely to spread as delta.

The earliest evidence for omicron's swift spread came from South Africa, where omicron rapidly grew to dominance in one province after another. In other countries, researchers were able to catch omicron earlier in its upswing, and the picture was the same: Omicron cases doubled every two to four days — a much faster rate than delta.

For a closer look at how well omicron spreads, British researchers also observed what happened in the households of 121 people who had been infected with the variant. They found that omicron was 3.2 times as likely to cause a household infection as delta was.

Scientists don't yet know what makes omicron so good at spreading, but a few clues have emerged from preliminary research. A team of British scientists found that omicron is particularly good at infecting cells in the nose, for example. When people breathe out through their noses, they can release new viruses.

But several studies suggest that people with omicron infections do not have higher viral loads than those infected by delta.

Instead, many scientists believe that omicron may spread so swiftly because it is adept at dodging antibodies produced by vaccines and previous infections. That allows it to spread quickly even in highly vaccinated populations.

Omicron also appears to have a shorter incubation period than other variants do. People who are infected with omicron typically develop symptoms just three days after infection, on average, compared with four days for delta and five days for earlier variants.

Much of the research to date has focused on the BA.1 subvariant, but evidence suggests that BA.2 is even more transmissible than BA.1.

What are the symptoms of omicron?

Some symptom differences have emerged from preliminary data. For instance, one possible difference is that omicron may be less likely than earlier variants to cause a loss of taste and smell.

Data released in December from South Africa's largest private health insurer, for instance, suggest that South Africans with omicron often develop a scratchy or sore throat along with nasal congestion, a dry cough and muscle pain, especially low back pain.

But these are all symptoms of delta and of the original coronavirus, too. It's likely that the symptoms of omicron will resemble delta's more than they differ.

Does immunity from previous infections stop omicron?

While it likely provides protection against severe disease, immunity from previous infections does little to hinder infections with omicron. The first clues that omicron could evade immunity came from South Africa, where scientists estimate that at least 70% of people have had COVID-19 at some point in the pandemic. An unexpectedly large fraction of omicron cases involved people who had previously been infected.

When omicron surged in England, British researchers similarly found that many people infected with the new variant had already survived COVID. The researchers estimated that the risk of reinfection with omicron was about five times that of other variants.

Similar results came from Denmark, where scientists compared more than 2,200 households where someone got infected with omicron to some 6,300 delta-infected households. Omicron was 3.6 times more likely to infect people with boosters sharing the same house than delta. But it was barely more likely to infect unvaccinated people.

How much do vaccines protect against an omicron infection?

Several studies indicate that full vaccination plus a booster shot provides strong protection against infection with omicron, at least in the short term. Without a booster, however, two doses of a vaccine like Pfizer-BioNTech's or Moderna's provide much less protection. (Still, two doses of a vaccine do appear to protect against severe disease from omicron.)

Scientists drew blood from fully vaccinated people and mixed their antibodies with omicron in a petri dish loaded with human cells. Every vaccine tested has done a worse job at neutralizing omicron than other variants. And antibodies from people who received two doses of the AstraZeneca or one dose of Johnson & Johnson vaccines don't seem to do anything at all against Omicron.

But when researchers tested antibodies from people who had received boosters of Moderna or Pfizer-BioNTech vaccines, they saw a different picture. Boosted antibodies blocked many omicron viruses from infecting cells.

Researchers found a similar response when they looked at people who had been fully vaccinated with two doses after a COVID-19 infection: Their antibodies were extremely potent against omicron.

Real-world studies support the results of these experiments. In South Africa, researchers found that two doses of the Pfizer-BioNTech vaccine had effectiveness against omicron infection of just 33%. Against other variants, they found its effectiveness is 80%.

In Britain, researchers found that people who had received two doses of the AstraZeneca vaccine enjoyed no protection at all from infection from omicron six months after vaccination. Two doses of Pfizer-BioNTech had effectiveness of just 34%. But a Pfizer-BioNTech booster had effectiveness of 75% against infection.

Results like these reinvigorated vaccination efforts and have spurred widespread booster campaigns.

But booster shots may lose some of their effectiveness against infection over time. In one British study, scientists found that the protection boosters offer against symptomatic omicron infections wanes within 10 weeks. And data from Israel suggest that a fourth shot may not offer much additional protection against omicron infections, according to a study published in mid-March.

Can vaccines reduce the severity of COVID?

Yes. In a large study of more than 1 million cases of COVID, British researchers found that people who had received booster doses were 81% less likely to be admitted to the hospital, compared with unvaccinated people. The risk of being admitted to a hospital for omicron cases was 65% lower for those who had received two doses of a vaccine.

And booster doses of the Pfizer and Moderna shots are 90% effective at preventing hospitalization from omicron infections, the CDC reported in January. The benefits were especially pronounced for older adults.

The protection that vaccines afford against severe disease with omicron has left its mark on hospitals. When omicron fueled a new surge of cases, the people coming to hospitals in New York City were overwhelmingly unvaccinated.

Vaccinated people are at risk of infection with omicron because the variant can evade antibodies produced by vaccines and start multiplying in the nose and throat. But vaccines do more than just trigger the production of antibodies against coronaviruses. They also stimulate the growth of T cells that help fight a particular disease. T cells learn to recognize when other cells are infected with specific viruses and then destroy them, slowing the infection.

Scientists have also examined the T cells produced by COVID-19 vaccines to see how well they fare against omicron. Early studies suggest that these T cells still recognize the omicron variant.

This preliminary evidence suggests that omicron infections cannot get past the T-cell line of defense. By killing infected cells, T cells may make it harder for omicron to reach deep into the airway, where it can cause serious disease.

How bad are COVID cases caused by omicron?

While omicron can cause deadly infections in some people, it is less severe overall than the delta variant.

Scientists measure the severity of a coronavirus variant by examining how many people infected by it end up in the hospital. The delta variant turned out to be substantially more severe than earlier variants. But the reverse is true for omicron. A British study found that the risk of hospitalization due to omicron is half that of delta.

When the omicron variant began surging in the United States, hospitals observed the same reduced risk. A study from California found that compared with delta, omicron infections were less likely to send people to the hospital or the ICU. And despite record-breaking new cases, new hospitalizations rose at a slower rate during the omicron surge. Although it's a relief that omicron is not as severe as delta, the new variant still put tremendous strain on hospitals, thanks to its extraordinary contagiousness.

The lower severity of omicron likely has several causes. Many of the people that omicron is infecting are vaccinated or recovered from previous infections. Their immunity lowers their chances of ending up in a hospital with COVID.

But preliminary studies on animals and cells also suggests that omicron has a different biology than other variants. Its strategy for infecting cells works well in the upper airway, but not in the lungs.

Is COVID caused by omicron still treatable?

Yes. In late December, the Food and Drug Administration authorized two new antiviral pills for COVID, called Paxlovid and molnupiravir. Preliminary experiments indicate that both treatments should work against omicron. People who are at high risk of developing severe COVID can be prescribed either drug in the first few days after a diagnosis.

Another effective treatment is a drug called sotrovimab, made by GSK and Vir. It is a monoclonal antibody that can attach to the omicron variant and prevent it from infecting cells. Unlike Paxlovid and molnupiravir, which are packaged as pills, sotrovimab has to be given as an infusion in a hospital or clinic.

Two other widely used monoclonal antibodies, made by Regeneron and Eli Lilly, won't work because omicron is resistant to them. As omicron came to dominate the United States, the federal government scrambled to secure more doses of sotrovimab.

Another option for people infected with omicron is an antiviral drug called remdesivir. Like sotrovimab, it is effective at preventing severe COVID.

For people hospitalized with omicron infections, a wide range of other treatments are also available. For example, a steroid called dexamethasone has been demonstrated to be very effective at reining in lung-damaging inflammation.

Why is it called omicron?

When the WHO began to name emerging variants of the coronavirus, they turned to the Greek alphabet — alpha, beta, gamma, delta and so on — to make them easier to describe. The first "variant of concern," alpha, was identified in Britain in late 2020, soon followed by beta in South Africa.

Next in line? Omicron.

Melinda Wenner Moyer contributed reporting. This article originally appeared in The New York Times.