Omicron Poses A Puzzle: It's Infectious But Not As Deadly As Delta

Scientists are scouring patchy evidence from around the world to better understand Omicron, the new SARS-CoV-2 variant, and what it might mean for the next phase of the pandemic.

Photo Insert: Early signs that Omicron causes less severe symptoms than previous variants offer some reassurance.

Three weeks after it was discovered, there are still mostly questions, but a few hints have emerged—some worrisome but others encouraging, Kai Kupferschmidt and Gretchen Vogel reported for Science.

Researchers are focusing on three key questions: Can Omicron evade immunity from vaccines or previous infections? How transmissible is it? And how much severe disease will it cause?

The most solid clues so far pertain to the first question—and they are not reassuring. The genome alone—with more than 30 mutations in the all-important spike protein—suggested the variant might well be the best yet at dodging our immune defenses.

And early data from South Africa seem to confirm that worry: A study published last week that analyzed 35,670 reinfections among nearly 2.8 million positive tests carried out through late November suggested an earlier infection with COVID-19 only offers half as much protection against the new variant as it does against Delta.

Whether Omicron is more transmissible than its predecessors—as both Alpha and Delta were—is harder to judge. Omicron cases in South Africa have risen steeply in the past few weeks. But Jeremy Farrar, head of the Wellcome Trust, sees cause for concern.

“The evidence that this is more transmissible is getting stronger every day,” he says. In the United Kingdom, the number of positive polymerase chain reaction tests in which the gene encoding the spike protein cannot be detected (a sign of a likely Omicron infection) is increasing rapidly.

In Oslo, a company Christmas party at a restaurant became a super spreading event, with at least 120 people testing positive; 19 cases so far have been confirmed as Omicron. (All attendees were vaccinated and had tested negative before the event.) In Denmark, 53 of 150 high school students who attended a party went on to test positive for Omicron.

“None of this alone tells us that this is more transmissible,” says Kristian Andersen, an infectious disease researcher at Scripps Research. Superspreading events, for instance, have been a hallmark of SARS-CoV-2 from the start.

“But Omicron is really rare still, so the fact that we see early cases being associated with super spreading events is quite concerning,” Andersen says. Early signs that Omicron causes less severe symptoms than previous variants offer some reassurance. Doctors in South Africa are reportedly seeing a larger proportion of mild COVID-19 cases in the hospital than at the start of earlier waves.

Data through 6 December indicate the number who needed oxygen support was lower than in previous waves, suggesting fewer patients are suffering the serious lung damage from COVID-19 that has put so many in the hospital during the pandemic.

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