Health experts estimate omicron could be 25% to 50% more contagious than delta, and World Health Organization director general Tedros Adhanom Ghebreyesus said omicron "is spreading at a rate we have not seen with any previous variant." Now, new data offers insight into why the variant is so transmissible—but experts caution it's still too early to tell how this transmissibility translates to severity. 

Why omicron is spreading so rapidly

Since it was first identified in late November, the omicron variant been found in 77 countries and is quickly leading to new infections in those areas.

For example, Britain has seen a rapid uptick in Covid-19 cases from the omicron variant, and officials estimate that 200,000 people are being infected by omicron every day. And in Denmark, omicron cases are doubling around every two days.

Although delta remains the dominant variant in the United States, omicron may soon overtake it, the New York Times reports. Omicron now makes up 2.9% of all Covid-19 cases nationwide, up from 0.4% earlier this month. And some areas of the country, such as New York and New Jersey, are seeing much higher omicron case rates.

"No part of the country will be safe from omicron," said Shweta Bansal, a disease ecologist at Georgetown University.

According to early research, the reason behind omicron's rapid spread worldwide may be its high transmissibility, which is likely even greater than delta's.

In a preprint study published in medRxiv, researchers found that omicron's spike protein was more effective at entering human cells than delta's spike protein or that of the original coronavirus. This finding suggests that omicron can infect cells more quickly and easily.

"Strikingly, omicron was 4-fold more infectious than [the original coronavirus] and 2-fold more infectious than delta," wrote Wilfredo Garcia-Beltran, a fellow at the Ragon Institute of Massachusetts General Hospital, and colleagues who conducted the study.

Separately, another preliminary study from researchers at the University of Hong Kong found that omicron was able to replicate 70 times faster in human respiratory tissue than delta, which may help the variant spread more rapidly between people. There were also higher levels of the omicron variant in respiratory tissue 48 hours after infection than the delta variant.

The researchers also found that omicron replicated around 10 times slower in the lungs compared with the original version of the coronavirus—suggesting that it may lead to less severe illness overall even if it spreads quickly.

However, scientists still need to measure the viral loads inside infected people's respiratory tracts, added Garcia-Beltran. "I want to see what the viral loads look like for omicron," he says. "Samples from people who are actually infected—that's the gold standard. That's where the action is."

What omicron's transmissibility means for its overall threat

So far, several experts have said it is too soon to tell how omicron's rapid spread will affect the trajectory of the pandemic, particularly regarding hospitalizations and deaths, the Times reports.

"The most challenging question is severity," said Christopher Murray, director of the Institute for Health Metrics and Evaluations at the University of Washington. According to the Times, data on Covid-19 hospitalizations and deaths typically lags several weeks behind infections, meaning it will take time before definitive conclusions about omicron's severity can be drawn.

Currently, early data suggests omicron may be milder than previous variants, but some experts believe that omicron's increased transmissibility could still result in high rates of severe disease and death. In the United States, many hospitals are already struggling to handle surges of the delta variant, and a wave of omicron infections could quickly overwhelm them.

"By infecting many more people, a very infectious virus may cause more severe disease and death even though the virus itself may be less pathogenic," said Michael Chan Chi-Wai, the virologist who led the Hong Kong study. He added that recent studies suggesting omicron can partially evade vaccine immunity mean that the overall threat from the variant "is likely to be very significant."

"I think we need to be prepared for the possibility that this could be at least as bad as any previous wave that we've seen," said Justin Lessler, an epidemiologist at the University of North Carolina at Chapel Hill.  (Lapid, Reuters, 12/16; Anthes, New York Times, 12/16; Kay, Bloomberg, 12/15; Doucleff, "Goats and Soda," NPR, 12/15; Schnell, The Hill, 12/16)