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Omicron is spreading fast, already outstripping delta in some countries.

Omicron’s high number of mutations in important viral proteins, and signs that the variant was behind a surge of COVID-19 cases in South Africa, quickly raised red flags, hurling the pandemic into yet another tsunami of uncertainty (SN: 12/1/21).

In the weeks since omicron emerged, the variant has been identified in more than 85 countries. Some, like Denmark, have identified some cases that date before South African researchers revealed omicron’s presence to the world — a hint that the variant had already slipped across borders from wherever it originated before its November discovery. In many of these places, omicron infections are rising fast.

Omicron is responsible for nearly all new COVID-19 cases in South Africa, and is already the predominant version of the coronavirus in London. The European Centre for Disease Prevention and Control estimates that omicron will be the most common variant across the European Union by mid-January.

In the United States, omicron now appears to reign. The variant was responsible for an estimated 73.2 percent of new infections across the country for the week ending December 18, according to the U.S. Centers for Disease Control and Prevention’s predictions. That’s up from an estimated 12.6 percent the previous week and 0.7 percent the week ending December 4. Omicron now accounts for an estimated 92 percent of new cases in New York and New Jersey and 96.3 percent in Washington, Oregon and Idaho.

Previous data suggested that estimates of omicron’s prevalence from earlier in December were lower. It takes time to collect and analyze viruses from patient samples, Bronwyn MacInnis, director of pathogen surveillance at the Broad Institute of MIT and Harvard, said in a December 14 call with journalists. So the numbers can “change quickly as more data comes in off machines in real time.”

With that in mind, omicron is likely to worsen the surge that is unfolding across the United States. Some places, including New York City, are already seeing large spikes in COVID-19 cases with numbers rising fast. It currently takes about two days for the number of omicron cases to double, CDC director Rochelle Walensky said December 15 in a White House news briefing. Highly infectious delta, in comparison, doubled every two weeks at the beginning of its surge in the United States (SN: 7/2/21).

It was a huge question whether omicron would compete with delta for global dominance. Now, some real-world studies show that omicron is coming on strong in many regions. Preliminary data from the United Kingdom show that omicron is around 3.2 times as likely to spread among households as delta is, researchers with Public Health England, a U.K. health agency, reported December 9.

And people exposed to omicron may get sick faster — and therefore be able to spread the virus sooner — than people exposed to other variants. An analysis of an omicron outbreak at a company Christmas party in Norway found that the median time that a person exposed at the party developed symptoms was three days, researchers reported December 16 in Eurosurveillance. It takes slightly longer for delta infections to cause symptoms — around four days — and about five days for non-delta variants.    

The reasons behind omicron’s swift spread are still fuzzy. It could be because omicron is more transmissible than delta or because it can dodge parts of the immune response. Or, more likely, it could be a mixture of both, says Kartik Chandran, a virologist at Albert Einstein College of Medicine in New York City. Some preliminary studies done in lab-grown cells hint that omicron may turn out to be more transmissible than delta, though how much more is unclear. One reason may be because the new variant might make more copies of itself inside host cells than other variants do.

Omicron may also replicate particularly well in bronchial cells — which line the tubes that deliver air to the lungs — compared with how well it grows in lung tissue, researchers reported December 15 in preliminary data from the University of Hong Kong. If the virus is growing well in bronchial cells, symptoms like coughing could release a lot of viruses into the air.