Researchers testing coronavirus samples in Washington state have recorded a rapid rise in cases with a mutation that is characteristic of the omicron variant, mirroring trends that have emerged in countries like South Africa, Britain and Denmark.

Researchers at the University of Washington found that 13% of 217 positive coronavirus case specimens collected Wednesday had the mutation. That was up from about 7% of samples they had tested from the day before, and 3% from the day before that — in a region that had its first identified cases only two weeks ago.

“It’s clearly looking like it’s rising really quickly,” said Dr. Pavitra Roychoudhury, a researcher at the University of Washington.

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Roychoudhury cautioned that the relatively small sample sizes provided only a limited window into the variant’s trajectory. But even so, she said, the results add to worries that omicron may be highly transmissible.

As omicron spreads around the globe, scientists have seen evidence that the variant can partially evade existing immune defenses. But they are still trying to determine how often the variant causes severe disease.

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Dr. Trevor Bedford, who studies the spread and evolution of viruses at the Fred Hutchinson Cancer Research Center in Seattle, said the omicron numbers are still too small to have a large impact on overall case numbers, but he said that would likely change by next week as it continues to displace the virulent delta version of the virus.

“There is an inevitable very large wave of omicron,” Bedford said. “It’s going to happen.”

What’s much less certain is how much the new variant could trigger hospitalizations. Bedford said there are signs that the variant is producing less severe infections, but he said the range of possibilities for the variant remains wide.

Washington state may have a clearer window into omicron’s spread than other parts of the country do, because of research groups in Seattle that have invested in testing and sequencing the virus’s genomic structure. Those researchers helped identify the initial reported outbreak of the coronavirus in the United States early in 2020.

The UW Medicine virology lab has been screening samples of the virus from across the state, looking for a single mutation that indicates omicron’s presence — a technique that Roychoudhury said helps provide rapid warning of the variant’s spread. Detailed genomic sequencing of each sample is definitive, but can take longer, she said, adding that so far, all the samples that were flagged by spotting the mutation last week and then were sequenced have come back confirmed as omicron.

The lab gets about half its samples from King County, which includes Seattle and is by far the most populous in the state. A small portion comes from outside Washington state.

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