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YOLO COUNTY, Calif. (KTXL) — Yolo County said Wednesday it has seen a “significant increase” in the COVID-19 omicron variant, including several new cases linked to a holiday party.

Over the course of last week, between Dec. 12 and 18, the UC Davis Genome Center identified 42 new omicron cases, the county reported. The positive tests came from residents in Davis, El Macero, West Sacramento and Woodland.

“The Omicron variant is spreading quickly in Yolo County, and accounted for 41% of new cases detected by the UC Davis Genome Center last week,” said Yolo County Public Health Officer Dr. Aimee Sisson, Yolo County Public Health Officer.

County officials said 10 of those cases came from an outbreak that stemmed from a workplace holiday party in Davis.

Of the 50 party attendees, 16 people have tested positive for COVID-19 so far. All those infected are fully vaccinated and eight have received a booster shot.

Public health officials identified the first case of the omicron variant in Yolo County on Dec. 9. Since then, River City High School in West Sacramento reported a suspected omicron outbreak involving at least five of its students and a student’s family member.

Omicron has also been detected in wastewater samples from Davis, the county said.

There has been a steady increase in the number of COVID-19 cases across the country.

“We were already expecting an increased number of cases because of winter. The virus is traditionally transmitted during winter due to favorable temperature and humidity,” said Dr. Dean Blumberg with UC Davis Health.

Sacramento County reported its first cases of the variant Tuesday.

“With omicron, it’s somewhere between two to five times more transmissible than delta,” Blumberg continued.

With the upcoming holidays and an increase in travel, doctors at UC Davis Health said they believe the number of COVID-19 cases will peak sometime in late January.