"We are presuming this is an omicron outbreak in Marin," said County Public Health Officer Dr. Matt Willis.
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Dr. Willis says omicron is exploding in size across the country, even in Marin County. He says it's likely responsible for an outbreak at a holiday party held at Farm House Local restaurant in Larkspur on December 11.
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Twenty-eight attendees tested positive for COVID, some have shown mild symptoms.
"It's a harbinger of what we might expect for the next chapter of this pandemic, even in the coming days and weeks, we will see a lot more cases of COVID unfortunately," said Willis.
Willis says the party hosts required guests be vaccinated, and even tested for coronavirus before the event.
"The organizers did everything right. This isn't about blame, as much as it is the nature of the omicron variant, the contagiousness is so much higher," he added.
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Restaurant owner David Monson says his staff followed protocols after being notified about the outbreak.
"When that happens, we at the restaurant get tested, everyone was negative except one, they tested positive on Saturday," Monson said.
That employee is now isolating at home. Health officials aren't saying to cancel your holiday plans, but just make sure those you gather with are vaccinated and boosted.
"If you are not boosted, you are highly vulnerable to being infected," Willis added.
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The hosts of the party did not want to be identified and declined our interview request. But they provided a statement to ABC7 News which read in part, "We are extremely sorry this happened. But we hope that this is a cautionary tale of what can go wrong even when you try to do everything right. Even if you're lucky enough to not get terribly sick, testing positive for COVID is painfully disruptive for not only yourself but also the other members of your family. It is our hope that people can learn from this and protect our vulnerable population."