By Katie Nielsen

OAKLAND (CBS SF) — A number of staff members at Kaiser’s Oakland Medical Center have tested positive for the new Omicron COVID variant. They did not get sick at work, but instead tested positive after all 11 attended a wedding in Wisconsin at the end of November.

Interestingly, all had been vaccinated and had a booster shot, but still got sick. Their symptoms are described as mild and none have been hospitalized.

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The news comes on the same day that Pfizer released new data about how effective their vaccine is against Omicron.

Dr. John Swartzberg, an infectious disease expert at UC Berkeley, said he was encouraged by the information released on Wednesday by Pfizer. A preliminary study shows the initial two shots of the vaccine appear to be much less effective against Omicron than other variants, but a booster dose offers significantly more protection.

“Protection really means it’s going to protect you against getting hospitalized or dying,” Swartzberg said.

According to the CDC, nationwide only about 25% of people who are eligible for a booster shot have gotten one. COVID hospitalizations jumped up 30% in the past month – all due to the Delta variant.

“While Omicron is getting all the press, Delta is our biggest worry right now,” Swartzberg said.

He says the best say to be safe while gathering for the holidays is to keep it small – immediate friends and family only – skip the larger holiday parties, especially if they’re indoors, and go get a booster shot if eligible.

“What we do to prevent Delta is going to be the same things we do to prevent Omicron,” he said

Experts also stress all the information about Omicron is still very preliminary and could change in the coming weeks and months.

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When it comes to the Kaiser outbreak, the medical center initially identified a COVID case among staff on November 30 and notified the county health department.

On December 3, the health department announced 12 people had contracted COVID and that five of the cases were confirmed to be the omicron variant with other results pending. A sixth case of the omicron variant was subsequently confirmed.

The Kaiser Oakland staff members, ranging in age from 18 to 49, had attended a wedding in Wisconsin during Thanksgiving weekend. One of those who attended the wedding had also just returned from international travel, according to public health officials.

“These staff members’ exposure to COVID-19 happened at a wedding out-of-state, not through their work at the medical center,” said Kaiser spokesman Karl Sonkin.”They are isolated at home with mild symptoms as reported by Alameda County, which is consistent with the reported severity experienced by other people who are vaccinated and contracted this illness.”

Sonkin said Kaiser worked with the county health department to identify potential exposures among patients and identified eight patients and eight staff potentially exposed. Among them 13 tested negative for COVID and the three others are in process.

“Consistent with CDC guidelines, each of the affected staff were isolated at home, after they developed symptoms or received a positive test,” said Sonkin. “Any employee or physician confirmed to have COVID-19 or suspected of having COVID-19 due to symptoms will not come to work, in adherence to Kaiser Permanente and CDC isolation protocols.”

Scientists don’t know whether omicron is more transmissible or makes people sicker. Researchers need more time and data, but a UC Berkeley epidemiologist predicted more holiday gatherings will likely lead to bad outcomes.

“Unfortunately, we’re going to see [more COVID cases] this winter,” said Dr. Arthur Reingold. “Hospitals are going to be stretched thin and it’s going to create problems. Now [will it be] the same magnitude as last winter? I certainly hope not.”

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The Centers for Disease Control and Prevention urges full vaccination against COVID-19 for everyone five years of age and older. Health officials also recommend prevention measures such as wearing a mask in public indoor settings, frequent handwashing, getting tested at the first sign of symptoms, and staying home when sick.