Santa Clara County Health Officer Dr. Sara Cody said Thursday she expects a “deluge” of omicron cases in the county as the COVID-19 variant begins sweeping across the Bay Area and residents start traveling and gathering for the holidays.

“What I see is perhaps one of the most challenging moments we’ve had yet in the pandemic,” Cody said at a news conference. “And I think it’s challenging because it’s not what we’re expecting. We’ve all come to live with COVID over the last two years, and we’re all a bit tired. But I want to let you know, when I look around the corner, I see a lot of COVID, and a lot of omicron.”

Although the number of omicron cases in Santa Clara County remains low — 10 as of Thursday — there are indications they  will soon rise, Cody said. Last week, the county announced a single omicron case but noted it also found the new variant in a wastewater plant. By Thursday, Cody said the county had detected omicron in all four of its wastewater plants, which she said serve a majority of the region’s population.

And while Santa Clara County’s vaccination numbers remain among the highest in the country — 80% of all eligible individuals in the entire region are inoculated with at least two doses — worldwide evidence suggests the county may not be able to escape a widespread outbreak of the new variant.

That is another reason everyone age 16 and older should get a booster shot, she urged.

She said Norway and Denmark, which have vaccination rates similar to Santa Clara County’s, are seeing high numbers of omicron cases.

But Dr. Marty Fenstersheib, the county’s COVID-19 vaccine officer, said the region is “not doing as well” with boosters as it did with first and second doses. He said 44% of those 18 and older in the county have received a booster, which leaves a quarter of a million people 50 and older who haven’t and are without additional protection.

Of the 10 positive cases in Santa Clara County, four individuals were unvaccinated, five were vaccinated with two doses and one had also received a booster, Cody said. Most of the individuals were symptomatic, though no one had to be hospitalized.

“None of our prevention tools replace any others,” Fenstersheib said. “Vaccinations are our most important. And our boosters are our most important. But again, there’s still the need to use all of our tools in our toolbox.”

In addition to getting boosters, Cody and Fenstersheib recommended that people take COVID tests before getting together in groups for the holidays.

While the vaccine’s effectiveness against the highly transmissible omicron is still being determined, the latest research is somewhat troubling. A study out of South Africa this week revealed that a two-dose Pfizer-BioNTech vaccination provided only 33% protection against the new variant.  And a Massachusetts-based study found that people who got two doses of Moderna, Pfizer-BioNTech and the J&J vaccines were unable to neutralize omicron in their blood.

However, Israel has reported that boosters there offered a hundredfold increase in protection against omicron, and two doses still protect against severe cases.

The first U.S. case of the variant — which is considered twice as infectious as delta and four times as much as the original strain — was detected in San Francisco on Dec. 1. As of Thursday, Alameda County is reporting seven cases, while San Mateo and Marin counties are reporting none.

In Contra Costa County, “Omicron was detected in a local wastewater sample last week, but the new variant hasn’t been found in any of our genetic sequencing yet,” health department spokesman Will Harper said in an email. “But it’s just a matter of time until we do.”

San Francisco’s health department did not immediately respond to a request for its omicron case counts.

Stanford University announced Thursday it would start with remote classes the first two weeks of the winter quarter because of concerns surrounding omicron.

And on Wednesday, California reinstated mask requirements for everyone including toddlers in indoor settings, regardless of their vaccination status. San Francisco, Alameda and Contra Costa counties are staying with their own local rules, however, which allow individuals to unmask in certain indoor settings.