Sonoma County is the first in the Bay Area to reinstate a ban on gatherings and issue new recommendations to stay home in response to the omicron surge. Shelter-in-place orders were used statewide before vaccinations became widely available to curtail spread of disease, in particular during the first wave of disease in March 2020 and during last winter’s surge.

The directive comes as the county, along with the rest of the Bay Area, is assailed by a massive spike in coronavirus cases that health officials said is threatening hospital capacity. Over the past two weeks, the county’s case rate has increased five-fold — to more than 121 cases a day per 100,000 residents — and is expected to keep climbing. Its positive test rate is at a record-high 16.5%; the previous high was 9.7%.

“Our case rates are at their highest level since the pandemic began and our hospitalizations are climbing at an alarming rate as well,” Dr. Sundari Mase, the Sonoma County health officer, said in a statement. “We are seeing widespread transmission occurring within unvaccinated groups as well as some transmission among vaccinated individuals.”

COVID hospitalizations in the county jumped from 28 on Jan. 3 to 76 less than a week later. At the peak of last winter’s surge, about 100 people were hospitalized. Health officials said that models suggest without further mitigation efforts, hospitalizations in this surge could top 380 a day.

“We know what we need to do to prevent our hospitals from being overwhelmed,” Mase said. “The next 30 days will be key to helping us stop this rapid spread of this highly contagious variant in our community.”

For the next 30 days, residents are advised to only leave their homes for essential purposes, such as going to work or school, shopping or seeking health care. The order specifies that for people who are at high risk of severe illness from COVID, gatherings must be limited to 12 people, except for family groups.

Gatherings are defined as any large group convening in places such as auditoriums, gymnasiums, stadiums, arenas, conference or wedding venues, or any other indoor or outdoor meeting spaces.

The ban does not apply to people in schools, workplace settings, places of worship, cafeterias or other places that are currently open to the public like museums or malls.

The ban on gatherings came in response to high proportion of cases being tied to large get-togethers. Of cases where the coronavirus exposure could be determined, more than half were from large gatherings, health officials said.